Publikationen

• Ehrlich, A., Crewell, S., Herber, A., Klingebiel, M., Lüpkes, C., Mech, M., Becker, S., Borrmann, S., Bozem, H., Buschmann, M., Clemen, H.-C., De La Torre Castro, E., Dorff, H., Dupuy, R., Eppers, O., Ewald, F., George, G., Giez, A., Grawe, S., Gourbeyre, C., Hartmann, J., Jäkel, E., Joppe, P., Jourdan, O., Jurányi, Z., Kirbus, B., Lucke, J., Luebke, A. E., Maahn, M., Maherndl, N., Mallaun, C., Mayer, J., Mertes, S., Mioche, G., Moser, M., Müller, H., Pörtge, V., Risse, N., Roberts, G., Rosenburg, S., Röttenbacher, J., Schäfer, M., Schaefer, J., Schäfler, A., Schirmacher, I., Schneider, J., Schnitt, S., Stratmann, F., Tatzelt, C., Voigt, C., Walbröl, A., Weber, A., Wetzel, B., Wirth, M., and Wendisch, M.: A comprehensive in-situ and remote sensing data set collected during the HALO(AC)3 aircraft campaign, Earth Syst. Sci. Data Discuss. [preprint], doi:10.5194/essd-2024-281, in review, 2024.
• Schirmacher, I., Schnitt, S., Klingebiel, M., Maherndl, N., Kirbus, B., Ehrlich, A., Mech, M., and Crewell, S.: Clouds and precipitation in the initial phase of marine cold air outbreaks as observed by airborne remote sensing, EGUsphere [preprint], doi:10.5194/egusphere-2024-850, 2024.
• Arteaga, D., et al. incl. Ehrlich, A., Wendisch, M.: Arctic mixed-phase clouds simulated by the WRF model: Comparisons with ACLOUD radar and in situ airborne observations and sensitivity of microphysics properties, Atmos.Res., in review, 2023.
• Mahecha, M. D., et al. incl. Wendisch, M., Ehrlich, A., Schäfer, M.: Biodiversity and climate extremes: known interactions and research gaps, ESS Open Archive, doi:10.22541/essoar.169462031.19744802/v1, in review, 2023.
• Walbröl, A., Michaelis, J., Becker, S., Dorff, H., Gorodetskaya, I., Kirbus, B., Lauer, M., Maherndl, N., Maturilli, M., Mayer, J., Müller, H., Neggers, R. A. J., Paulus, F. M., Röttenbacher, J., Rückert, J. E., Schirmacher, I., Slättberg, N., Ehrlich, A., Wendisch, M., and Crewell, S.: Environmental conditions in the North Atlantic sector of the Arctic during the HALO–(AC)³ campaign, EGUsphere [preprint], doi:10.5194/egusphere-2023-668, 2023.

• Foth, L., Dorn, W., Rinke, A., Jäkel, E., and Niehaus, H.: On the importance to consider the cloud dependence in parameterizing the albedo of snow on sea ice, The Cryosphere, 18, 4053–4064, doi:10.5194/tc-18-4053-2024, 2024.
• Wendisch, M., et al.: Overview: Quasi-Lagrangian observations of Arctic air mass transformations – Introduction and initial results of the HALO–(AC)³ aircraft campaign, Atmos. Chem. Phys., 24, 8865–8892, doi:10.5194/acp-24-8865-2024, 2024.
• Röttenbacher, J., Ehrlich, A., Müller, H., Ewald, F., Luebke, A. E., Kirbus, B., Hogan, R. J., and Wendisch, M.: Evaluating the Representation of Arctic Cirrus Solar Radiative Effects in the IFS with Airborne Measurements, Atmos. Chem. Phys., 24, 8085–8104, doi:10.5194/acp-24-8085-2024, 2024.
• Tao, R., Nicolaus, M., Katlein, C., Anhaus, P., Hoppmann, M., Spreen, G., Niehaus, H., Jäkel, E., Wendisch, M., and Haas, C.: Seasonality of spectral radiative fluxes and optical properties of Arctic sea ice during the spring–summer transition. Elementa: Science of the Anthropocene, 12 (1): 00130. doi: 10.1525/elementa.2023.00130, 2024.
• Wolf, K., Bellouin, N., and Boucher, O.: Distribution and morphology of non-persistent contrail and persistent contrail formation areas in ERA5, Atmos. Chem. Phys., 24, 5009–5024, doi:10.5194/acp-24-5009-2024, 2024.
• Müller, H., Ehrlich, A., Jäkel, E., Röttenbacher, J., Kirbus, B., Schäfer, M., Hogan, R. J., and Wendisch, M.: Evaluation of downward and upward solar irradiances simulated by the Integrated Forecasting System of ECMWF using airborne observations above Arctic low-level clouds, Atmos. Chem. Phys., 24, 4157–4175, doi:10.5194/acp-24-4157-2024, 2024.
• Wang, X., Wolf, K., Boucher, O., and Bellouin, N.: Radiative effect of two contrail cirrus outbreaks over Western Europe estimated using geostationary satellite observations and radiative transfer calculations. Geophysical Research Letters, 51, e2024GL108452. doi:10.1029/2024GL108452, 2024.
• Kirbus, B., Schirmacher, I., Klingebiel, M., Schäfer, M., Ehrlich, A., Slättberg, N., Lucke, J., Moser, M., Müller, H., and Wendisch, M.: Thermodynamic and cloud evolution in a cold-air outbreak during HALO-(AC)³: quasi-Lagrangian observations compared to the ERA5 and CARRA reanalyses, Atmos. Chem. Phys., 24, 3883–3904, doi:10.5194/acp-24-3883-2024, 2024.
• Jäkel, E., Becker, S., Sperzel, T. R., Niehaus, H., Spreen, G., Tao, R., Nicolaus, M., Dorn, W., Rinke, A., Brauchle, J., and Wendisch, M.: Observations and modeling of areal surface albedo and surface types in the Arctic, The Cryosphere, 18, 1185–1205, doi:10.5194/tc-18-1185-2024, 2024.
• Lonardi, M., Akansu, E. F., Ehrlich, A., Mazzola, M., Pilz, C., Shupe, M. D., Siebert, H., and Wendisch, M.: Tethered balloon-borne observations of thermal-infrared irradiance and cooling rate profiles in the Arctic atmospheric boundary layer, Atmos. Chem. Phys., 24, 1961–1978, doi:10.5194/acp-24-1961-2024, 2024.

• Awoleye, P.O., Ogunjobi, K.O., Balogun, I.A., and Wendisch, M.: Quantification of Aerosol Particle Radiative Forcing Under Cloud-Free Condition During Dry Season Period Over West Africa. Aerosol Sci. Eng., doi:10.1007/s41810-023-00202-8, 2023.
• Klingebiel, M., Ehrlich, A., Ruiz-Donoso, E., Risse, N., Schirmacher, I., Jäkel, E., Schäfer, M., Wolf, K., Mech, M., Moser, M., Voigt, C., and Wendisch, M.: Variability and properties of liquid-dominated clouds over the ice-free and sea-ice-covered Arctic Ocean, Atmos. Chem. Phys., 23, 15289–15304, doi:10.5194/acp-23-15289-2023, 2023.
• Kirbus, B., Chylik, J., Ehrlich, A., Becker, S., Schäfer, M., Neggers, R., and Wendisch, M.: Analysis of an Arctic cold air outbreak during autumn and related air mass transformations forced by surface changes and advection in higher altitudes, Elementa: Science of the Anthropocene, 11 (1): 00079, doi:10.1525/elementa.2023.00079, 2023.
• Wolf, K., Bellouin, N., and Boucher, O.: Sensitivity of cirrus and contrail radiative effect on cloud microphysical and environmental parameters, Atmos. Chem. Phys., 23, 14003–14037, doi:10.5194/acp-23-14003-2023, 2023.
• Yeom, J.M., Szodry, K.-E., Siebert, H., Ehrlich, A., Mellado, J.P., Shaw, R.A. et al.: High-resolution measurements of microphysics and entrainment in marine stratocumulus clouds. Quarterly Journal of the Royal Meteorological Society, 1–17, doi:10.1002/qj.4586, 2023.
• Schirmacher, I., Kollias, P., Lamer, K., Mech, M., Pfitzenmaier, L., Wendisch, M., and Crewell, S.: Assessing Arctic low-level clouds and precipitation from above – a radar perspective, Atmos. Meas. Tech., 16, 4081–4100, doi:10.5194/amt-16-4081-2023, 2023.
• Sperzel, T.R., Jäkel, E., Pätzold, F., Lampert, A., Niehaus, H., Spreen, G., Rosenburg, S., Birnbaum, G., Neckel, N., Wendisch, M.: Surface albedo measurements and surface type classification from helicopter-based observations during MOSAiC, Sci Data 10, 584, doi:10.1038/s41597-023-02492-6, 2023.
• Wendisch, M., Stapf, J., Becker, S., Ehrlich, A., Jäkel, E., Klingebiel, M., Lüpkes, C., Schäfer, M., and Shupe, M. D.: Effects of variable ice–ocean surface properties and air mass transformation on the Arctic radiative energy budget, Atmos. Chem. Phys., 23, 9647–9667, doi:10.5194/acp-23-9647-2023, 2023.
• Linke, O., Quaas, J., Baumer, F., Becker, S., Chylik, J., Dahlke, S., Ehrlich, A., Handorf, D., Jacobi, C., Kalesse-Los, H., Lelli, L., Mehrdad, S., Neggers, R. A. J., Riebold, J., Saavedra Garfias, P., Schnierstein, N., Shupe, M. D., Smith, C., Spreen, G., Verneuil, B., Vinjamuri, K. S., Vountas, M., and Wendisch, M.: Constraints on simulated past Arctic amplification and lapse rate feedback from observations, Atmos. Chem. Phys., 23, 9963–9992, doi:10.5194/acp-23-9963-2023, 2023.
• Rosenburg, S., Lange, C., Jäkel, E., Schäfer, M., Ehrlich, A., and Wendisch, M.: Retrieval of snow layer and melt pond properties on Arctic sea ice from airborne imaging spectrometer observations, Atmos. Meas. Tech., 16, 3915–3930, doi:10.5194/amt-16-3915-2023, 2023.
• Akansu, E. F., Dahlke, S., Siebert, H., and Wendisch, M.:Evaluation of methods to determine the surface mixing layer height of the atmospheric boundary layer in the central Arctic during polar night and transition to polar day in cloudless and cloudy conditions, Atmos. Chem. Phys., 23, 15473–15489, doi:10.5194/acp-23-15473-2023, 2023.
• Pilz, C., Lonardi, M., Egerer, U., Siebert, H., Ehrlich, A., Heymsfield, A. J., Schmitt, C. G., Shupe, M. D., Wehner, B., and Wendisch, M.: Profile observations of the Arctic atmospheric boundary layer with the BELUGA tethered balloon during MOSAiC. Sci Data 10, 534, doi:10.1038/s41597-023-02423-5, 2023.
• Moser, M., Voigt, C., Jurkat-Witschas, T., Hahn, V., Mioche, G., Jourdan, O., Dupuy, R., Gourbeyre, C., Schwarzenboeck, A., Lucke, J., Boose, Y., Mech, M., Borrmann, S., Ehrlich, A., Herber, A., Lüpkes, C., and Wendisch, M.: Microphysical and thermodynamic phase analyses of Arctic low-level clouds measured above the sea ice and the open ocean in spring and summer, Atmos. Chem. Phys., 23, 7257–7280, doi:10.5194/acp-23-7257-2023, 2023.
• Becker, S., Ehrlich, A., Schäfer, M., and Wendisch, M.: Airborne observations of the surface cloud radiative effect during different seasons over sea ice and open ocean in the Fram Strait, Atmos. Chem. Phys., 23, 7015–7031, doi:10.5194/acp-23-7015-2023, 2023.
• Egerer, U., Cassano, J. J., Shupe, M. D., de Boer, G., Lawrence, D., Doddi, A., Siebert, H., Jozef, G., Calmer, R., Hamilton, J., Pilz, C., and Lonardi, M.: Estimating turbulent energy flux vertical profiles from uncrewed aircraft system measurements: exemplary results for the MOSAiC campaign, Atmos. Meas. Tech., 16, 2297–2317, doi:10.5194/amt-16-2297-2023, 2023.
• Kirbus, B., Tiedeck, S., Camplani, A., Chylik, J., Crewell, S., Dahlke, S., Ebell, K., Gorodetskaya, I., Griesche, H., Handorf, D., Höschel, I., Lauer, M., Neggers, R., Rückert, J., Shupe, M.D., Spreen, G., Walbröl, A., Wendisch, M. and Rinke, A.: Surface impacts and associated mechanisms of a moisture intrusion into the Arctic observed in mid-April 2020 during MOSAiC. Front. Earth Sci. 11:1147848, doi:10.3389/feart.2023.1147848, 2023.
• Lochmann, M., Kalesse-Los, H., Schäfer, M., Heinrich, I., Leinweber, R.: Analysing wind power ramp events and improving very short-term wind power predictions by including wind speed observations. Wind Energy., 1- 16. doi:10.1002/we.2816, 2023.
• Ehrlich, A., Zöger, M., Giez, A., Nenakhov, V., Mallaun, C., Maser, R., Röschenthaler, T., Luebke, A. E., Wolf, K., Stevens, B., Wendisch, M.: A new airborne broadband radiometer system and an efficient method to correct dynamic thermal offsets, Atmos. Meas. Tech., 16, 1563–1581, doi:10.5194/amt-16-1563-2023, 2023.
• Niehaus, H., Spreen, G., Birnbaum, G., Istomina, L., Jäkel, E.,  Linhardt, F., Neckel, N., Nicolaus, M., Sperzel, T., Webster, M., Wright, N.: Sea ice melt pond fraction derived from Sentinel-2 data: Along the MOSAiC drift and Arctic-wide. Geophysical Research Letters, 50, e2022GL102102, doi:10.1029/2022GL102102, 2023.
• Chechin, D. G., Lüpkes, C., Hartmann, J., Ehrlich, A., Wendisch, M.: Turbulent structure of the Arctic boundary layer in early summer driven by stability, wind shear and cloud top radiative cooling: ACLOUD airborne observations, Atmos. Chem. Phys., 23, 4685–4707, https://doi.org/10.5194/acp-23-4685-2023, 2023.
• Liu, Y., Su, H., Wang, S., Wei, C., Tao, W., Pöhlker, M. L., Pöhlker, C., Holanda, B. A., Krüger, O. O., Hoffmann, T., Wendisch, M., Artaxo, P., Pöschl, U., Andreae, M. O., Cheng, Y.: Strong particle production and condensational growth in the upper troposphere sustained by biogenic VOCs from the canopy of the Amazon Basin, Atmos. Chem. Phys., 23, 251–272, doi:10.5194/acp-23-251-2023, 2023.
• Jurányi, Z., Zanatta, M., Lund, M.T., Samset, B. H., Skeie, R. B., Sharma, S., Wendisch, M., and  Herber, A.: Atmospheric concentrations of black carbon are substantially higher in spring than summer in the Arctic. Commun. Earth Environ., 4, 91, doi:10.1038/s43247-023-00749-x, 2023.
• Holanda, B. A., et al., with Wendisch, M.: African biomass burning affects aerosol cycling over the Amazon. Commun. Earth Environ., 4, 154, doi:10.1038/s43247-023-00795-5, 2023.
• Romps, D. M., Latimer, K., Zhu, Q., Jurkat-Witschas, T., Mahnke, C., Prabhakaran, T., Weigel, R., Wendisch, M.: Air pollution unable to intensify storms via warm-phase invigoration. Geophysical Research Letters, 50, e2022GL100409, doi:10.1029/2022GL100409, 2023.
• Wendisch, M., et al., with Brückner, M., Ehrlich, A., Donth, T., Gottschalk, M., Jäkel, E., Klingebiel, M., Ruiz Donoso, E., Schäfer, M., Stapf, J.: Atmospheric and Surface Processes, and Feedback Mechanisms Determining Arctic Amplification: A Review of First Results and Prospects of the (AC)3 Project, Bull. Amer. Meteorol., 104 (1), E208–E242, doi:10.1175/BAMS-D-21-0218.1, 2023.

• Dauhut, T., Couvreux, F., Bouniol, D., Beucher, F., Volkmer, L., Pörtge, V., Schäfer, M., Ayet, A., Brilouet, P.-E., Jacob, M., and Wirth, M.: Flower trade-wind clouds are shallow mesoscale convective systems. Quarterly Journal of the Royal Meteorological Society, 149( 750), 325– 347, doi:10.1002/qj.4409, 2023.

• Mech, M., Ehrlich, A., Herber, A., Lüpkes, C., Wendisch, M., Becker, S., Boose, Y., Chechin, D.,  Crewell, S.,  Dupuy, R., Gourbeyre, C., Hartmann, J., Jäkel, E., Jourdan, O., Leif- Kliesch, L., Klingebiel, M., Kulla, B. S., Mioche, G., Moser, M., Risse, N., Ruiz-Donoso, E., Schäfer, M., Stapf, J., and Voigt, C.: MOSAiC-ACA and AFLUX - Arctic airborne campaigns characterizing the exit area of MOSAiC. Sci Data 9, 790, doi:10.1038/s41597-022-01900-7, 2022.
• Wendisch, M., et al., with Brückner, M., Ehrlich, A., Donth, T., Gottschalk, M., Jäkel, E., Klingebiel, M., Ruiz Donoso, E., Schäfer, M., Stapf, J.: Atmospheric and Surface Processes, and Feedback Mechanisms Determining Arctic Amplification: A Review of First Results and Prospects of the (AC)3 Project, BAMS (published online ahead of print 2022), doi:10.1175/BAMS-D-21-0218.1, 2022.
• Mahecha, M.D., Bastos, A., Bohn, F.J., Eisenhauer, N., Feilhauer, H., Hartmann, H., Hickler, T., Kalesse-Los, H., Migliavacca, M., Otto, F.E.L., Peng, J., Quaas, J., Tegen, I., Weigelt, A., Wendisch, M., Wirth, C.: Biodiversity loss and climate extremes — study the feedbacks. Nature. 612, 7938, 30-32. doi:10.1038/d41586-022-04152-y, 2022.
• Braga, R. C., Rosenfeld, D., Andreae, M. O., Pöhlker, C., Pöschl, U., Voigt, C., Weinzierl, B., Wendisch, M., Pöhlker, M. L. and Harrison, D.: Detrainment dominates CCN concentrations around non-precipitating convective clouds over the Amazon. Geophysical Research Letters, 49, e2022GL100411, doi:10.1029/2022GL100411, 2022.
• Lonardi, M., Pilz, C., Akansu, E.F., Dahlke, S., Egerer, U., Ehrlich, A., Griesche, H., Heymsfield, A.J., Kirbus, B.,  Schmitt, C.G., Shupe, M.D., Siebert, H., Wehner, B., Wendisch, M.: Tethered balloon-borne profile measurements of atmospheric properties in the cloudy atmospheric boundary layer over the Arctic sea ice during MOSAiC: Overview and first results, Elem Sci Anth, 10(1), 000120, doi:10.1525/elementa.2021.000120, 2022.
• Becker, S., Ehrlich, A., Jäkel, E., Carlsen, T., Schäfer, M., and Wendisch, M.: Airborne measurements of directional reflectivity over the Arctic marginal sea ice zone, Atmos. Meas. Tech., 15, 2939–2953, doi:10.5194/amt-15-2939-2022, 2022.
• Schäfer, M., Wolf, K., Ehrlich, A., Hallbauer, C., Jäkel, E., Jansen, F., Luebke, A. E., Müller, J., Thoböll, J., Röschenthaler, T., Stevens, B., and Wendisch, M.: VELOX – A new thermal infrared imager for airborne remote sensing of cloud and surface properties, Atmos. Meas. Tech., 15, 1491–1509, doi:10.5194/amt-2021-341, 2022.
• Luebke, A., Ehrlich, A., Schäfer, M., Wolf, K. and Wendisch, M.: An assessment of macrophysical and microphysical cloud properties driving radiative forcing of shallow trade-wind clouds, Atmos. Chem. Phys., 22, 2727–2744, doi:10.5194/acp-22-2727-2022, 2022.
• Geerts, B., Giangrande, S. E., McFarquhar, G. M., Xue, L., Abel, S. J., Comstock, J. M., Crewell, S., DeMott, P. J., Ebell, K., Field, P., Hill, T. C. J., Hunzinger, A., Jensen, M. P., Johnson, K. L., Juliano, T. W., Kollias, P., Kosovic, B., Lackner, C., Luke, E., Lüpkes, C., Matthews, A. A., Neggers, R., Ovchinnikov, M., Powers, H., Shupe, M. D., Spengler, T., Swanson, B. E., Tjernström, M., Theisen, A. K., Wales, N. A., Wang, Y., Wendisch, M., Wu, P.: The COMBLE Campaign: A Study of Marine Boundary Layer Clouds in Arctic Cold-Air Outbreaks, Bulletin of the American Meteorological Society, 103(5), E1371-E1389, doi:10.1175/BAMS-D-21-0044.1, 2022.
• Shupe, M. D., et al., with Ehrlich, A., Klingebiel, M., Lonardi, M., Wendisch, M.,: Overview of the MOSAiC expedition: Atmosphere. Elem Sci Anth, 10(1), 00060, doi:10.1525/elementa.2021.00060, 2022.
• Rabe, B., et al., with Wendisch, M.: Overview of the MOSAiC expedition: Physical oceanography. Elem Sci Anth, 10(1), 00062, doi:10.1525/elementa.2021.00062, 2022.
• Nicolaus, M., et al., with Wendisch, M.: Overview of the MOSAiC expedition: Snow and sea ice. Elem Sci Anth, 10(1), 000046, doi:10.1525/elementa.2021.000046, 2022.

• Shrivastava, M., Rasool, Q. Z., Zhao, B., Octaviani, M., Zaveri, R.A., Zelenyuk, A., Gaudet, B., Liu, Y., Shilling, J.E., Schneider, J., Schulz, C., Zöger, M., Martin, C.T., Ye, J., Guenther, A., Souza, R.F., Wendisch, M., Pöschl, U.: Tight Coupling of Surface and In-Plant Biochemistry and Convection Governs Key Fine Particulate Components over the Amazon Rainforest, ACS Earth and Space Chemistry , 6 (2), 380-390, doi:10.1021/acsearthspacechem.1c00356, 2022.

• Jäkel E., Carlsen T., Ehrlich A., Wendisch M., Schäfer M., Rosenburg S., Nakoudi K., Zanatta M., Birnbaum G., Helm V., Herber A., Istomina L., Mei L., Rohde A.: Measurements and Modeling of Optical-Equivalent Snow Grain Sizes under Arctic Low-Sun Conditions, Remote Sensing, 13(23), 4904, doi:10.3390/rs13234904, 2021.
• Braga, R.C., Ervens, B., Rosenfeld, D., Andreae, M.O., Förster, J.-D., Fütterer, D., Pardo, L.H., Holanda, B.A., Jurkat-Witschas, T., Krüger, O.O., Lauer, O.,  Machado, L.A.T., Pöhlker, Ch., Sauer, D., Voigt, Ch., Walser, A. Wendisch, M., Pöschl, U. and Pöhlker, M.L.: Cloud droplet formation at the base of tropical convective clouds: closure between modeling and measurement results of ACRIDICON–CHUVA, Atmos. Chem. Phys., 21, 17513–17528, doi:10.5194/acp-21-17513-2021, 2021.
• Braga, R.C., Rosenfeld, D., Krüger, O.O., Ervens, B., Holanda, B.A., Wendisch, M., Krisna, T., Pöschl, U., Andreae, M.O., Voigt, Ch. and Pöhlker, M.L.: Linear relationship between effective radius and precipitation water content near the top of convective clouds: measurement results from ACRIDICON–CHUVA campaign, Atmos. Chem. Phys., 21, 14079–14088, doi:10.5194/acp-21-14079-2021, 2021.
• Stevens, B., Ehrlich, A., Klingebiel, M., Röttenbacher, J., Schäfer, M., Wendisch, M., Wolf, K., Schmidt, J., et al.: EUREC⁴A, Earth Syst. Sci. Data, 13, 4067–4119, doi:10.5194/essd-13-4067-2021, 2021.
• George, G., Stevens, B., Bony, S., Pincus, R., Fairall, C., Schulz, H., Kölling, T., Kalen, Q. T., Klingebiel, M., Konow, H., Lundry, A., Prange, M., and Radtke, J.: JOANNE: Joint dropsonde Observations of the Atmosphere in tropical North atlaNtic meso-scale Environments, Earth Syst. Sci. Data, 13, 5253–5272, doi:10.5194/essd-13-5253-2021, 2021.
• Konow, H., Ewald, F., George, G., Jacob, M., Klingebiel, M., Kölling, T., Luebke, A. E., Mieslinger, T., Pörtge, V., Radtke, J., Schäfer, M., Schulz, H., Vogel, R., Wirth, M., Bony, S., Crewell, S., Ehrlich, A., Forster, L., Giez, A., Gödde, F., Groß, S., Gutleben, M., Hagen, M., Hirsch, L., Jansen, F., Lang, T., Mayer, B., Mech, M., Prange, M., Schnitt, S., Vial, J., Walbröl, A., Wendisch, M., Wolf, K., Zinner, T., Zöger, M., Ament, F., and Stevens, B.: EUREC⁴A's HALO, Earth Syst. Sci. Data, 13, 5545–5563, doi:10.5194/essd-13-5545-2021, 2021.
• Mei, L., Rozanov, V., Jäkel, E., Cheng, X., Vountas, M., and Burrows, J. P.: The retrieval of snow properties from SLSTR Sentinel-3 – Part 2: Results and validation, The Cryosphere, 15, 2781–2802, doi:10.5194/tc-15-2781-2021, 2021.
• Foth, A., Zimmer, J., Lauermann, F., and Kalesse-Los, H.:, Evaluation of micro rain radar-based precipitation classification algorithms to discriminate between stratiform and convective precipitation, Atmos. Meas. Tech., 14, 4565-4574, doi:10.5194/amt-14-4565-2021, 2021.
• Sun, B., Schäfer, M., Ehrlich, A., Jäkel, E., and Wendisch, M.: Influence of atmospheric adjacency effect on top-of-atmosphere radiances and its correction in the retrieval of Lambertian surface reflectivity based on three-dimensional radiative transfer, Rem. Sens. of Envir., 263, 112543, doi:10.1016/j.rse.2021.112543, 2021.
• Egerer, U., Ehrlich, A., Gottschalk, M., Neggers, R. A. J., Siebert, H., and Wendisch, M., Case study of a humidity layer above Arctic stratocumulus and potential turbulent coupling with the cloud top, Atmos. Chem. Phys., doi:10.5194/acp-2020-584, 2021.
• Siebert, S., Szodry, K.-E., Egerer, U., Wehner, B., Henning, S., Chevalier, K., Lückerath, J., Welz, O., Weinhold, K., Lauermann, F., Gottschalk, M., Ehrlich, A., Wendisch, M., Fialho, P., Roberts, P. F. G., Allwayin, N., Schum, S., Shaw, R. A., Mazzoleni, C., Mazzoleni, L., Nowak, J. L., Malinowski, S., Karpinska, K., Kumala, W., Czyzewska, D., Luke, E. P., Kollias, P., Wood, R., Mellado, J. P., Observations of aerosol, cloud, turbulence, and radiation properties at the top of the marine boundary layer over the Eastern North Atlantic Ocean: The ACORES campaign, Bull. Am. Meteorol. Soc., 1–59, doi:10.1175/BAMS-D-19-0191.1, 2021.
• Stephan, C. C., Schnitt, S., Schulz, H., Bellenger, H., de Szoeke, S. P., Acquistapace, C., Baier, K., Dauhut, T., Laxenaire, R., Morfa-Avalos, Y., Person, R., Quiñones Meléndez, E., Bagheri, G., Böck, T., Daley, A., Güttler, J., Helfer, K. C., Los, S. A., Neuberger, A., Röttenbacher, J., Raeke, A., Ringel, M., Ritschel, M., Sadoulet, P., Schirmacher, I., Stolla, M. K., Wright, E., Charpentier, B., Doerenbecher, A., Wilson, R., Jansen, F., Kinne, S., Reverdin, G., Speich, S., Bony, S., and Stevens, B.: Ship- and island-based atmospheric soundings from the 2020 EUREC⁴A field campaign, Earth Syst. Sci. Data, 13, 491–514, doi:10.5194/essd-13-491-2021, 2021.

• Quaas, J., Arola, A., Cairns, B., Christensen, M., Deneke, H., Ekman, A. M. L., Feingold, G., Fridlind, A., Gryspeerdt, E., Hasekamp, O., Li, Z., Lipponen, A., Ma, P.-L., Mülmenstädt, J., Nenes, A., Penner, J., Rosenfeld, D., Schrödner, R., Sinclair, K., Sourdeval, O., Stier, P., Tesche, M., van Diedenhoven, B., and Wendisch, M., Constraining the Twomey effect from satellite observations: Issues and perspectives, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2020-279, in review, 2020.
• Carlsen, T., Birnbaum, G., Ehrlich, A., Helm, V., Jäkel, E., Schäfer, M., and Wendisch, M., Parameterizing anisotropic reflectance of snow surfaces from airborne digital camera observations in Antarctica, The Cryosphere, 14, 3959–3978, doi:10.5194/tc-14-3959-2020, 2020.
• Kretzschmar, J., Stapf, J., Klocke, D., Wendisch, M., and Quaas, J., Employing airborne radiation and cloud microphysics observations to improve cloud representation in ICON at kilometer-scale resolution in the Arctic, Atmos. Chem. Phys., 20, 13145–13165, doi:10.5194/acp-20-13145-2020, 2020.
• Li, L., Li, Z., Chang, W., Ou, Y., Goloub, P., Li, C., Li, K., Hu, Q., Wang, J., and Wendisch, M., Aerosol solar radiative forcing near the Taklimakan Desert based on radiative transfer and regional meteorological simulations during the Dust Aerosol Observation-Kashi campaign, Atmos. Chem. Phys., 20, 10845–10864, doi:10.5194/acp-20-10845-2020, 2020.
• Stapf, J., Ehrlich, A., Jäkel, E., Lüpkes, C., and Wendisch, M., Reassessment of shortwave surface cloud radiative forcing in the Arctic: Consideration of surface-albedo–cloud interactions, Atmos. Chem. Phys., 20, 9895–9914, doi:10.5194/acp-20-9895-2020, 2020.
• Wolf, K., A. Ehrlich, M. Mech, R. J. Hogan, and M. Wendisch, Evaluation of ECMWF Radiation Scheme Using Aircraft Observations of Spectral Irradiance above Clouds, J. Atmos. Sci., 77 (8), 2665–2685, doi:10.1175/JAS-D-19-0333.1, 2020.
  A novel approach to compare airborne observations of solar spectral irradiances measured above clouds with along-track radiative transfer simulations (RTS) is presented. The irradiance measurements were obtained with the Spectral Modular Airborne Radiation Measurement System (SMART) installed on the High Altitude and Long Range Research Aircraft (HALO). The RTS were conducted using the operational ecRad radiation scheme of the Integrated Forecast System (IFS), operated by the European Centre for Medium-Range Weather Forecasts (ECMWF), and a stand-alone radiative transfer solver, the library for Radiative transfer (libRadtran). Profiles of observed and simulated radar reflectivity were provided by the HALO Microwave Package (HAMP) and the Passive and Active Microwave Transfer Model (PAMTRA), respectively. The comparison aims to investigate the capability of the two models to reproduce the observed radiation field. By analyzing spectral irradiances above clouds, different ice cloud optical parameterizations in the models were evaluated. Simulated and observed radar reflectivity fields allowed the vertical representation of the clouds modeled by the IFS to be evaluated, and enabled errors in the IFS analysis data (IFS AD) and the observations to be separated. The investigation of a North Atlantic low pressure system showed that the RTS, in combination with the IFS AD, generally reproduced t he observed radiation field. For heterogeneously distributed liquid water clouds, an underestimation of upward irradiance by up to 27% was found. Simulations of ice-topped clouds, using a specific ice optics parameterization, indicated a systematic underestimation of broadband cloud-top albedo, suggesting major deficiencies in the ice optics parameterization between 1242 and 1941 nm wavelength.
• Donth, T., Jäkel, E., Ehrlich, A., Heinold, B., Schacht, J., Herber, A., Zanatta, M., and Wendisch, M., Combining atmospheric and snow layer radiative transfer models to assess the solar radiative effects of black carbon in the Arctic, Atmos. Chem. Phys., 20, 8139–8156, doi:10.5194/acp-20-8139-2020, 2020.
• Hartmann, M., Adachi, K., Eppers, O., Haas, C., Herber, A., Holzinger, R., Hünerbein, A., Jäkel, E., Jentzsch, C., van Pinxteren, M., Wex, H., Willmes, S., Stratmann, F., Wintertime airborne measurements of ice nucleating particles in the high Arctic: a hint to a marine, biogenic source for Ice Nucleating Particles, Geophys. Res. Lett., 47, e2020GL087770, doi:10.1029/2020GL087770, 2020.
• Nakoudi, K., Ritter, C., Böckmann, C., Kunkel, D., Eppers, O., Rozanov, V., Mei, L., Pefanis, V., Jäkel, E., Herber, A.; Maturilli, M.; Neuber, R., Does the Intra-Arctic Modification of Long-Range Transported Aerosol Affect the Local Radiative Budget? (A Case Study), Remote Sens., 12(13), 2112 doi:10.3390/rs12132112, 2020.
• Ruiz-Donoso, E., Ehrlich, A., Schäfer, M., Jäkel, E., Schemann, V., Crewell, S., Mech, M., Kulla, B. S., Kliesch, L.-L., Neuber, R., and Wendisch, M., Small-scale structure of thermodynamic phase in Arctic mixed-phase clouds observed by airborne remote sensing during a cold air outbreak and a warm air advection event, Atmos. Chem. Phys., 20, 5487–5511, doi:10.5194/acp-20-5487-2020, in review, 2020.
• Holanda, B. A., Pöhlker, M. L., Walter, D., Saturno, J., Sörgel, M., Ditas, J., Ditas, F., Schulz, C., Franco, M. A., Wang, Q., Donth, T., Artaxo, P., Barbosa, H. M. J., Borrmann, S., Braga, R., Brito, J., Cheng, Y., Dollner, M., Kaiser, J., Klimach, T., Knote, C., Krüger, O. O., Fütterer, D., Lavrič, J. V., Ma, N., Machado, L. A. T., Ming, J., Morais, F., Paulsen, H., Sauer, D., Schlager, H., Schneider, J., Su, H., Weinzierl, B., Walser, A., Wendisch, M., Ziereis, H., Zöger, M., Pöschl, U., Andreae, M. O., and Pöhlker, C., Influx of African biomass burning aerosol during the Amazonian dry season through layered transatlantic transport of black carbon-rich smoke, Atmos. Chem. Phys., 20, 4757–4785, doi:10.5194/acp-20-4757-2020, 2020.
• Ehrlich, A., Schäfer, M., Ruiz-Donoso, E., and Wendisch, M., Airborne Remote Sensing of Arctic Clouds, In: Kokhanovsky A. (eds) Springer Series in Light Scattering, Volume 5, 39-66, doi:10.1007/978-3-030-38696-2_2, 2020.
• Mei, F., Wang, J., Comstock, J. M., Weigel, R., Krämer, M., Mahnke, C., Shilling, J. E., Schneider, J., Schulz, C., Long, C. N., Wendisch, M., Machado, L. A. T., Schmid, B., Krisna, T., Pekour, M., Hubbe, J., Giez, A., Weinzierl, B., Zoeger, M., Pöhlker, M. L., Schlager, H., Cecchini, M. A., Andreae, M. O., Martin, S. T., de Sá, S. S., Fan, J., Tomlinson, J., Springston, S., Pöschl, U., Artaxo, P., Pöhlker, C., Klimach, T., Minikin, A., Afchine, A., and Borrmann, S., Comparison of Aircraft Measurements during GoAmazon2014/5 and ACRIDICON-CHUVA, Atmos. Meas. Tech., 13, 661–684, doi:10.5194/amt-13-661-2020, 2020.
• Polonik, P., Knote, C., Zinner, T., Ewald, F., Kölling, T., Mayer, B., Andreae, M. O., Jurkat-Witschas, T., Klimach, T., Mahnke, C., Molleker, S., Pöhlker, C., Pöhlker, M. L., Pöschl, U., Rosenfeld, D., Voigt, C., Weigel, R., and Wendisch, M., The challenge of simulating the sensitivity of the Amazonian clouds microstructure to cloud condensation nuclei number concentrations, Atmos. Chem. Phys., 20, 1591–1605, doi:10.5194/acp-20-1591-2020, 2020.
• Kokhanovsky, A., Tomasi, C., Smirnov, A., Herber, A., Neuber, R., Ehrlich, A., Lupi, A., Petkov, B. H., Mazzola, M., Ritter, C., Toledano, C., Carl, R., Vitale, V., and Holben, B., Zielinski, T., Bélanger, S., Larouche, P., Kinne, S., Radionov, V., Wendisch, M., Tackett, J. L., and Winker, D. M., Remote Sensing of Arctic Atmospheric Aerosols, in: Kokhanovsky A., Tomasi C. (eds) Physics and Chemistry of the Arctic Atmosphere, 505-589, Springer Polar Sciences. Springer, Cham, doi:10.1007/978-3-030-33566-3_9, 2020.
• Pohl, C., Istomina, L., Tietsche, S., Jäkel, E., Stapf, J., Spreen, G., and Heygster, G., Broadband albedo of Arctic sea ice from MERIS optical data, The Cryosphere, 14, 165–182, doi:10.5194/tc-14-165-2020, 2020.
• Cohen, J., Zhang, X., Francis, J., Jung, T., Kwok, R., Overland, J., Ballinger, T., Bhatt, U.S., Chen, H. W., Coumou, D., Feldstein, S., Handorf, D., Henderson, G., Ionita, M., Kretschmer, M., Laliberte, F., Lee, S., Linderholm, H. W., Maslowski, W., Peings, Y., Pfeiffer, K., Rigor, I., Semmler, T., Stroeve, J., Taylor, P.C., Vavrus, S., Vihma, T., Wang, S., Wendisch, M., Wu, Y., and Yoon, J., Divergent consensuses on Arctic amplification influence on midlatitude severe winter weather, Nat. Clim. Chang., 10, 20–29, doi:10.1038/s41558-019-0662-y, 2020.
• Pohl, C., Rozanov, V. V., Wendisch, M., Spreen, G., and Heygster, G., Impact of the near-field effects on radiative transfer simulations of the bidirectional reflectance factor and albedo of a densely packed snow layer, J. Quant. Spectrosc. Radiat. Transf., 106704, doi:10.1016/j.jqsrt.2019.106704, 2020.
• Sun, B., Jäkel, E., Schäfer, M., and Wendisch, M., A Biased Sampling Approach to Accelerate Backward Monte Carlo Atmospheric Radiative Transfer Simulations and its Application to Arctic Heterogeneous Cloud and Surface Conditions, J. Quant. Spectrosc. Radiat. Transfer, 240, 106690, doi:10.1016/j.jqsrt.2019.106690, 2020.
  Electromagnetic radiation within the Earth-atmosphere system is inherently of three dimensional (3D) nature. To quantitatively investigate the various 3D radiative effects in arbitrary spatial resolution, the backward Monte Carlo approach provides an efficient and flexible technique. In this paper, a new importance sampling scheme of 3D backward Monte Carlo radiative transfer is introduced as Light Estimator Including Polarization, Surface Inhomogeneities, and Clouds (LEIPSIC) code (https://gitee.com/BinLeipzig/LEIPSIC). The new code is described, and its accuracy and efficiency are quantified. Two test cases of typical Arctic surface and atmosphere scenarios are investigated. Applying the new code it is shown that the neglect of 3D effects introduces a negative (cooling) net irradiance error of around 13 W m² integrated over the solar spectrum.

• Ehrlich, A., Wendisch, M., Lüpkes, C., Buschmann, M., Bozem, H., Chechin, D., Clemen, H.-C., Dupuy, R., Eppers, O., Hartmann, J., Herber, A., Jäkel, E., Järvinen, E., Jourdan, O., Kästner, U., Kliesch, L.-L., Köllner, F., Mech, M., Mertes, S., Neuber, R., Ruiz-Donoso, E., Schnaiter, M., Schneider, J., Stapf, J., and Zanatta, M., A comprehensive in situ and remote sensing data set from the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) campaign, Earth Syst. Sci. Data, 11, 1853–1881, doi:10.5194/essd-11-1853-2019, 2019.
  The Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) campaign was carried out north-west of Svalbard (Norway) between 23 May and 6 June 2017. The objective of ACLOUD was to study Arctic boundary layer and mid-level clouds and their role in Arctic amplification. Two research aircraft (Polar 5 and 6) jointly performed 22 research flights over the transition zone between open ocean and closed sea ice. Both aircraft were equipped with identical instrumentation for measurements of basic meteorological parameters, as well as for turbulent and radiative energy fluxes. In addition, on Polar 5 active and passive remote sensing instruments were installed, while Polar 6 operated in situ instruments to characterize cloud and aerosol particles as well as trace gases. A detailed overview of the specifications, data processing, and data quality is provided here. It is shown that the scientific analysis of the ACLOUD data benefits from the coordinated operation of both aircraft. By combining the cloud remote sensing techniques operated on Polar 5, the synergy of multi-instrument cloud retrieval is illustrated. The remote sensing methods were validated using truly collocated in situ and remote sensing observations. The data of identical instruments operated on both aircraft were merged to extend the spatial coverage of mean atmospheric quantities and turbulent and radiative flux measurement. Therefore, the data set of the ACLOUD campaign provides comprehensive in situ and remote sensing observations characterizing the cloudy Arctic atmosphere. All processed, calibrated, and validated data are published in the World Data Center PANGAEA as instrument-separated data subsets.
• Schacht, J., Heinold, B., Quaas, J., Backman, J., Cherian, R., Ehrlich, A., Herber, A., Huang, W. T. K., Kondo, Y., Massling, A., Sinha, P. R., Weinzierl, B., Zanatta, M., and Tegen, I., The importance of the representation of air pollution emissions for the modeled distribution and radiative effects of black carbon in the Arctic, Atmos. Chem. Phys., 19, 11159–11183, doi:10.5194/acp-19-11159-2019, 2019.
  Aerosol particles can contribute to the Arctic amplification (AA) by direct and indirect radiative effects. Specifically, black carbon (BC) in the atmosphere, and when deposited on snow and sea ice, has a positive warming effect on the top-of-atmosphere (TOA) radiation balance during the polar day. Current climate models, however, are still struggling to reproduce Arctic aerosol conditions. We present an evaluation study with the global aerosol-climate model ECHAM6.3-HAM2.3 to examine emission-related uncertainties in the BC distribution and the direct radiative effect of BC. The model results are comprehensively compared against the latest ground and airborne aerosol observations for the period 2005–2017, with a focus on BC. Four different setups of air pollution emissions are tested. The simulations in general match well with the observed amount and temporal variability in near-surface BC in the Arctic. Using actual daily instead of fixed biomass burning emissions is crucial for reproducing individual pollution events but has only a small influence on the seasonal cycle of BC. Compared with commonly used fixed anthropogenic emissions for the year 2000, an up-to-date inventory with transient air pollution emissions results in up to a 30 % higher annual BC burden locally. This causes a higher annual mean all-sky net direct radiative effect of BC of over 0.1 W m^-2 at the top of the atmosphere over the Arctic region (60–90 °N), being locally more than 0.2 W m^-2 over the eastern Arctic Ocean. We estimate BC in the Arctic as leading to an annual net gain of 0.5 W m^-2 averaged over the Arctic region but to a local gain of up to 0.8 W m^-2 by the direct radiative effect of atmospheric BC plus the effect by the BC-in-snow albedo reduction. Long-range transport is identified as one of the main sources of uncertainties for ECHAM6.3-HAM2.3, leading to an overestimation of BC in atmospheric layers above 500 hPa, especially in summer. This is related to a misrepresentation in wet removal in one identified case at least, which was observed during the ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) summer aircraft campaign. Overall, the current model version has significantly improved since previous intercomparison studies and now performs better than the multi-model average in the Aerosol Comparisons between Observation and Models (AEROCOM) initiative in terms of the spatial and temporal distribution of Arctic BC.
• Egerer, U., Gottschalk, M., Siebert, H., Ehrlich, A., and Wendisch, M., The new BELUGA setup for collocated turbulence and radiation measurements using a tethered balloon: First applications in the cloudy Arctic boundary layer, Atmos. Meas. Tech., 12, 4019-4038, doi:10.5194/amt-12-4019-2019, 2019.
  The new BELUGA (Balloon-bornE moduLar Utility for profilinG the lower Atmosphere) tethered balloon system is introduced. It combines a set of instruments to measure turbulent and radiative parameters and energy fluxes. BELUGA enables collocated measurements either at a constant altitude or as vertical profiles up to 1.5 km in height. In particular, the instrument payload of BELUGA comprises three modular instrument packages for high-resolution meteorological, wind vector and broadband radiation measurements. Collocated data acquisition allows for estimates of the driving parameters in the energy balance at various heights. Heating rates and net irradiances can be related to turbulent fluxes and local turbulence parameters such as dissipation rates. In this paper the technical setup, the instrument performance, and the measurement strategy of BELUGA are explained. Furthermore, the high vertical resolution due to the slow ascent speed is highlighted as a major advantage of tethered balloon-borne observations. Three illustrative case studies of the first application of BELUGA in the Arctic atmospheric boundary layer are presented. As a first example, measurements of a single-layer stratocumulus are discussed. They show a pronounced cloud top radiative cooling of up to 6 K h^-1. To put this into context, a second case elaborates respective measurements with BELUGA in a cloudless situation. In a third example, a multilayer stratocumulus was probed, revealing reduced turbulence and negligible cloud top radiative cooling for the lower cloud layer. In all three cases the net radiative fluxes are much higher than turbulent fluxes. Altogether, BELUGA has proven its robust performance in cloudy conditions of the Arctic atmospheric boundary layer.
• Jäkel., E., Stapf, J., Wendisch, M., Nicolaus, M., Dorn, W., and Rinke, A., Validation of the sea ice surface albedo scheme of the regional climate model HIRHAM-NAOSIM using aircraft measurements during the ACLOUD/PASCAL campaigns, The Cryosphere, 13, 1695-1708, doi:10.5194/tc-13-1695-2019, 2019.
  For large-scale and long-term Arctic climate simulations appropriate parameterization of the surface albedo is required. Therefore, the sea ice surface (SIS) albedo parameterization of the coupled regional climate model HIRHAM–NAOSIM was examined against broadband surface albedo measurements performed during the joint ACLOUD (Arctic CLoud Observations Using airborne measurements during polar Day) and PASCAL (Physical feedbacks of Arctic boundary layer, Sea ice, Cloud and AerosoL) campaigns, which were performed in May–June 2017 north of Svalbard. The SIS albedo parameterization was tested using measured quantities of the prognostic variables surface temperature and snow depth to calculate the surface albedo and the individual fractions of the ice surface subtypes (snow-covered ice, bare ice, and melt ponds) derived from digital camera images taken on board the Polar 5 and 6 aircraft. The selected low-altitude (less than 100 m) flight sections of overall 12 flights were performed over surfaces dominated by snow-covered ice. It was found that the range of parameterized SIS albedo for individual days is smaller than that of the measurements. This was attributed to the biased functional dependence of the SIS albedo parameterization on temperature. Furthermore, a time-variable bias was observed with higher values compared to the modeled SIS albedo (0.88 compared to 0.84 for 29 May 2017) in the beginning of the campaign, and an opposite trend towards the end of the campaign (0.67 versus 0.83 for 25 June 2017). Furthermore, the surface type fraction parameterization was tested against the camera image product, which revealed an agreement within 1 %. An adjustment of the variables, defining the parameterized SIS albedo, and additionally accounting for the cloud cover could reduce the root-mean-squared error from 0.14 to 0.04 for cloud free/broken cloud situations and from 0.06 to 0.05 for overcast conditions.
• Stevens, B., Ament, F., Bony, S., Crewell, S., Ewald, F., Gross, S., Hansen, A., Hirsch, L., Jacob, M., Kölling, T., Konow, H., Mayer, B., Wendisch, M., Wirth, M., Wolf, K., Bakan, S., Bauer-Pfundstein, B., Brueck, M., Delanoë, J., Ehrlich, A., Farrell, D., Forde, M., Gödde, F., Grob, H., Hagen, M., Jäkel, E., Jansen, F., Klepp, C., Klingebiel, M., Mech, M., Peters, G., Rapp, M., Wing, A. A., Zinner, T., A High-Altitude Long-Range Aircraft Configured as a Cloud Observatory: The NARVAL Expeditions, Bull. Am. Meteorol. Soc., 100, 1061–1077 doi:10.1175/BAMS-D-18-0198.1, 2019.
  A configuration of the High-Altitude Long-Range Research Aircraft (HALO) as a remote sensing cloud observatory is described, and its use is illustrated with results from the first and second Next-Generation Aircraft Remote Sensing for Validation (NARVAL) field studies. Measurements from the second NARVAL (NARVAL2) are used to highlight the ability of HALO, when configured in this fashion, to characterize not only the distribution of water condensate in the atmosphere, but also its impact on radiant energy transfer and the covarying large-scale meteorological conditions—including the large-scale velocity field and its vertical component. The NARVAL campaigns with HALO demonstrate the potential of airborne cloud observatories to address long-standing riddles in studies of the coupling between clouds and circulation and are helping to motivate a new generation of field studies.
• Wendisch, M., A. Macke, A. Ehrlich, C. Lüpkes, M. Mech, D. Chechin, K. Dethloff, C. Barrientos, H. Bozem, M. Brückner, H.-C. Clemen, S. Crewell, T. Donth, R. Dupuy, K. Ebell, U. Egerer, R. Engelmann, C. Engler, O. Eppers, M. Gehrmann, X. Gong, M. Gottschalk, C. Gourbeyre, H. Griesche, J. Hartmann, M. Hartmann, B. Heinold, A. Herber, H. Herrmann, G. Heygster, P. Hoor, S. Jafariserajehlou, E. Jäkel, E. Järvinen, O. Jourdan, U. Kästner, S. Kecorius, E. M. Knudsen, F. Köllner, J. Kretzschmar, L. Lelli, D. Leroy, M. Maturilli, L. Mei, S. Mertes, G. Mioche, R. Neuber, M. Nicolaus, T. Nomokonova, J. Notholt, M. Palm, M. van Pinxteren, J. Quaas, P. Richter, E. Ruiz-Donoso, M. Schäfer, K. Schmieder, M. Schnaiter, J. Schneider, A. Schwarzenböck, P. Seifert, M. D. Shupe, H. Siebert, G. Spreen, J. Stapf, F. Stratmann, T. Vogl, A. Welti, H. Wex, A. Wiedensohler, M. Zanatta, S. Zeppenfeld, The Arctic Cloud Puzzle: Using ACLOUD/PASCAL Multi-Platform Observations to Unravel the Role of Clouds and Aerosol Particles in Arctic Amplification, Bull. Am. Meteorol. Soc., 100, 841-871, doi:10.1175/BAMS-D-18-0072.1, 2019.
  Clouds play an important role in Arctic amplification. This term represents the recently observed enhanced warming of the Arctic relative to the global increase of near-surface air temperature. However, there are still important knowledge gaps regarding the interplay between Arctic clouds and aerosol particles, and surface properties, as well as turbulent and radiative fluxes that inhibit accurate model simulations of clouds in the Arctic climate system. In an attempt to resolve this so-called Arctic cloud puzzle, two comprehensive and closely coordinated field studies were conducted: the Arctic Cloud Observations Using Airborne Measurements during Polar Day (ACLOUD) aircraft campaign and the Physical Feedbacks of Arctic Boundary Layer, Sea Ice, Cloud and Aerosol (PASCAL) ice breaker expedition. Both observational studies were performed in the framework of the German Arctic Amplification: Climate Relevant Atmospheric and Surface Processes, and Feedback Mechanisms (AC)³ project. They took place in the vicinity of Svalbard, Norway, in May and June 2017. ACLOUD and PASCAL explored four pieces of the Arctic cloud puzzle: cloud properties, aerosol impact on clouds, atmospheric radiation, and turbulent dynamical processes. The two instrumented Polar 5 and Polar 6 aircraft; the icebreaker Research Vessel (R/V) Polarstern; an ice floe camp including an instrumented tethered balloon; and the permanent ground-based measurement station at Ny-Ålesund, Svalbard, were employed to observe Arctic low- and mid-level mixed-phase clouds and to investigate related atmospheric and surface processes. The Polar 5 aircraft served as a remote sensing observatory examining the clouds from above by downward-looking sensors; the Polar 6 aircraft operated as a flying in situ measurement laboratory sampling inside and below the clouds. Most of the collocated Polar 5/6 flights were conducted either above the R/V Polarstern or over the Ny-Ålesund station, both of which monitored the clouds from below using similar but upward-looking remote sensing techniques as the Polar 5 aircraft. Several of the flights were carried out underneath collocated satellite tracks. The paper motivates the scientific objectives of the ACLOUD/PASCAL observations and describes the measured quantities, retrieved parameters, and the applied complementary instrumentation. Furthermore, it discusses selected measurement results and poses critical research questions to be answered in future papers analyzing the data from the two field campaigns.
• Bilal, M., Nazeer, M., Nichol, J.E., Bleiweiss, M.P., Qiu, Z., Jäkel, E., Campbell, J.R., Atique, L., Huang, X., Lolli, S., A Simplified and Robust Surface Reflectance Estimation Method (SREM) for Use over Diverse Land Surfaces Using Multi-Sensor Data, Remote Sens., 11, 1344, doi:10.3390/rs11111344, 2019.
  Surface reflectance (SR) estimation is the most critical preprocessing step for deriving geophysical parameters in multi-sensor remote sensing. Most state-of-the-art SR estimation methods, such as the vector version of the Second Simulation of the Satellite Signal in the Solar Spectrum (6SV) radiative transfer (RT) model, depend on accurate information on aerosol and atmospheric gases. In this study, a Simplified and Robust Surface Reflectance Estimation Method (SREM) based on the equations from 6SV RT model, without integrating information of aerosol particles and atmospheric gasses, is proposed and tested using Landsat 5 Thematic Mapper (TM), Landsat 7 Enhanced Thematic Mapper plus (ETM+), and Landsat 8 Operational Land Imager (OLI) data from 2000 to 2018. For evaluation purposes, (i) the SREM SR retrievals are validated against in situ SR measurements collected by Analytical Spectral Devices (ASD) from the South Dakota State University (SDSU) site, USA; (ii) cross-comparison between the SREM and Landsat spectral SR products, i.e., Landsat Ecosystem Disturbance Adaptive Processing System (LEDAPS) and Landsat 8 Surface Reflectance Code (LaSRC), are conducted over 11 urban (2013–2018), 13 vegetated (2013–2018), and 11 desert/arid (2000 to 2018) sites located over different climatic zones at a global scale; (iii) the performance of the SREM spectral SR retrievals for low to high aerosol loadings is evaluated; (iv) spatio-temporal cross-comparison is conducted for six Landsat paths/rows located in Asia, Africa, Europe, and the United States of America from 2013 to 2018 to consider a large variety of land surfaces and atmospheric conditions; (v) cross-comparison is also performed for the Normalized Difference Vegetation Index (NDVI), the Enhanced Vegetation Index (EVI), and the Soil Adjusted Vegetation Index (SAVI) calculated from both the SREM and Landsat SR data; (vi) the SREM is also applied to the Sentinel-2A and Moderate Resolution Imaging Spectrometer (MODIS) data to explore its applicability; and (vii) errors in the SR retrievals are reported using the mean bias error (MBE), root mean squared deviation (RMSD), and mean systematic error (MSE). Results depict significant and strong positive Pearson’s correlation (r), small MBE, RMSD, and MSE for each spectral band against in situ ASD data and Landsat (LEDAPS and LaSRC) SR products. Consistency in SREM performance against Sentinel-2A (r = 0.994, MBE = −0.009, and RMSD = 0.014) and MODIS (r = 0.925, MBE = 0.007, and RMSD = 0.014) data suggests that SREM can be applied to other multispectral satellites data. Overall, the findings demonstrate the potential and promise of SREM for use over diverse surfaces and under varying atmospheric conditions using multi-sensor data on a global scale.
• Wolf, K., Ehrlich, A., Jacob, M., Crewell, S., Wirth, M., and Wendisch, M., Improvement of Airborne Retrievals of Cloud Droplet Number Concentration of Trade Wind Cumulus Using a Synergetic Approach, Atmos. Meas. Tech., 12, 1635-1658, doi:10.5194/amt-12-1635-2019, 2019.
  In situ measurements of cloud droplet number concentration N are limited by the sampled cloud volume. Satellite retrievals of N suffer from inherent uncertainties, spatial averaging, and retrieval problems arising from the commonly assumed strictly adiabatic vertical profiles of cloud properties. To improve retrievals of N it is suggested in this paper to use a synergetic combination of passive and active airborne remote sensing measurement, to reduce the uncertainty of N retrievals, and to bridge the gap between in situ cloud sampling and global averaging. For this purpose, spectral solar radiation measurements above shallow trade wind cumulus were combined with passive microwave and active radar and lidar observations carried out during the second Next Generation Remote Sensing for Validation Studies (NARVAL-II) campaign with the High Altitude and Long Range Research Aircraft (HALO) in August 2016. The common technique to retrieve N is refined by including combined measurements and retrievals of cloud optical thickness t, liquid water path (LWP), cloud droplet effective radius r_eff, and cloud base and top altitude. Three approaches are tested and applied to synthetic measurements and two cloud scenarios observed during NARVAL-II. Using the new combined retrieval technique, errors in N due to the adiabatic assumption have been reduced significantly.
• Li, L., Li, Z., Dubovik, O., Zheng, X., Li, Z., Ma, J., and Wendisch, M., Effects of the shape distribution of aerosol particles on their volumetric scattering properties and the radiative transfer through the atmosphere that includes polarization, Appl. Opt., 58, 1475-1484, doi:10.1364/AO.58.001475, 2019.
  In this paper, we investigate the effects of shape distribution of aerosol particles on the volumetric scattering properties, as well as the radiance and polarization distributions of skylight, by numerical simulations. The results demonstrate that the shape distribution indeed exerts a significant influence on the skylight degree of linear polarization. The skylight polarization calculated assuming the microscope-measured shape distributions is distinct from that using the inversion-based shape distributions. The significant effects will influence the retrieval of the sphericity of aerosols based on the sun–sky radiometer measurements. Our results suggest that using representative shape distributions obtained by direct microscopic observations of aerosol samples captured in the natural atmosphere has a high potential to improve the retrieval of the aerosol shape parameter.
• Zinner, T., Schwarz, U., Kölling, T., Ewald, F., Jäkel., E., Mayer, B., and Wendisch, M., Cloud geometry from oxygen-A band observations through an aircraft side window, Atmos. Meas. Tech., 12, 1167-1181, doi:10.5194/amt-12-1167-2019, 2019.
  During the ACRIDICON-CHUVA (Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems–Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud Resolving Modeling and to the GPM (Global Precipitation Measurement)) aircraft campaign in September 2014 over the Amazon, among other topics, aerosol effects on the development of cloud microphysical profiles during the burning season were studied. Hyperspectral remote sensing with the imaging spectrometer specMACS provided cloud microphysical information for sun-illuminated cloud sides. In order to derive profiles of phase or effective radius from cloud side observations, vertical location information is indispensable. For this purpose, spectral measurements of cloud-side-reflected radiation in the oxygen A absorption band collected by specMACS were used to determine absorption path length between cloud sides and the instrument aboard the aircraft. From these data, horizontal distance and eventually vertical height were derived. It is shown that, depending on aircraft altitude and sensor viewing direction, an unambiguous relationship of absorption and distance exists and can be used to retrieve cloud geometrical parameters. A comparison to distance and height information from stereo image analysis (using data of an independent camera) demonstrates the efficiency of the approach. Uncertainty estimates due to method, instrument and environmental factors are provided. The main sources of uncertainty are unknown in cloud absorption path contributions due to complex 3-D geometry or unknown microphysical properties, variable surface albedo and aerosol distribution. A systematic difference of 3.8 km between the stereo and spectral method is found which can be attributed to 3-D geometry effects not considered in the method's simplified cloud model. If this offset is considered, typical differences found are 1.6 km for distance and 230 m for vertical position at a typical distance around 20 km between sensor and convective cloud elements of typically 1–10 km horizontal and vertical extent.
• Ghate, V.P., P. Kollias, S. Crewell, A.M. Fridlind, T. Heus, U. Löhnert, M. Maahn, G.M. McFarquhar, D. Moisseev, M. Oue, M. Wendisch, and C. Williams, The Second ARM Training and Science Application Event: Training the Next Generation of Atmospheric Scientists, Bull. Am. Meteorol. Soc., 100, ES5-ES9, 10.1175/BAMS-D-18-0242.1, 2019.
  Twenty-four graduate students and early career scientists from all over the world gathered to learn about ARM data and the latest observation science between 14–21 July 2018 in Norman, Oklahoma.

• Knudsen, E. M., Heinold, B., Dahlke, S., Bozem, H., Crewell, S., Gorodetskaya, I. V., Heygster, G., Kunkel, D., Maturilli, M., Mech, M., Viceto, C., Rinke, A., Schmithüsen, H., Ehrlich, A., Macke, A., Lüpkes, C., and Wendisch, M., Meteorological conditions during the ACLOUD/PASCAL field campaign near Svalbard in early summer 2017, Atmos. Chem. Phys., 18, 17995-18022, doi:10.5194/acp-18-17995-2018, 2018.
  The two concerted field campaigns, Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) and the Physical feedbacks of Arctic planetary boundary level Sea ice, Cloud and AerosoL (PASCAL), took place near Svalbard from 23 May to 26 June 2017. They were focused on studying Arctic mixed-phase clouds and involved observations from two airplanes (ACLOUD), an icebreaker (PASCAL) and a tethered balloon, as well as ground-based stations. Here, we present the synoptic development during the 35-day period of the campaigns, using near-surface and upper-air meteorological observations, as well as operational satellite, analysis, and reanalysis data. Over the campaign period, short-term synoptic variability was substantial, dominating over the seasonal cycle. During the first campaign week, cold and dry Arctic air from the north persisted, with a distinct but seasonally unusual cold air outbreak. Cloudy conditions with mostly low-level clouds prevailed. The subsequent 2 weeks were characterized by warm and moist maritime air from the south and east, which included two events of warm air advection. These synoptical disturbances caused lower cloud cover fractions and higher-reaching cloud systems. In the final 2 weeks, adiabatically warmed air from the west dominated, with cloud properties strongly varying within the range of the two other periods. Results presented here provide synoptic information needed to analyze and interpret data of upcoming studies from ACLOUD/PASCAL, while also offering unprecedented measurements in a sparsely observed region.
• Li, L., Li, Z., Li, Q., Hua, X., and Wendisch, M., Retrieval of the polarized phase function of aerosol particles based on multi-angle multi-spectral measurements of Stokes parameters Q and U, Spectrosc. Spect. Anal., 38(12), 3699-3707, doi:10.3964/j.issn.1000-0593(2018)12-3699-09, 2018.
  The polarized phase function is one of the important optical parameters, which is very sensitive to the aerosol complex refractive index, particle size and shape. The multi-angle multi-spectral polarization remote sensing is an effective means for obtaining the aerosol polarized phase function. As a ground-based high-accuracy polarization instrument for aerosol remote sensing, the new generation CIMEL dual-polar sun-sky radiometer CE318-DP has been introduced into the worldwide AErosol RObotic NETwork (AERONET). Meanwhile, as the main instrument of the Sun/sky-radiometer Observation NETwork (SONET) with the extension of multi-wavelength polarization measurements, it has accumulated polarization data for many years over observation stations with different aerosol types. However, the retrieval has been based only on the degree of linear polarization or the polarized radiance up to now. Compared to the degree of linear polarization and the polarized radiance, the Stokes parameters Q and U contain information not only on intensity of linear polarization but also on the orientation of polarization. This study introduces an algorithm to retrieve the aerosol polarized phase function based on the Stokes parameters Q and U of skylight from the multi-angle multi-spectral polarization measurements of the CE318-DP. Considering that the Stokes parameter U is changeless with different aerosol properties for the CE318-DP standard polarization observation scenario PPP (Polarized Principal Plane), which is difficult to be utilized, a new ALMP (ALMucantar Polarization) observation scenario is tested to obtain the Stokes parameters Q and U, then to be applied in retrieval of polarized phase function. As to the typical biomass burning and water-soluble aerosols, the results of -P₁₂/P₁₁ in the channels centered at 340 to 1640 nm were presented and analyzed systematically. Moreover, the applicability of the inversion algorithm in clear and hazy sky conditions was also tested. For the visible and near-infrared channels, the results were in agreement with the truth values not only for the PPP but also for the ALMP geometries. One of the reasons for the obvious deviation of the results in the ultraviolet (UV) bands was also discussed, which was the assumption of the approximately equivalent "ratios of atmospheric single scattering and atmospheric scattering" based on the initial aerosol parameters and the real aerosol parameters could not be satisfied in the UV bands. The inversion model should be improved to be applied to the short-wave channels in further studies. On this basis, some subsequent researches can be engaged in to utilize the features of multi-spectral -P₁₂/P₁₁ to improve the retrieval of aerosol microphysical properties.
• Pithan, F., Svensson, G., Caballero, R., Chechin, D., Cronin, T. W., Ekman, A. M. L., Neggers, R., Shupe, M. D., Solomon, A., Tjernström, M., and Wendisch, M., Role of air-mass transformations in exchange between the Arctic and mid-latitudes, Nat. Geosci., 11, 805-812, doi:10.1038/s41561-018-0234-1, 2018.
  Pulses of warm and moist air from lower latitudes provide energy to the Arctic and form its main energy source outside of the summer months. These pulses can cause substantial surface warming and trigger ice melt. Air-mass transport in the opposite direction, away from the Arctic, leads to cold-air outbreaks. The outbreaks are often associated with cold extremes over continents, and extreme surface heat fluxes and occasional polar lows over oceans. Air masses advected across the strong Arctic-to-mid-latitude temperature gradient are rapidly transformed into colder and dryer or warmer and moister air masses by clouds, radiative and turbulent processes, particularly in the boundary layer. Phase changes from liquid to ice within boundary-layer clouds are critical in these air-mass transformations. The presence of liquid water determines the radiative effects of these clouds, whereas the presence of ice is crucial for subsequent cloud decay or dissipation, processes that are poorly represented in weather and climate models. We argue that a better understanding of how air masses are transformed on their way into and out of the Arctic is essential for improved prediction of weather and climate in the Arctic and mid-latitudes. Observational and modelling exercises should take an air-mass-following Lagrangian approach to attain these goals.
• Järvinen, E., Jourdan, O., Neubauer, D., Yao, B., Liu, C., Andreae, M. O., Lohmann, U., Wendisch, M., McFarquhar, G. M., Leisner, T., and Schnaiter, M., Additional Global Climate Cooling by Clouds due to Ice Crystal Complexity, Atmos. Chem. Phys., 18, 15767-15781, doi:10.5194/acp-18-15767-2018, 2018.
  Ice crystal submicron structures have a large impact on the optical properties of cirrus clouds and consequently on their radiative effect. Although there is growing evidence that atmospheric ice crystals are rarely pristine, direct in situ observations of the degree of ice crystal complexity are largely missing. Here we show a comprehensive in situ data set of ice crystal complexity coupled with measurements of the cloud angular scattering functions collected during a number of observational airborne campaigns at diverse geographical locations. Our results demonstrate that an overwhelming fraction (between 61% and 81%) of atmospheric ice crystals sampled in the different regions contain mesoscopic deformations and, as a consequence, a similar flat and featureless angular scattering function is observed. A comparison between the measurements and a database of optical particle properties showed that severely roughened hexagonal aggregates optimally represent the measurements in the observed angular range. Based on this optical model, a new parameterization of the cloud bulk asymmetry factor was introduced and its effects were tested in a global climate model. The modelling results suggest that, due to ice crystal complexity, ice-containing clouds can induce an additional short-wave cooling effect of -1.12 W m² on the top-of-the-atmosphere radiative budget that has not yet been considered.
• Schulz, C., Schneider, J., Amorim Holanda, B., Appel, O., Costa, A., de Sá, S. S., Dreiling, V., Fütterer, D., Jurkat-Witschas, T., Klimach, T., Krämer, M., Martin, S. T., Mertes, S., Pöhlker, M. L., Sauer, D., Voigt, C., Walser, A., Weinzierl, B., Ziereis, H., Zöger, M., Andreae, M. O., Artaxo, P., Machado, L. A. T., Pöschl, U., Wendisch, M., and Borrmann, S., Aircraft-based observations of isoprene-epoxydiol-derived secondary organic aerosol (IEPOX-SOA) in the tropical upper troposphere over the Amazon region, Atmos. Chem. Phys., 18, 14979-15001, doi:10.5194/acp-18-14979-2018, 2018.
  During the ACRIDICON-CHUVA field project (September-October 2014; based in Manaus, Brazil) aircraft-based in situ measurements of aerosol chemical composition were conducted in the tropical troposphere over the Amazon using the High Altitude and Long Range Research Aircraft (HALO), covering altitudes from the boundary layer (BL) height up to 14.4 km. The submicron non-refractory aerosol was characterized by flash-vaporization/electron impact-ionization aerosol particle mass spectrometry. The results show that significant secondary organic aerosol (SOA) formation by isoprene oxidation products occurs in the upper troposphere (UT), leading to increased organic aerosol mass concentrations above 10km altitude. The median organic mass concentrations in the UT above 10 km range between 1.0 and 2.5 μg m^-3 (referring to standard temperature and pressure; STP) with interquartile ranges of 0.6 to 3.2 μg m^-3 (STP), representing 78% of the total submicron non-refractory aerosol particle mass. The presence of isoprene-epoxydiol-derived secondary organic aerosol (IEPOX-SOA) was confirmed by marker peaks in the mass spectra. We estimate the contribution of IEPOX-SOA to the total organic aerosol in the UT to be about 20%. After isoprene emission from vegetation, oxidation processes occur at low altitudes and/or during transport to higher altitudes, which may lead to the formation of IEPOX (one oxidation product of isoprene). Reactive uptake or condensation of IEPOX on preexisting particles leads to IEPOX-SOA formation and subsequently increasing organic mass in the UT. This organic mass increase was accompanied by an increase in the nitrate mass concentrations, most likely due to NO_x production by lightning. Analysis of the ion ratio of NO⁺ to NO₂⁺ indicated that nitrate in the UT exists mainly in the form of organic nitrate. IEPOX-SOA and organic nitrates are coincident with each other, indicating that IEPOX-SOA forms in the UT either on acidic nitrate particles forming organic nitrates derived from IEPOX or on already neutralized organic nitrate aerosol particles.
• Schäfler, A., G. Craig, H. Wernli, P. Arbogast, J.D. Doyle, R. McTaggart-Cowan, J. Methven, G. Rivière, F. Ament, M. Boettcher, M. Bramberger, Q. Cazenave, R. Cotton, S. Crewell, J. Delanoë, A. Dörnbrack, A. Ehrlich, F. Ewald, A. Fix, C.M. Grams, S.L. Gray, H. Grob, S. Groß, M. Hagen, B. Harvey, L. Hirsch, M. Jacob, T. Kölling, H. Konow, C. Lemmerz, O. Lux, L. Magnusson, B. Mayer, M. Mech, R. Moore, J. Pelon, J. Quinting, S. Rahm, M. Rapp, M. Rautenhaus, O. Reitebuch, C.A. Reynolds, H. Sodemann, T. Spengler, G. Vaughan, M. Wendisch, M. Wirth, B. Witschas, K. Wolf, and T. Zinner, The North Atlantic Waveguide and Downstream Impact Experiment, Bull. Am. Meteorol. Soc., 8, 1607-1637, doi:10.1175/BAMS-D-17-0003.1, 2018.
  The North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX) explored the impact of diabatic processes on disturbances of the jet stream and their influence on downstream high-impact weather through the deployment of four research aircraft, each with a sophisticated set of remote sensing and in situ instruments, and coordinated with a suite of ground-based measurements. A total of 49 research flights were performed, including, for the first time, coordinated flights of the four aircraft: the German High Altitude and Long Range Research Aircraft (HALO), the Deutsches Zentrum für Luft- und Raumfahrt (DLR) Dassault Falcon 20, the French Service des Avions Francais Instrumentes pour la Recherche en Environnement (SAFIRE) Falcon 20, and the British Facility for Airborne Atmospheric Measurements (FAAM) BAe 146. The observation period from 17 September to 22 October 2016 with frequently occurring extratropical and tropical cyclones was ideal for investigating midlatitude weather over the North Atlantic. NAWDEX featured three sequences of upstream triggers of waveguide disturbances, as well as their dynamic interaction with the jet stream, subsequent development, and eventual downstream weather impact on Europe. Examples are presented to highlight the wealth of phenomena that were sampled, the comprehensive coverage, and the multifaceted nature of the measurements. This unique dataset forms the basis for future case studies and detailed evaluations of weather and climate predictions to improve our understanding of diabatic influences on Rossby waves and the downstream impacts of weather systems affecting Europe.
• Schäfer, M., Loewe, K., Ehrlich, A., Hoose, C., and Wendisch, M., Simulated and observed horizontal inhomogeneities of optical thickness of Arctic stratus, Atmos. Chem. Phys., 18, 13115-13133, doi:10.5194/acp-18-13115-2018, 2018.
  Two-dimensional horizontal fields of cloud optical thickness t derived from airborne measurements of solar spectral, cloud-reflected radiance are compared with semi-idealized large eddy simulations (LESs) of Arctic stratus performed with the Consortium for Small-scale Modeling (COSMO) atmospheric model. The measurements were collected during the Vertical Distribution of Ice in Arctic Clouds (VERDI) campaign carried out in Inuvik, Canada, in April/May 2012. The input for the LESs is obtained from collocated airborne dropsonde observations of a persistent Arctic stratus above the sea-ice-free Beaufort Sea. Simulations are performed for spatial resolutions of 50m (1.6 km x 1.6 km domain) and 100 m (6.4 km x 6.4 km domain). Macrophysical cloud properties, such as cloud top altitude and vertical extent, are well captured by the COSMO simulations. However, COSMO produces rather homogeneous clouds compared to the measurements, in particular for the simulations with coarser spatial resolution. For both spatial resolutions, the directional structure of the cloud inhomogeneity is well represented by the model. Differences between the individual cases are mainly associated with the wind shear near cloud top and the vertical structure of the atmospheric boundary layer. A sensitivity study changing the wind velocity in COSMO by a vertically constant scaling factor shows that the directional, small-scale cloud inhomogeneity structures can range from 250 to 800 m, depending on the mean wind speed, if the simulated domain is large enough to capture also large-scale structures, which then influence the small-scale structures. For those cases, a threshold wind velocity is identified, which determines when the cloud inhomogeneity stops increasing with increasing wind velocity.
• Saturno, J., Ditas, F., Penning de Vries, M., Holanda, B. A., Pöhlker, M. L., Carbone, S., Walter, D., Bobrowski, N., Brito, J., Chi, X., Gutmann, A., Hrabe de Angelis, I., Machado, L. A. T., Moran-Zuloaga, D., Rüdiger, J., Schneider, J., Schulz, C., Wang, Q., Wendisch, M., Artaxo, P., Wagner, T., Pöschl, U., Andreae, M. O., and Pöhlker, C., African volcanic emissions influencing atmospheric aerosols over the Amazon rain forest, Atmos. Chem. Phys., 18, 10391-10405, doi:10.5194/acp-18-10391-2018, 2018.
  The long-range transport (LRT) of trace gases and aerosol particles plays an important role for the composition of the Amazonian rain forest atmosphere. Sulfate aerosols originate to a substantial extent from LRT sources and play an important role in the Amazonian atmosphere as strongly light-scattering particles and effective cloud condensation nuclei. The transatlantic transport of volcanic sulfur emissions from Africa has been considered as a source of particulate sulfate in the Amazon; however, direct observations have been lacking so far. This study provides observational evidence for the influence of emissions from the Nyamuragira-Nyiragongo volcanoes in Africa on Amazonian aerosol properties and atmospheric composition during September 2014. Comprehensive ground-based and airborne aerosol measurements together with satellite observations are used to investigate the volcanic event. Under the volcanic influence, hourly mean sulfate mass concentrations in the submicron size range reached up to 3.6 μg m^-3 at the Amazon Tall Tower Observatory, the highest value ever reported in the Amazon region. The substantial sulfate injection increased the aerosol hygroscopicity with k values up to 0.36, thus altering aerosol-cloud interactions over the rain forest. Airborne measurements and satellite data indicate that the transatlantic transport of volcanogenic aerosols occurred in two major volcanic plumes with a sulfate-enhanced layer between 4 and 5 km of altitude. This study demonstrates how African aerosol sources, such as volcanic sulfur emissions, can substantially affect the aerosol cycling and atmospheric processes in Amazonia.
• Machado, L. A. T., Calheiros, A. J. P., Biscaro, T., Giangrande, S., Silva Dias, M. A. F., Cecchini, M. A., Albrecht, R., Andreae, M. O., Araujo, W. F., Arttaxo, P., Borrmann, S., Braga, R., Burleyson, C., Eichholz, C. W., Fan, J., Feng, Z., Fisch, G. F., Jensen, M. P., Martin, S. T., Pöschl, U., Pöhlker, C., Pöhlker, M. L., Ribaud, J.-F., Rosenfeld, D., Saraiva, J. M. B., Schumacher, C., Thalman, R., Walter, D., and Wendisch, M., Overview: Precipitation characteristics and sensitivities to the environmental conditions during GoAmazon2014/5 and ACRIDICON-CHUVA, Atmos. Chem. Phys., 18, 6461-6482, doi:10.5194/acp-18-6461-2018, 2018.
  This study provides an overview of precipitation processes and their sensitivities to environmental conditions in the Central Amazon Basin near Manaus during the GoAmazon2014/5 and ACRIDICON-CHUVA experiments. This study takes advantage of the numerous measurement platforms and instrument systems operating during both campaigns to sample cloud structure and environmental conditions during 2014 and 2015; the rainfall variability among seasons, aerosol loading, land surface type, and topography has been carefully characterized using these data. Differences between the wet and dry seasons were examined from a variety of perspectives. The rainfall rates distribution, total amount of rainfall, and raindrop size distribution (the mass-weighted mean diameter) were quantified over both seasons. The dry season generally exhibited higher rainfall rates than the wet season and included more intense rainfall periods. However, the cumulative rainfall during the wet season was 4 times greater than that during the total dry season rainfall, as shown in the total rainfall accumulation data. The typical size and life cycle of Amazon cloud clusters (observed by satellite) and rain cells (observed by radar) were examined, as were differences in these systems between the seasons. Moreover, monthly mean thermodynamic and dynamic variables were analysed using radiosondes to elucidate the differences in rainfall characteristics during the wet and dry seasons. The sensitivity of rainfall to atmospheric aerosol loading was discussed with regard to mass-weighted mean diameter and rain rate. This topic was evaluated only during the wet season due to the insignificant statistics of rainfall events for different aerosol loading ranges and the low frequency of precipitation events during the dry season. The impacts of aerosols on cloud droplet diameter varied based on droplet size. For the wet season, we observed no dependence between land surface type and rain rate. However, during the dry season, urban areas exhibited the largest rainfall rate tail distribution, and deforested regions exhibited the lowest mean rainfall rate. Airplane measurements were taken to characterize and contrast cloud microphysical properties and processes over forested and deforested regions. Vertical motion was not correlated with cloud droplet sizes, but cloud droplet concentration correlated linearly with vertical motion. Clouds over forested areas contained larger droplets than clouds over pastures at all altitudes. Finally, the connections between topography and rain rate were evaluated, with higher rainfall rates identified at higher elevations during the dry season.
• Grosvenor, D.P., O. Souderval, P. Zuidema, A.S. Ackerman, M.D. Alexandrov, R. Bennartz, R. Boers, B. Cairns, J.C. Chiu, M. Christensen, H. Deneke, M. Diamond, G. Feingold, A. Fridlind, A. Hünerbein, C. Knist, P. Kollias, A. Marshak, D. McCoy, D. Merk, D. Painemal, J. Rausch, D. Rosenfeld, H. Russchenberg, P. Seifert, K. Sinclair, P. Stier, B. van Diedenhoven, Wendisch, M., F. Werner, R. Wood, Z. Zhang, and J. Quaas, Remote sensing of cloud droplet number concentration: Review of current and perspectives for new approaches, Rev. Geophys., 56, 409-453, doi:10.1029/2017RG000593, 2018.
  The cloud droplet number concentration (N_d) of central interest to improve the understanding of cloud physics and for quantifying the effective radiative forcing by aerosol-cloud interactions. Current standard satellite retrievals do not operationally provide N_d, but it can be inferred from retrievals of cloud optical depth (t_c) cloud droplet effective radius (r_e) and cloud top temperature. This review summarizes issues with this approach and quantifies uncertainties. A total relative uncertainty of 78% is inferred for pixel-level retrievals for relatively homogeneous, optically thick and unobscured stratiform clouds with favorable viewing geometry. The uncertainty is even greater if these conditions are not met. For averages over 1°x1° regions the uncertainty is reduced to 54% assuming random errors for instrument uncertainties. In contrast, the few evaluation studies against reference in situ observations suggest much better accuracy with little variability in the bias. More such studies are required for a better error characterization. N_d uncertainty is dominated by errors in re, and therefore, improvements in re retrievals would greatly improve the quality of the N_d retrievals. Recommendations are made for how this might be achieved. Some existing N_d data sets are compared and discussed, and best practices for the use of N_d data from current passive instruments (e.g., filtering criteria) are recommended. Emerging alternative N_d estimates are also considered. First, new ideas to use additional information from existing and upcoming spaceborne instruments are discussed, and second, approaches using high-quality ground-based observations are examined.
• Krisna, T. C., Wendisch, M., Ehrlich, A., Jäkel, E., Werner, F., Weigel, R., Borrmann, S., Mahnke, C., Pöschl, U., Andreae, M. O., Voigt, C., and Machado, L. A. T., Comparing airborne and satellite retrievals of cloud optical thickness and particle effective radius using a spectral radiance ratio technique: two case studies for cirrus and deep convective clouds, Atmos. Chem. Phys., 18, 4439-4462, doi:10.5194/acp-18-4439-2018, 2018.
  Solar radiation reflected by cirrus and deep convective clouds (DCCs) was measured by the Spectral Modular Airborne Radiation Measurement System (SMART) installed on the German High Altitude and Long Range Research Aircraft (HALO) during the Mid-Latitude Cirrus (ML-CIRRUS) and the Aerosol, Cloud, Precipitation, and Radiation Interaction and Dynamic of Convective Clouds System - Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud Resolving Modelling and to the Global Precipitation Measurement (ACRIDICON-CHUVA) campaigns. On particular flights, HALO performed measurements closely collocated with overpasses of the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite. A cirrus cloud located above liquid water clouds and a DCC topped by an anvil cirrus are analyzed in this paper. Based on the nadir spectral upward radiance measured above the two clouds, the optical thickness t and particle effective radius r_eff of the cirrus and DCC are retrieved using a radiance ratio technique, which considers the cloud thermodynamic phase, the vertical profile of cloud microphysical properties, the presence of multilayer clouds, and the heterogeneity of the surface albedo. For the cirrus case, the comparison of t and reff retrieved on the basis of SMART and MODIS measurements yields a normalized mean absolute deviation of up to 1.2% for t and 2.1% for r_eff. For the DCC case, deviations of up to 3.6% for t and 6.2% for r_eff are obtained. The larger deviations in the DCC case are mainly attributed to the fast cloud evolution and three-dimensional (3-D) radiative effects. Measurements of spectral upward radiance at near-infrared wavelengths are employed to investigate the vertical profile of r_eff in the cirrus. The retrieved values of r_eff are compared with corresponding in situ measurements using a vertical weighting method. Compared to the MODIS observations, measurements of SMART provide more information on the vertical distribution of particle sizes, which allow reconstructing the profile of r_eff close to the cloud top. The comparison between retrieved and in situ r_eff yields a normalized mean absolute deviation, which ranges between 1.5 and 10.3%, and a robust correlation coefficient of 0.82.
• C. Emde, V. Barlakas, C. Cornet, F. Evans, Z. Wang, L. C.-Labonotte, A. Macke, B. Mayer, and Wendisch, M., IPRT polarized radiative transfer model intercomparison project - Three-dimensional test cases (phase B), J. Quant. Spectrosc. Radiat. Transfer, 209, 19-44, doi:10.1016/j.jqsrt.2018.01.024, 2018.
  Initially unpolarized solar radiation becomes polarized by scattering in the Earth's atmosphere. In particular molecular scattering (Rayleigh scattering) polarizes electromagnetic radiation, but also scattering of radiation at aerosols, cloud droplets (Mie scattering) and ice crystals polarizes. Each atmospheric constituent produces a characteristic polarization signal, thus spectro-polarimetric measurements are frequently employed for remote sensing of aerosol and cloud properties. Retrieval algorithms require efficient radiative transfer models. Usually, these apply the plane-parallel approximation (PPA), assuming that the atmosphere consists of horizontally homogeneous layers. This allows to solve the vector radiative transfer equation (VRTE) efficiently. For remote sensing applications, the radiance is considered constant over the instantaneous field-of-view of the instrument and each sensor element is treated independently in plane-parallel approximation, neglecting horizontal radiation transport between adjacent pixels (Independent Pixel Approximation, IPA). In order to estimate the errors due to the IPA approximation, three-dimensional (3D) vector radiative transfer models are required. So far, only a few such models exist. Therefore, the International Polarized Radiative Transfer (IPRT) working group of the International Radiation Commission (IRC) has initiated a model intercomparison project in order to provide benchmark results for polarized radiative transfer. The group has already performed an intercomparison for one-dimensional (1D) multi-layer test cases [phase A, 1]. This paper presents the continuation of the intercomparison project (phase B) for 2D and 3D test cases: a step cloud, a cubic cloud, and a more realistic scenario including a 3D cloud field generated by a Large Eddy Simulation (LES) model and typical background aerosols.
• Li, L., Li, Z., Li, K., Sun, B., Wu, Y., Xu, H., Xie, Y., Goloub, P., and Wendisch, M., Uncertainties of atmospheric polarimetric measurements with sun-sky radiometers induced by errors of relative orientations of polarizers, J. Quant. Spectrosc. Radiat. Transfer, 209, 10-18, doi:10.1016/j.jqsrt.2018.01.013, 2018.
  In this study errors of the relative orientations of polarizers in the Cimel polarized sun-sky radiometers are measured and introduced into the Mueller matrix of the instrument. The linearly polarized light with different polarization directions from 0° to 180° (or 360°) is generated by using a rotating linear polarizer in front of an integrating sphere. Through measuring the referential linearly polarized light, the errors of relative orientations of polarizers are determined. The efficiencies of the polarizers are obtained simultaneously. By taking the error of relative orientation into consideration in the Mueller matrix, the accuracies of the calculated Stokes parameters, the degree of linear polarization, and the angle of polarization are remarkably improved. The method may also apply to other polarization instruments of similar types.
• Andreae, M. O., Afchine, A., Albrecht, R., Holanda, B. A., Artaxo, P., Barbosa, H. M. J., Bormann, S., Cecchini, M. A., Costa, A., Dollner, M., Fütterer, D., Järvinen, E., Jurkat, T., Klimach, T., Konemann, T., Knote, C., Krämer, M., Krisna, T., Machado, L. A. T., Mertes, S., Minikin, A., Pöhlker, C., Pöhlker, M. L., Pöschl, U., Rosenfeld, D., Sauer, D., Schlager, H., Schnaiter, M., Schneider, J., Schulz, C., Spanu, A., Sperling, V. B., Voigt, C., Walser, A., Wang, J., Weinzierl, B., Wendisch, M., and Ziereis, H., Aerosol characteristics and particle production in the upper troposphere over the Amazon Basin, Atmos. Chem. Phys., 18, 921-961, doi:10.5194/acp-18-921-2018, 2018.
  Airborne observations over the Amazon Basin showed high aerosol particle concentrations in the upper troposphere (UT) between 8 and 15 km altitude, with number densities (normalized to standard temperature and pressure) often exceeding those in the planetary boundary layer (PBL) by 1 or 2 orders of magnitude. The measurements were made during the German-Brazilian cooperative aircraft campaign ACRIDICON-CHUVA, where ACRIDICON stands for Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems and CHUVA is the acronym for Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud Resolving Modeling and to the GPM (global precipitation measurement), on the German High Altitude and Long Range Research Aircraft (HALO). The campaign took place in September-October 2014, with the objective of studying tropical deep convective clouds over the Amazon rainforest and their interactions with atmospheric trace gases, aerosol particles, and atmospheric radiation.
  Aerosol enhancements were observed consistently on all flights during which the UT was probed, using several aerosol metrics, including condensation nuclei (CN) and cloud condensation nuclei (CCN) number concentrations and chemical species mass concentrations. The UT particles differed sharply in their chemical composition and size distribution from those in the PBL, ruling out convective transport of combustion-derived particles from the boundary layer (BL) as a source. The air in the immediate outflow of deep convective clouds was depleted of aerosol particles, whereas strongly enhanced number concentrations of small particles (<90 nm diameter) were found in UT regions that had experienced outflow from deep convection in the preceding 5-72 h. We also found elevated concentrations of larger (>90nm) particles in the UT, which consisted mostly of organic matter and nitrate and were very effective CCN.
  Our findings suggest a conceptual model, where production of new aerosol particles takes place in the continental UT from biogenic volatile organic material brought up by deep convection and converted to condensable species in the UT. Subsequently, downward mixing and transport of upper tropospheric aerosol can be a source of particles to the PBL, where they increase in size by the condensation of biogenic volatile organic compound (BVOC) oxidation products. This may be an important source of aerosol particles for the Amazonian PBL, where aerosol nucleation and new particle formation have not been observed. We propose that this may have been the dominant process supplying secondary aerosol particles in the pristine atmosphere, making clouds the dominant control of both removal and production of atmospheric particles.

• Cecchini, M. A., Machado, L. A. T., Wendisch, M., Costa, A., Krämer, M., Andreae, M. O., Afchine, A., Albrecht, R. I., Artaxo, P., Borrmann, S., Fütterer, D., Klimach, T., Mahnke, C., Martin, S. T., Minikin, A., Molleker, S., Pardo, L. H., Pöhlker, C., Pöhlker, M. L., Pöschl, U., Rosenfeld, D., and Weinzierl, B. Illustration of microphysical processes in Amazonian deep convective clouds in the gamma phase space: introduction and potential applications, Atmos. Chem. Phys., 17, 14727-14746, doi:10.5194/acp-17-14727-2017, 2017.
• Braga, R. C., Rosenfeld, D., Weigel, R., Jurkat, T., Andreae, M. O., Wendisch, M., Pöschl, U., Voigt, C., Mahnke, C., Borrmann, S., Albrecht, R. I., Molleker, S., Vila, D. A., Machado, L. A. T., and Grulich, L., Further evidence for CCN aerosol concentrations determining the height of warm rain and ice initiation in convective clouds over the Amazon basin, Atmos. Chem. Phys., 17, 14433-14456, doi:10.5194/acp-17-14433-2017, 2017.
• Carlsen, T., Birnbaum, G., Ehrlich, A., Freitag, J., Heygster, G., Istomina, L., Kipfstuhl, S., Orsi, A., Schäfer, M., and, M. Wendisch, Comparison of different methods to retrieve optical-equivalent snow grain size in central Antarctica, The Cryosphere, 11, 2727-2741, doi:10.5194/tc-11-2727-2017, 2017.
  The optical-equivalent snow grain size affects the reflectivity of snow surfaces and, thus, the local surface energy budget in particular in polar regions. Therefore, the specific surface area (SSA), from which the optical snow grain size is derived, was observed for a 2-month period in central Antarctica (Kohnen research station) during austral summer 2013/14. The data were retrieved on the basis of ground-based spectral surface albedo measurements collected by the COmpact RAdiation measurement System (CORAS) and airborne observations with the Spectral Modular Airborne Radiation measurement sysTem (SMART). The snow grain size and pollution amount (SGSP) algorithm, originally developed to analyze spaceborne reflectance measurements by the MODerate Resolution Imaging Spectroradiometer (MODIS), was modified in order to reduce the impact of the solar zenith angle on the retrieval results and to cover measurements in overcast conditions. Spectral ratios of surface albedo at 1280 and 1100 nm wavelength were used to reduce the retrieval uncertainty. The retrieval was applied to the ground-based and airborne observations and validated against optical in situ observations of SSA utilizing an IceCube device. The SSA retrieved from CORAS observations varied between 27 and 89 m²kg^-1. Snowfall events caused distinct relative maxima of the SSA which were followed by a gradual decrease in SSA due to snow metamorphism and wind-induced transport of freshly fallen ice crystals. The ability of the modified algorithm to include measurements in overcast conditions improved the data coverage, in particular at times when precipitation events occurred and the SSA changed quickly. SSA retrieved from measurements with CORAS and MODIS agree with the in situ observations within the ranges given by the measurement uncertainties. However, SSA retrieved from the airborne SMART data slightly underestimated the ground-based results.
• Costa, A., Meyer, J., Afchine, A., Luebke, A., Günther, G., Dorsey, J. R., Gallagher, M. W., Ehrlich, A., Wendisch, M., Baumgardner, D., Wex, H., and Krämer, M., Classification of Arctic, midlatitude and tropical clouds in the mixed-phase temperature regime, Atmos. Chem. Phys., 17, 12219-12238, doi:10.5194/acp-17-12219-2017, 2017.
  The degree of glaciation of mixed-phase clouds constitutes one of the largest uncertainties in climate prediction. In order to better understand cloud glaciation, cloud spectrometer observations are presented in this paper, which were made in the mixed-phase temperature regime between 0 and -38 °C (273 to 235 K), where cloud particles can either be frozen or liquid. The extensive data set covers four airborne field campaigns providing a total of 139000 1Hz data points (38.6 h within clouds) over Arctic, midlatitude and tropical regions. We develop algorithms, combining the information on number concentration, size and asphericity of the observed cloud particles to classify four cloud types: liquid clouds, clouds in which liquid droplets and ice crystals coexist, fully glaciated clouds after the Wegener-Bergeron-Findeisen process and clouds where secondary ice formation occurred. We quantify the occurrence of these cloud groups depending on the geographical region and temperature and find that liquid clouds dominate our measurements during the Arctic spring, while clouds dominated by the Wegener-Bergeron-Findeisen process are most common in midlatitude spring. The coexistence of liquid water and ice crystals is found over the whole mixed-phase temperature range in tropical convective towers in the dry season. Secondary ice is found at midlatitudes at -5 to -10 °C (268 to 263 K) and at higher a ltitudes, i.e. lower temperatures in the tropics. The distribution of the cloud types with decreasing temperature is shown to be consistent with the theory of evolution of mixed-phase clouds. With this study, we aim to contribute to a large statistical database on cloud types in the mixed-phase temperature regime.
• Bony, S., B. Stevens, F. Ament, S. Bigorre, P. Chazette, S. Crewell, J. Delanoë, K. Emanuel, D. Farrell, C. Flamant, S. Gross, L. Hirsch, J. Karstensen, B. Mayer, L. Nuijens, J. H. Ruppert Jr., I. Sandu, P. Siebesma, S. Speich, F. Szczap, J. Totems, R. Vogel, M. Wendisch, and M. Wirth, EUREC4A: A Field Campaign to Elucidate the Couplings Between Clouds, Convection and Circulation, Surv. Geophys., 1573-0956, doi:10.1007/s10712-017-9428-0, 2017.
  Trade-wind cumuli constitute the cloud type with the highest frequency of occurrence on Earth, and it has been shown that their sensitivity to changing environmental conditions will critically influence the magnitude and pace of future global warming. Research over the last decade has pointed out the importance of the interplay between clouds, convection and circulation in controling this sensitivity. Numerical models represent this interplay in diverse ways, which translates into different responses of trade-cumuli to climate perturbations. Climate models predict that the area covered by shallow cumuli at cloud base is very sensitive to changes in environmental conditions, while process models suggest the opposite. To understand and resolve this contradiction, we propose to organize a field campaign aimed at quantifying the physical properties of trade-cumuli (e.g., cloud fraction and water content) as a function of the large-scale environment. Beyond a better understanding of clouds-circulation coupling processes, the campaign will provide a reference data set that may be used as a benchmark for advancing the modelling and the satellite remote sensing of clouds and circulation. It will also be an opportunity for complementary investigations such as evaluating model convective parameterizations or studying the role of ocean mesoscale eddies in air-sea interactions and convective organization.
• Klingebiel, M., Ehrlich, A., Finger, F., Röschenthaler, T., Jakirlic, S., Voigt, M., Müller, S., Maser, R., Wendisch, M., Hoor, P., Spichtinger, P., and Borrmann, S., A tandem approach for collocated measurements of microphysical and radiative cirrus properties, Atmos. Meas. Tech., 10, 3485-3498 doi:10.5194/amt-10-3485-2017, 2017.
  Microphysical and radiation measurements were collected with the novel AIRcraft TOwed Sensor Shuttle (AIRTOSS) - Learjet tandem platform. The platform is a combination of an instrumented Learjet 35A research aircraft and an aerodynamic bird, which is detached from and retracted back to the aircraft during flight via a steel wire with a length of 4000 m. Both platforms are equipped with radiative, cloud microphysical, trace gas, and meteorological instruments. The purpose of the development of this tandem set-up is to study the inhomogeneity of cirrus as well as other stratiform clouds. Sophisticated numerical flow simulations were conducted in order to optimally integrate an axially asymmetric Cloud Combination Probe (CCP) inside AIRTOSS. The tandem platform was applied during measurements at altitudes up to 36000 ft (10970 m) in the framework of the AIRTOSS - Inhomogeneous Cirrus Experiment (AIRTOSS-ICE). Ten flights were performed above the North Sea and Baltic Sea to probe frontal and in situ formed cirrus, as well as anvil outflow cirrus. For one flight, cirrus microphysical and radiative properties displayed significant inhomogeneities resolved by both measurement platforms. The CCP data show that the maximum of the observed particle number size distributions shifts with decreasing altitude from 30 to 300 μm, which is typical for frontal, midlatitude cirrus. Theoretical considerations imply that cloud particle aggregation inside the studied cirrus is very unlikely. Consequently, diffusional growth was identified to be the dominant microphysical growth process. Measurements of solar downward and upward irradiances at 670 nm wavelength were conducted above, below, and in the cirrus on both the Learjet and AIRTOSS. The observed variability of the downward irradiance below the cirrus reflects the horizontal heterogeneity of the observed thin cirrus. Vertically resolved solar heating rates were derived by either using single-platform measurements at different altitudes or by making use of the collocated irradiance measurements at different altitudes of the tandem platform. Due to unavoidable biases of the measurements between the individual flight legs, the single-platform approach failed to provide a realistic solar heating rate profile, while the uncertainties of the tandem approach are reduced. Here, the solar heating rates range up to 6 K day^-1 at top of the cirrus layer.
• Ehrlich, A., Bierwirth, E., Istomina, L., and Wendisch, M., Combined retrieval of Arctic liquid water cloud and surface snow properties using airborne spectral solar remote sensing, Atmos. Meas. Tech., 10, 3215-3230, doi:10.5194/amt-10-3215-2017, 2017.
  The passive solar remote sensing of cloud properties over highly reflecting ground is challenging, mostly due to the low contrast between the cloud reflectivity and that of the underlying surfaces (sea ice and snow). Uncertainties in the retrieved cloud optical thickness t and cloud droplet effective radius r_eff,C may arise from uncertainties in the assumed spectral surface albedo, which is mainly determined by the generally unknown effective snow grain size r_eff,S. Therefore, in a first step the effects of the assumed snow grain size are systematically quantified for the conventional bispectral retrieval technique of t and r_eff,C for liquid water clouds. In general, the impact of uncertainties of r_eff,S is largest for small snow grain sizes. While the uncertainties of retrieved t are independent of the cloud optical thickness and solar zenith angle, the bias of retrieved r_eff,C increases for optically thin clouds and high Sun. The largest deviations between the retrieved and true original values are found with 83% for t and 62% for r_eff,C.
  In the second part of the paper a retrieval method is presented that simultaneously derives all three parameters (t, r_eff,C, r_eff,S) and therefore accounts for changes in the snow grain size. Ratios of spectral cloud reflectivity measurements at the three wavelengths λ₁=1040nm (sensitive to reff,S), λ₂=1650nm (sensitive to t), and λ₃=2100nm (sensitive to r_eff,C) are combined in a trispectral retrieval algorithm. In a feasibility study, spectral cloud reflectivity measurements collected by the Spectral Modular Airborne Radiation measurement sysTem (SMART) during the research campaign Vertical Distribution of Ice in Arctic Mixed-Phase Clouds (VERDI, April/May 2012) were used to test the retrieval procedure. Two cases of observations above the Canadian Beaufort Sea, one with dense snow-covered sea ice and another with a distinct snow-covered sea ice edge are analysed. The retrieved values of t, r_eff,C, and r_eff,S show a continuous transition of cloud properties across snow-covered sea ice and open water and are consistent with estimates based on satellite data. It is shown that the uncertainties of the trispectral retrieval increase for high values of t, and low r_eff,S but nevertheless allow the effective snow grain size in cloud-covered areas to be estimated.
• Cecchini, M. A., L. A. T. Machado, M. O. Andreae, S. T. Martin, R. I. Albrecht, P. Artaxo, H. M. J. Barbosa, S. Borrmann, D. Fütterer, T. Jurkat, C. Mahnke, A. Minikin, S. Molleker, M. L. Pöhlker, U. Pöschl, D. Rosenfeld, C. Voigt, B. Weinzierl, and M. Wendisch, Sensitivities of Amazonian clouds to aerosols and updraft speed, Atmos. Chem. Phys. , 17, 10037-10050, doi:10.5194/acp-17-10037-2017, 2017.
  The effects of aerosol particles and updraft speed on warm-phase cloud microphysical properties are studied in the Amazon region as part of the ACRIDICON-CHUVA experiment. Here we expand the sensitivity analysis usually found in the literature by concomitantly considering cloud evolution, putting the sensitivity quantifications into perspective in relation to in-cloud processing, and by considering the effects on droplet size distribution (DSD) shape. Our in situ aircraft measurements over the Amazon Basin cover a wide range of particle concentration and thermodynamic conditions, from the pristine regions over coastal and forested areas to the southern Amazon, which is highly polluted from biomass burning. The quantitative results show that particle concentration is the primary driver for the vertical profiles of effective diameter and droplet concentration in the warm phase of Amazonian convective clouds, while updraft speeds have a modulating role in the latter and in total condensed water. The cloud microphysical properties were found to be highly variable with altitude above cloud base, which we used as a proxy for cloud evolution since it is a measure of the time droplets that were subject to cloud processing. We show that DSD shape is crucial in understanding cloud sensitivities. The aerosol effect on DSD shape was found to vary with altitude, which can help models to better constrain the indirect aerosol effect on climate.
• Jäkel, E., Wendisch, M., Krisna, T. C., Ewald, F., Kölling, T., Jurkat, T., Voigt, C., Cecchini, M. A., Machado, L. A. T., Afchine, A., Costa, A., Krämer, M., Andreae, M. O., Pöschl, U., Rosenfeld, D., and Yuan, T., Vertical distribution of the particle phase in tropical deep convective clouds as derived from cloud-side reflected solar radiation measurements, Atmos. Chem. Phys., 17, 9049-9066, doi:10.5194/acp-17-9049-2017, 2017.
  Vertical profiles of cloud particle phase in tropical deep convective clouds (DCCs) were investigated using airborne solar spectral radiation data collected by the German High Altitude and Long Range Research Aircraft (HALO) during the ACRIDICON-CHUVA campaign, which was conducted over the Brazilian rainforest in September 2014. A phase discrimination retrieval based on imaging spectroradiometer measurements of DCC side spectral reflectivity was applied to clouds formed in different aerosol conditions. From the retrieval results the height of the mixed-phase layer of the DCCs was determined. The retrieved profiles were compared with in situ measurements and satellite observations. It was found that the depth and vertical position of the mixed-phase layer can vary up to 900 m for one single cloud scene. This variability is attributed to the different stages of cloud development in a scene. Clouds of mature or decaying stage are affected by falling ice particles resulting in lower levels of fully glaciated cloud layers compared to growing clouds. Comparing polluted and moderate aerosol conditions revealed a shift of the lower boundary of the mixed-phase layer from 5.6 ± 0.2 km (269 K; moderate) to 6.2 ± 0.3 km (267 K; polluted), and of the upper boundary from 6.8 ± 0.2 km (263 K; moderate) to 7.4 ± 0.4 km (259 K; polluted), as would be expected from theory.
• Li, L., Zheng, X., Li, Z., Dubovik, O., Chen, X. and Wendisch, M. Studying aerosol light scattering based on aspect ratio distribution observed by fluorescence microscope, Opt. Express., 25, A813-A823, doi:10.1364/OE.25.00A813, 2017.
  Particle shape is crucial to the properties of light scattered by atmospheric aerosol particles. A method of fluorescence microscopy direct observation was introduced to determine the aspect ratio distribution of aerosol particles. The result is comparable with that of the electron microscopic analysis. The measured aspect ratio distribution has been successfully applied in modeling light scattering and further in simulation of polarization measurements of the sun/sky radiometer. These efforts are expected to improve shape retrieval from skylight polarization by using directly measured aspect ratio distribution.
• Rösch, C., D. K. Wissenbach, U. Franck, M. Wendisch, and U. Schlink, Degradation of indoor limonene by outdoor ozone: A cascade of secondary organic aerosols, Environ. Pollut., 226, 463-472, doi:10.1016/j.envpol.2017.04.030, 2017.
  In indoor air, terpene-ozone reactions can form secondary organic aerosols (SOA) in a transient process. 'Real world' measurements conducted in a furnished room without air conditioning were modelled involving the indoor background of airborne particulate matter, outdoor ozone infiltrated by natural ventilation, repeated transient limonene evaporations, and different subsequent ventilation regimes. For the given setup, we disentangled the development of nucleated, coagulated, and condensed SOA fractions in the indoor air and calculated the time dependence of the aerosol mass fraction (AMF) by means of a process model. The AMF varied significantly between 0.3 and 5.0 and was influenced by the ozone limonene ratio and the background particles which existed prior to SOA formation. Both influencing factors determine whether nucleation or adsorption processes are preferred; condensation is strongly intensified by particulate background. The results provide evidence that SOA levels in natural indoor environments can surpass those known from chamber measurements. An indicator for the SOA forming potential of limonene was found to be limona ketone. Multiplying its concentration (in μg/m3) by 450(±100) provides an estimate of the concentration of the reacted limonene. This can be used to detect a high particle formation potential due to limonene pollution, e.g. in epidemiological studies considering adverse health effects of indoor air pollutants.
• Braga, R., D. Rosenfeld, R. Weigel, T. Jurkat, M. Andreae, M. Wendisch, M. Pöhlker, T. Klimach, U. Pöschl, C. Pöhlker, C. Voigt, C. Mahnke, S. Borrmann, R. Albrecht, S. Molleker, D. Vila, L. Machado, and P. Artaxo, Comparing parameterized versus measured microphysical properties of tropical convective cloud bases during the ACRIDICON-CHUVA campaign, Atmos. Chem. Phys., 17, 7365-7386, doi:10.5194/acp-17-7365-2017, 2017.
  The objective of this study is to validate parameterizations that were recently developed for satellite retrievals of cloud condensation nuclei supersaturation spectra, N_CCN(S), at cloud base alongside more traditional parameterizations connecting N_CCN(S) with cloud base updrafts and drop concentrations. This was based on the HALO aircraft measurements during the ACRIDICON-CHUVA campaign over the Amazon region, which took place in September 2014. The properties of convective clouds were measured with a cloud combination probe (CCP), a cloud and aerosol spectrometer (CAS-DPOL), and a CCN counter onboard the HALO aircraft. An intercomparison of the cloud drop size distributions (DSDs) and the cloud water content (CWC) derived from the different instruments generally shows good agreement within the instrumental uncertainties. To this end, the directly measured cloud drop concentrations (N_d) near cloud base were compared with inferred values based on the measured cloud base updraft velocity (W_b) and N_CCN(S) spectra. The measurements of N_d at cloud base were also compared with drop concentrations (N_a) derived on the basis of an adiabatic assumption and obtained from the vertical evolution of cloud drop effective radius (r_e) above cloud base. The measurements of N_CCN(S) and Wb reproduced the observed Nd within the measurements uncertainties when the old (1959) Twomey's parameterization was used. The agreement between the measured and calculated Nd was only within a factor of 2 with attempts to use cloud base S, as obtained from the measured W_b, N_d, and N_CCN(S). This underscores the yet unresolved challenge of aircraft measurements of S in clouds. Importantly, the vertical evolution of r_e with height reproduced the observation-based nearly adiabatic cloud base drop concentrations, N_a. The combination of these results provides aircraft observational support for the various components of the satellite-retrieved methodology that was recently developed to retrieve N_CCN(S) under the base of convective clouds. This parameterization can now be applied with the proper qualifications to cloud simulations and satellite retrievals.
• Korolev, A., G. McFarquhar, P.R. Field, C. Franklin, P. Lawson, Z. Wang, E. Williams, S.J. Abel, D. Axisa, S. Borrmann, J. Crosier, J. Fugal, M. Krämer, U. Lohmann, O. Schlenczek, and M. Wendisch, Ice Formation and Evolution in Clouds and Precipitation: Measurement and Modeling Challenges. Chapter 5: Mixed-phase clouds: progress and challenges., Meteorological Monographs, 58, 5.1-5.50, doi:10.1175/AMSMONOGRAPHS-D-17-0001.1, 2017.
• Bühl, J., S. Alexander, S. Crewell, A. Heymsfield, H. Kalesse, A. Khain, M. Maahn, K. Van Tricht, and M. Wendisch, Ice Formation and Evolution in Clouds and Precipitation: Measurement and Modeling Challenges. Chapter 10: Remote Sensing., Meteorological Monographs, 58, 10.1-10.21, doi:10.1175/AMSMONOGRAPHS-D-16-0015.1, 2017.
  State-of-the-art remote sensing techniques applicable to the investigation of ice formation and evolution are described. Ground-based and spaceborne measurements with lidar, radar, and radiometric techniques are discussed together with a global view on past and ongoing remote sensing measurement campaigns concerned with the study of ice formation and evolution. This chapter has the intention of a literature study and should illustrate the major efforts that are currently taken in the field of remote sensing of atmospheric ice. Since other chapters of this monograph mainly focus on aircraft in situ measurements, special emphasis is put on active remote sensing instruments and synergies between aircraft in situ measurements and passive remote sensing methods. The chapter concentrates on homogeneous and heterogeneous ice formation in the troposphere because this is a major topic of this monograph. Furthermore, methods that deliver direct, process-level information about ice formation are elaborated with a special emphasis on active remote sensing methods. Passive remote sensing methods are also dealt with but only in the context of synergy with aircraft in situ measurements.
• Martin, S. T., P. Artaxo, L. Machado, A.O. Manzi, R. A. F. Souza, C. Schumacher, J. Wang, T. Biscaro, J. Brito, A. Calheiros, K. Jardine, A. Medeiros, B. Portela, S. de Sá, K. Adachi, A.C. Aiken, R. Albrecht, L. Alexander, M.O. Andreae, H.M.J. Barbosa, P. Buseck, D. Chand, J.M. Comstock, D.A. Day, M. Dubey, J. Fan, J. Fast, G. Fisch, E. Fortner, S. Giangrande, M. Gilles, A.H. Goldstein, A. Guenther, J. Hubbe, M. Jensen, J.L. Jimenez, F.N. Keutsch, S. Kim, C. Kuang, A. Laskin, K. McKinney, F. Mei, M. Miller, R. Nascimento, T. Pauliquevis, M. Pekour, J. Peres, T. Petäjä, C. Pöhlker, U. Pöschl, L. Rizzo, B. Schmid, J.E. Shilling, M.A. Silva Dias, J.N. Smith, J.M. Tomlinson, J. Tóta, and M. Wendisch, and U. Schlink, The Green Ocean Amazon Experiment (GoAmazon2014/5) Observes pollution affecting gases, aerosols, clouds, and rainfall over the rain forest, Bull. Am. Meteorol. Soc., 98, 981-997, doi:10.1175/BAMS-D-15-00221.1, 2017.
  The Observations and Modeling of the Green Ocean Amazon 2014-2015 (GoAmazon2014/5) experiment took place around the urban region of Manaus in central Amazonia across 2 years. The urban pollution plume was used to study the susceptibility of gases, aerosols, clouds, and rainfall to human activities in a tropical environment. Many aspects of air quality, weather, terrestrial ecosystems, and climate work differently in the tropics than in the more thoroughly studied temperate regions of Earth. GoAmazon2014/5, a cooperative project of Brazil, Germany, and the United States, employed an unparalleled suite of measurements at nine ground sites and on board two aircraft to investigate the flow of background air into Manaus, the emissions into the air over the city, and the advection of the pollution downwind of the city. Herein, to visualize this train of processes and its effects, observations aboard a low-flying aircraft are presented. Comparative measurements within and adjacent to the plume followed the emissions of biogenic volatile organic carbon compounds (BVOCs) from the tropical forest, their transformations by the atmospheric oxidant cycle, alterations of this cycle by the influence of the pollutants, transformations of the chemical products into aerosol particles, the relationship of these particles to cloud condensation nuclei (CCN) activity, and the differences in cloud properties and rainfall for background compared to polluted conditions. The observations of the GoAmazon2014/5 experiment illustrate how the hydrologic cycle, radiation balance, and carbon recycling may be affected by present-day as well as future economic development and pollution over the Amazonian tropical forest.
• Schnitt, S., E. Orlandi, M. Mech, A. Ehrlich, and S. Crewell, Characterization of water vapor and clouds during the Next-generation Aircraft Remote-sensing for VALidation (NARVAL)-south studies, IEEE JSTARS, 99, 1-11, doi:10.1109/JSTARS.2017.2687943, 2017.
  Shallow trade wind clouds pose one of the largest uncertainties in climate models. Due to the difficulties in assessing these clouds with routine observations the next-generation aircraft remote-sensing for validation campaign with the German High Altitude and LOng range research aircraft (HALO) took place in December 2013. Here we take advantage of the synergy of the HALO active and passive microwave package as well as spectrally resolved solar radiation (SR) measured by HALO-SR to characterize shallow clouds in the Caribbean. Based on a cloud mask developed from HALO-SR, about 12 000 cloudy profiles within ~4100 individual clouds could be detected with about 70% of them having a length of less than 2 km. Corresponding measurements with passive microwave measurements reveal that these small clouds also contain little water with 36% of the clouds showing a liquid water path (LWP) of less than 50 g m^-2. We show that these small and thin clouds are difficult to characterize with satellite observations by the special sensor microwave imager/sounder due to its coarse resolution. Moderate imaging spectroradiometer measurements are able to identify the smaller clouds but suffer in terms of LWP when clouds start precipitating, which is the case for about 7% of the clouds as detected by the airborne 35 GHz radar.
• Wolf, K., A. Ehrlich, T. Hüneke, K. Pfeilsticker, F. Werner, M. Wirth, and, M. Wendisch, Potential of remote sensing of cirrus optical thickness by airborne spectral radiance measurements at different sideward viewing angles, Atmos. Chem. Phys., 17, 4283-4303, doi:10.5194/acp-17-4283-2017, 2017.
  Spectral radiance measurements collected in nadir and sideward viewing directions by two airborne passive solar remote sensing instruments, the Spectral Modular Airborne Radiation measurement sysTem (SMART) and the Differential Optical Absorption Spectrometer (mini-DOAS), are used to compare the remote sensing results of cirrus optical thickness t. The comparison is based on a sensitivity study using radiative transfer simulations (RTS) and on data obtained during three airborne field campaigns: the North Atlantic Rainfall VALidation (NARVAL) mission, the Mid-Latitude Cirrus Experiment (ML-CIRRUS) and the Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems (ACRIDICON) campaign. Radiative transfer simulations are used to quantify the sensitivity of measured upward radiance I with respect to t, ice crystal effective radius r_eff, viewing angle of the sensor θ_V, spectral surface albedo α, and ice crystal shape. From the calculations it is concluded that sideward viewing measurements are generally better suited than radiance data from the nadir direction to retrieve t of optically thin cirrus, especially at wavelengths larger than λ=900 nm. Using sideward instead of nadir-directed spectral radiance measurements significantly improves the sensitivity and accuracy in retrieving t, in particular for optically thin cirrus of t ≤ 2.
  The comparison of retrievals of t based on nadir and sideward viewing radiance measurements from SMART, mini-DOAS and independent estimates of t from an additional active remote sensing instrument, the Water Vapor Lidar Experiment in Space (WALES), shows general agreement within the range of measurement uncertainties. For the selected example a mean t of 0.54 ± 0.2 is derived from SMART, and 0.49 ± 0.2 by mini-DOAS nadir channels, while WALES obtained a mean value of t= 0.32 ± 0.02 at 532 nm wavelength, respectively. The mean of t derived from the sideward viewing mini-DOAS channels is 0.26 ± 0.2. For the few simultaneous measurements, the mini-DOAS sideward channel measurements systematically underestimate (-17.6%) the nadir observations from SMART and mini-DOAS. The agreement between mini-DOAS sideward viewing channels and WALES is better, showing the advantage of using sideward viewing measurements for cloud remote sensing for t ≤ 1. Therefore, we suggest sideward viewing measurements for retrievals of t of thin cirrus because of the significantly enhanced capability of sideward viewing compared to nadir measurements.
• Voigt, C., U. Schumann, A. Minikin, A. Abdelmonem, A. Afchine, S. Borrmann, M. Boettcher, B. Buchholz, L. Bugliaro, A. Costa, J. Curtius, M. Dollner, A. Dörnbrack, V. Dreiling, V. Ebert, A. Ehrlich, A. Fix, L. Forster, F. Frank, D. Fütterer, A. Giez, K. Graf, J.-U. Grooß, S. Groß, K. Heimerl, B. Heinold, T. Hüneke, E. Järvinen, T. Jurkat, S. Kaufmann, M. Kenntner, M. Klingebiel, T. Klimach, R. Kohl, M. Krämer, T. C. Krisna, A. Luebke, B. Mayer, S. Mertes, S. Molleker, A. Petzold, K. Pfeilsticker, M. Port, M. Rapp, P. Reutter, C. Rolf, D. Rose, D. Sauer, A. Schäfler, R. Schlage, M. Schnaiter, J. Schneider, N. Spelten, P. Spichtinger, P. Stock, A. Walser, R. Weigel, B. Weinzierl, M. Wendisch, F. Werner, H. Wernli, M. Wirth, A. Zahn, H. Ziereis, and M. Zöger, ML-CIRRUS - The airborne experiment on natural cirrus and contrail cirrus with the high-altitude long-range research aircraft HALO, Bull. Am. Meteorol. Soc., 98,271-288, doi:10.1175/BAMS-D-15-00213.1, 2017.
  The Midlatitude Cirrus experiment (ML-CIRRUS) deployed the High Altitude and Long Range Research Aircraft (HALO) to obtain new insights into nucleation, life cycle, and climate impact of natural cirrus and aircraft-induced contrail cirrus. Direct observations of cirrus properties and their variability are still incomplete, currently limiting our understanding of the clouds' impact on climate. Also, dynamical effects on clouds and feedbacks are not adequately represented in today's weather prediction models.
  Here, we present the rationale, objectives, and selected scientific highlights of ML-CIRRUS using the G-550 aircraft of the German atmospheric science community. The first combined in situ-remote sensing cloud mission with HALO united state-of-the-art cloud probes, a lidar and novel ice residual, aerosol, trace gas, and radiation instrumentation. The aircraft observations were accompanied by remote sensing from satellite and ground and by numerical simulations.
  In spring 2014, HALO performed 16 flights above Europe with a focus on anthropogenic contrail cirrus and midlatitude cirrus induced by frontal systems including warm conveyor belts and other dynamical regimes (jet streams, mountain waves, and convection). Highlights from ML-CIRRUS include 1) new observations of microphysical and radiative cirrus properties and their variability in meteorological regimes typical for midlatitudes, 2) insights into occurrence of in situ-formed and lifted liquid-origin cirrus, 3) validation of cloud forecasts and satellite products, 4) assessment of contrail predictability, and 5) direct observations of contrail cirrus and their distinction from natural cirrus. Hence, ML-CIRRUS provides a comprehensive dataset on cirrus in the densely populated European midlatitudes with the scope to enhance our understanding of cirrus clouds and their role for climate and weather.
• Schäfer, M., Bierwirth, E., Ehrlich, A., Jäkel, E., Werner, F., and Wendisch, M. ,Directional, horizontal inhomogeneities of cloud optical thickness fields retrieved from ground-based and airbornespectral imaging, Atmos. Chem. Phys., 17, 2359-2372, doi:10.5194/acp-17-2359-2017, 2017.
  Clouds exhibit distinct horizontal inhomogeneities of their optical and microphysical properties, which complicate their realistic representation in weather and climate models. In order to investigate the horizontal structure of cloud inhomogeneities, 2-D horizontal fields of optical thickness (t) of subtropical cirrus and Arctic stratus are investigated with a spatial resolution of less than 10 μm. The 2-D t-fields are derived from (a) downward (transmitted) solar spectral radiance measurements from the ground beneath four subtropical cirrus and (b) upward (reflected) radiances measured from aircraft above 10 Arctic stratus. The data were collected during two field campaigns: (a) Clouds, Aerosol, Radiation, and tuRbulence in the trade wind regime over BArbados (CARRIBA) and (b) VERtical Distribution of Ice in Arctic clouds (VERDI). One-dimensional and 2-D autocorrelation functions, as well as power spectral densities, are derived from the retrieved t-fields. The typical spatial scale of cloud inhomogeneities is quantified for each cloud case. Similarly, the scales at which 3-D radiative effects influence the radiance field are identified. In most of the investigated cloud cases considerable cloud inhomogeneities with a prevailing directional structure are found. In these cases, the cloud inhomogeneities favour a specific horizontal direction, while across this direction the cloud is of homogeneous character. The investigations reveal that it is not sufficient to quantify horizontal cloud inhomogeneities using 1-D inhomogeneity parameters; 2-D parameters are necessary.
• Wendisch, M., M. Brückner, J. P. Burrows, S. Crewell, K. Dethloff, K. Ebell, Ch. Lüpkes, A. Macke, J. Notholt, J. Quaas, A. Rinke, and I. Tegen, ArctiC Amplification: Climate Relevant Atmospheric and SurfaCe Processes, and Feedback Mechanisms: (AC)3, Eos, 98, doi:10.1029/2017EO064803, 2017.
  For meteorologists and climate scientists, the Arctic is one of the most interesting regions on Earth. There climate changes currently take place at an unprecedented pace and intensity, and the reported dramatic changes have not been completely anticipated. The Arctic is warming more rapidly than the rest of the world, a process referred to as the Arctic amplification.
  Over the past 25 years, scientists have observed a remarkable increase of near-surface air temperatures, which exceeds the global warming by a factor of 2 to 3. To find out why this is happening, in January 2016 the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) launched a new Transregional Collaborative Research Center (TR 172) called "Arctic Amplification: Climate Relevant Atmospheric and Surface Processes, and Feedback Mechanisms." This effort, known by the abbreviation (AC)³, has as its overarching scientific objectives the identification, investigation, and evaluation of key processes involved in Arctic amplification; improving the understanding of the major feedback mechanisms; and quantifying the relative importance of these mechanisms.
  Our current understanding of the rapid changes in the Arctic climate implies that atmospheric processes likely dominate the short-term warming mechanisms involved. Thus, research in (AC)³ has an atmospheric focus during Phase I, which was approved to obtain funding by DFG from January 2016 to December 2019. In Phases II and III (planned for January 2020 to December 2027) the researchers of TR 172 plan to investigate the interactions between oceanic and atmospheric components more thoroughly.

• Wex, H., Dieckmann, K., Roberts, G. C., Conrath, T., Izaguirre, M. A., Hartmann, S., Herenz, P., Schäfer, M., Ditas, F., Schmeissner, T., Henning, S., Wehner, B., Siebert, H., Stratmann, F., Aerosol arriving on the Caribbean island of Barbados: physical properties and origin, Atmos. Chem. Phys., 16 (2016), 14107-14130, doi:10.5194/acp-15-8147-2015.
  The marine aerosol arriving at Barbados (Ragged Point) was characterized during two 3-week long measurement periods in November 2010 and April 2011, in the context of the measurement campaign CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over BArbados). Through a comparison between ground-based and airborne measurements it was shown that the former are representative of the marine boundary layer at least up to cloud base. In general, total particle number concentrations (N_total) ranged from as low as 100 up to 800 cm^-3, while number concentrations for cloud condensation nuclei (N_CCN) at a supersaturation of 0.26% ranged from some 10 to 600 cm^-3. N_total and N_CCN depended on the air mass origin. Three distinct types of air masses were found. One type showed elevated values for both Ntotal and NCCN and could be attributed to long-range transport from Africa, by which biomass burning particles from the Sahel region and/or mineral dust particles from the Sahara were advected. The second and third type both had values for N_CCN below 200 cm^-3 and a clear minimum in the particle number size distribution (NSD) around 70 to 80 nm Hoppel minimum). While for one of these two types the accumulation mode was dominating (albeit less so than for air masses advected from Africa), the Aitken mode dominated the other and contributed more than 50% of all particles. These Aitken mode particles likely were formed by new particle formation no more than 3 days prior to the measurements. Hygroscopicity of particles in the CCN size range was determined from CCN measurements to be κ=0.66 on average, which suggests that these particles contain mainly sulfate and do not show a strong influence from organic material, which might generally be the case for the months during which measurements were made. The average κ could be used to derive N_CCN from measured number size distributions, showing that this is a valid approach to obtain N_CCN. Although the total particulate mass sampled on filters was found to be dominated by Na⁺ and Cl^-, this was found to be contributed by a small number of large particles (> 500 nm, mostly even in the super-micron size range). Based on a three-modal fit, a sea spray mode observed in the NSDs was found to contribute 90% to the total particulate mass but only 4 to 10% to N_total and up to 15% to N_CCN. This is in accordance with finding no correlation between N_total and wind speed.
• Barlakas, V., Macke, A., and Wendisch, M., SPARTA - Solver for Polarized Atmospheric Radiative Transfer Applications: Introduction and application in Saharan dust fields, J. Quant. Spectr. & Rad. Trans., 178, 77-92, doi:10.1016/j.jqsrt.2016.02.019, 2016.
  Non-spherical particles in the atmosphere absorb and scatter solar radiation. They change the polarization state of solar radiation depending on their shape, size, chemical composition and orientation. To quantify polarization effects, a new three-dimensional (3D) vector radiative transfer model, SPARTA (Solver for Polarized Atmospheric Radiative Transfer Applications) is introduced and validated against benchmark results. SPARTA employs the statistical forward Monte Carlo technique for efficient column-response pixel-based radiance calculations including polarization for 3D inhomogeneous cloudless and cloudy atmospheres. A sensitivity study has been carried out and exemplarily results are presented for two lidar-based mineral dust fields. The scattering and absorption properties of the dust particles have been computed for spheroids and irregular shaped particles. Polarized radiance fields in two-dimensional (2D) and one-dimensional (1D) inhomogeneous Saharan dust fields have been calculated at 532 nm wavelength. The domain-averaged results of the normalized reflected radiance are almost identical for the 1D and 2D modes. In the areas with large spatial gradient in optical thickness with expected significant horizontal photon transport, the radiance fields of the 2D mode differ by about ±12% for the first Stokes component (radiance, I) and ±8% for the second Stokes component (linear polarization, Q) from the fields of the 1D mode.
• Wendisch, M., U. Pöschl, M. O. Andreae, L. A. T. Machado, R. Albrecht, H. Schlager, D. Rosenfeld, S. T. Martin, A. Abdelmonem, A. Afchine, A. Araujo, P. Artaxo, H. Aufmhoff, H. M. J. Barbosa, S. Borrmann, R. Braga, B. Buchholz, M. A. Cecchini, A. Costa, J. Curtius, M. Dollner, M. Dorf, V. Dreiling, V. Ebert, A. Ehrlich, F. Ewald, G. Fisch, A. Fix, F. Frank, D. Fütterer, C. Heckl, F. Heidelberg, T. Hüneke, E. Jäkel, E. Järvinen, T. Jurkat, S. Kanter, U. Kästner, M. Kenntner, J. Kesselmeier, T. Klimach, M. Knecht, R. Kohl, T. Kölling, M. Krämer, M. Krüger, T. C. Krisna, J. V. Lavric, K. Longo, C. Mahnke, A. O. Manzi, B. Mayer, S. Mertes, A. Minikin, S. Molleker, S. Münch, Björn Nillius, K. Pfeilsticker, C. Pöhlker, A.-E. Roiger, D. Rose, D. Rosenow, D. Sauer, M. Schnaiter, J. Schneider, C. Schulz, R. A. F. de Souza, A. Spanu, P. Stock, D. Vila, C. Voigt, A. Walser, D. Walter, R. Weigel, B. Weinzierl, F. Werner, M. A. Yamasoe, H. Ziereis, T. Zinner, M. Zöger, The ACRIDICON-CHUVA campaign: Studying tropical deep convective clouds and precipitation over Amazonia using the new German research aircraft HALO, Bull. Am. Meteorol. Soc., 97, 1885-1908, doi:10.1175/BAMS-D-14-00255.1, 2016.
  Comprehensive in-situ and remote sensing observations of deep convective clouds from below base to anvils using the new German research jet aircraft HALO (High Altitude and LOng Range research aircraft) have been performed over Amazonia to study the influence of anthropogenic aerosols on the cloud life cycle and precipitation formation processes.
  Between 1 September and 4 October 2014 a combined airborne and ground-based measurement campaign was conducted to study tropical deep convective clouds over the Brazilian Amazon rainforest. The new German research aircraft HALO, a modified Gulfstream G550, and extensive ground-based instrumentation were deployed in and near Manaus (State of Amazonas). The campaign was part of the German-Brazilian ACRIDICON-CHUVA venture to quantify aerosol-cloud-precipitation interactions and their thermodynamic, dynamic, and radiative effects by in-situ and remote sensing measurements over Amazonia. The ACRIDICON-CHUVA field observations were carried out in cooperation with the second Intensive Operating Period of GoAmazon2014/5. In this paper we focus on the airborne data measured on HALO, which was equipped with about 30 in-situ and remote sensing instruments for meteorological, trace gas, aerosol, cloud, precipitation, and spectral solar radiation measurements. Fourteen research flights with a total duration of 96 flight hours were performed. Five scientific topics were pursued: (a) cloud vertical evolution and life cycle (cloud profiling), (b) cloud processing of aerosol particles and trace gases (inflow and outflow), (c) satellite and radar validation (cloud products), (d) vertical transport and mixing (tracer experiment), and (e) cloud formation over forested/deforested areas. Data were collected in near-pristine atmospheric conditions and in environments polluted by biomass burning and urban emissions. The paper presents a general introduction of the ACRIDICON-CHUVA campaign (motivation and addressed research topics) and of HALO with its extensive instrument package, as well as a presentation of a few selected measurement results acquired during the flights for some selected scientific topics.
• Finger, F., Werner, F., Klingebiel, M., Ehrlich, A., Jäkel, E., Voigt, M., Borrmann, S., Spichtinger, P., and Wendisch, M., Spectral optical layer properties of cirrus from collocated airborne measurements and simulations, Atmos. Chem. Phys., 16, 7681-7693, doi:10.5194/acp-16-7681-2016, 2016.
  Spectral upward and downward solar irradiances from vertically collocated measurements above and below a cirrus layer are used to derive cirrus optical layer properties such as spectral transmissivity, absorptivity, reflectivity, and cloud top albedo. The radiation measurements are complemented by in situ cirrus crystal size distribution measurements and radiative transfer simulations based on the microphysical data. The close collocation of the radiative and microphysical measurements, above, beneath, and inside the cirrus, is accomplished by using a research aircraft (Learjet 35A) in tandem with the towed sensor platform AIRTOSS (AIRcraft TOwed Sensor Shuttle). AIRTOSS can be released from and retracted back to the research aircraft by means of a cable up to a distance of 4 km. Data were collected from two field campaigns over the North Sea and the Baltic Sea in spring and late summer 2013. One measurement flight over the North Sea proved to be exemplary, and as such the results are used to illustrate the benefits of collocated sampling. The radiative transfer simulations were applied to quantify the impact of cloud particle properties such as crystal shape, effective radius r_eff, and optical thickness t on cirrus spectral optical layer properties. Furthermore, the radiative effects of low-level, liquid water (warm) clouds as frequently observed beneath the cirrus are evaluated. They may cause changes in the radiative forcing of the cirrus by a factor of 2. When low-level clouds below the cirrus are not taken into account, the radiative cooling effect (caused by reflection of solar radiation) due to the cirrus in the solar (shortwave) spectral range is significantly overestimated
• Martin, S. T., Artaxo, P., Machado, L. A. T., Manzi, A. O., Souza, R. A. F., Schumacher, C., Wang, J., Andreae, M. O., Barbosa, H. M. J., Fan, J., Fisch, G., Goldstein, A. H., Guenther, A., Jimenez, J. L., Pöschl, U., Silva Dias, M. A., Smith, J. N., and Wendisch, M., Introduction: Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5), Atmos. Chem. Phys., 15, 4785-4797, doi:10.5194/acp-16-4785-2016, 2016.
  The Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) Experiment was carried out in the environs of Manaus, Brazil, in the central region of the Amazon basin for 2 years from 1 January 2014 through 31 December 2015. The experiment focused on the complex interactions among vegetation, atmospheric chemistry, and aerosol production on the one hand and their connections to aerosols, clouds, and precipitation on the other. The objective was to understand and quantify these linked processes, first under natural conditions to obtain a baseline and second when altered by the effects of human activities. To this end, the pollution plume from the Manaus metropolis, superimposed on the background conditions of the central Amazon basin, served as a natural laboratory. The present paper, as the introduction to the special issue of GoAmazon2014/5, presents the context and motivation of the GoAmazon2014/5 Experiment. The nine research sites, including the characteristics and instrumentation of each site, are presented. The sites range from time point zero (T0) upwind of the pollution, to T1 in the midst of the pollution, to T2 just downwind of the pollution, to T3 furthest downwind of the pollution (70 km). In addition to the ground sites, a low-altitude G-159 Gulfstream I (G-1) observed the atmospheric boundary layer and low clouds, and a high-altitude Gulfstream G550 (HALO) operated in the free troposphere. During the 2-year experiment, two Intensive Operating Periods (IOP1 and IOP2) also took place that included additional specialized research instrumentation at the ground sites as well as flights of the two aircraft. GoAmazon2014/5 IOP1 was carried out from 1 February to 31 March 2014 in the wet season. GoAmazon2014/5 IOP2 was conducted from 15 August to 15 October 2014 in the dry season. The G-1 aircraft flew during both IOP1 and IOP2, and the HALO aircraft flew during IOP2. In the context of the Amazon basin, the two IOPs also correspond to the clean and biomass burning seasons, respectively. The Manaus plume is present year-round, and it is transported by prevailing northeasterly and easterly winds in the wet and dry seasons, respectively. This introduction also organizes information relevant to many papers in the special issue. Information is provided on the vehicle fleet, power plants, and industrial activities of Manaus. The mesoscale and synoptic meteorologies relevant to the two IOPs are presented. Regional and long-range transport of emissions during the two IOPs is discussed based on satellite observations across South America and Africa. Fire locations throughout the airshed are detailed. In conjunction with the context and motivation of GoAmazon2014/5 as presented in this introduction, research articles including thematic overview articles are anticipated in this special issue to describe the detailed results and findings of the GoAmazon2014/5 Experiment.

• Jäkel, E., Mey, B., Levy, R., Gu, X., Yu, T., Li, Z., Althausen, D., Heese, B., and Wendisch, M., Adaption of the MODIS aerosol retrieval algorithm using airborne spectral surface reflectance measurements over urban areas: a case study, Atmos. Meas. Tech., 8, 5237-5249, doi:10.5194/amt-8-5237-2015, 2015.
  MODIS (MOderate-resolution Imaging Spectroradiometer) retrievals of aerosol optical depth (AOD) are biased over urban areas, primarily because the reflectance characteristics of urban surfaces are different than that assumed by the retrieval algorithm. Specifically, the operational "dark-target" retrieval is tuned towards vegetated (dark) surfaces and assumes a spectral relationship to estimate the surface reflectance in blue and red wavelengths. From airborne measurements of surface reflectance over the city of Zhongshan, China, were collected that could replace the assumptions within the MODIS retrieval algorithm. The subsequent impact was tested upon two versions of the operational algorithm, Collections 5 and 6 (C5 and C6). AOD retrieval results of the operational and modified algorithms were compared for a specific case study over Zhongshan to show minor differences between them all. However, the Zhongshan-based spectral surface relationship was applied to a much larger urban sample, specifically to the MODIS data taken over Beijing between 2010 and 2014. These results were compared directly to ground-based AERONET (AErosol RObotic NETwork) measurements of AOD. A significant reduction of the differences between the AOD retrieved by the modified algorithms and AERONET was found, whereby the mean difference decreased from 0.27±0.14 for the operational C5 and 0.19±0.12 for the operational C6 to 0.10±0.15 and -0.02±0.17 by using the modified C5 and C6 retrievals. Since the modified algorithms assume a higher contribution by the surface to the total measured reflectance from MODIS, consequently the overestimation of AOD by the operational methods is reduced. Furthermore, the sensitivity of the MODIS AOD retrieval with respect to different surface types was investigated. Radiative transfer simulations were performed to model reflectances at top of atmosphere for predefined aerosol properties. The reflectance data were used as input for the retrieval methods. It was shown that the operational MODIS AOD retrieval over land reproduces the AOD reference input of 0.85 for dark surface types (retrieved AOD = 0.87 (C5)). An overestimation of AOD = 0.99 is found for urban surfaces, whereas the modified C5 algorithm shows a good performance with a retrieved value of AOD = 0.86.
• Wehner, B., Werner, F., Ditas, F., Shaw, R. A., Kulmala, M., and Siebert, H., Observations of new particle formation in enhanced UV irradiance zones near cumulus clouds, Atmos. Chem. Phys., 15, 11701-11711, doi:10.5194/acp-15-11701-2015, 2015.
  During the CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over BArbados) campaign, the interaction between aerosol particles and cloud microphysical properties was investigated in detail, which also includes the influence of clouds on the aerosol formation. During two intensive campaigns in 2010 and 2011, helicopter-borne measurement flights were performed to investigate the thermodynamic, turbulent, microphysical, and radiative properties of trade-wind cumuli over Barbados. During these flights, 91 cases with increased aerosol particle number concentrations near clouds were detected. The majority of these cases are also correlated with enhanced irradiance in the ultraviolet (UV) spectral wavelength range. This enhancement reaches values up to a factor of 3.3 greater compared to background values. Thus, cloud boundaries provide a perfect environment for the production of precursor gases for new particle formation. Another feature of cloud edges is an increased turbulence, which may also enhance nucleation and particle growth. The observed events have a mean length of 100 m, corresponding to a lifetime of less than 300 s. This implies that particles with diameters of at least 7 nm grew several nanometers per minute, which corresponds to the upper end of values in the literature (Kulmala et al., 2004). Such high values cannot be explained by sulfuric acid alone; thus extremely low volatility organic compounds (ELVOCs) are probably involved here.
• Ehrlich, A. and Wendisch, M., Reconstruction of high-resolution time series from slow-response broadband terrestrial irradiance measurements by deconvolution, Atmos. Meas. Tech., 8, 3671-3684, doi:10.5194/amt-8-3671-2015, 2015.
  Broadband solar and terrestrial irradiance measurements of high temporal resolution are needed to study inhomogeneous clouds or surfaces and to derive vertical profiles of heating/cooling rates at cloud top. An efficient method to enhance the temporal resolution of slow-response measurements of broadband terrestrial irradiance using pyrgeometer is introduced. It is based on the deconvolution theorem of Fourier transform to restore amplitude and phase shift of high frequent fluctuations. It is shown that the quality of reconstruction depends on the instrument noise, the pyrgeometer response time and the frequency of the oscillations.
  The method is tested in laboratory measurements for synthetic time series including a boxcar function and periodic oscillations using a CGR-4 pyrgeometer with response time of 3 s. The originally slow-response pyrgeometer data were reconstructed to higher resolution and compared to the predefined synthetic time series. The reconstruction of the time series worked up to oscillations of 0.5 Hz frequency and 2 W m^-2 amplitude if the sampling frequency of the data acquisition is 16 kHz or higher. For oscillations faster than 2 Hz, the instrument noise exceeded the reduced amplitude of the oscillations in the measurements and the reconstruction failed.
  The method was applied to airborne measurements of upward terrestrial irradiance from the VERDI (Vertical Distribution of Ice in Arctic Clouds) field campaign. Pyrgeometer data above open leads in sea ice and a broken cloud field were reconstructed and compared to KT19 infrared thermometer data. The reconstruction of amplitude and phase shift of the deconvoluted data improved the agreement with the KT19 data. Cloud top temperatures were improved by up to 1 K above broken clouds of 80-800 m size (1-10 s flight time) while an underestimation of 2.5 W m^-2 was found for the upward irradiance over small leads of about 600 m diameter (10 s flight time) when using the slow-response data. The limitations of the method with respect to instrument noise and digitalization of measurements by the data acquisition are discussed.
• Schäfer, M., Bierwirth, E., Ehrlich, A., Jäkel, E., and Wendisch, M., Airborne observations and simulations of three-dimensional radiative interactions between Arctic boundary layer clouds and ice floes, Atmos. Chem. Phys., 15, 8147-8163, doi:10.5194/acp-15-8147-2015, 2015.
  Based on airborne spectral imaging observations, three-dimensional (3-D) radiative effects between Arctic boundary layer clouds and highly variable Arctic surfaces were identified and quantified. A method is presented to discriminate between sea ice and open water under cloudy conditions based on airborne nadir reflectivity γ_&lambda measurements in the visible spectral range. In cloudy cases the transition of γ_&lambda from open water to sea ice is not instantaneous but horizontally smoothed. In general, clouds reduce γ_&lambda above bright surfaces in the vicinity of open water, while γ_&lambda above open sea is enhanced. With the help of observations and 3-D radiative transfer simulations, this effect was quantified to range between 0 and 2200 m distance to the sea ice edge (for a dark-ocean albedo of a_water = 0.042 and a sea-ice albedo of a_ice = 0.91 at 645 nm wavelength). The affected distance ΔL was found to depend on both cloud and sea ice properties. For a low-level cloud at 0-200 m altitude, as observed during the Arctic field campaign VERtical Distribution of Ice in Arctic clouds (VERDI) in 2012, an increase in the cloud optical thickness t from 1 to 10 leads to a decrease in ΔL from 600 to 250 m. An increase in the cloud base altitude or cloud geometrical thickness results in an increase in ΔL; for t = 1/10 ΔL = 2200 m/1250 m in case of a cloud at 500-1000 m altitude. To quantify the effect for different shapes and sizes of ice floes, radiative transfer simulations were performed with various albedo fields (infinitely long straight ice edge, circular ice floes, squares, realistic ice floe field). The simulations show that ΔL increases with increasing radius of the ice floe and reaches maximum values for ice floes with radii larger than 6 km (500-1000 m cloud altitude), which matches the results found for an infinitely long, straight ice edge.
  Furthermore, the influence of these 3-D radiative effects on the retrieved cloud optical properties was investigated. The enhanced brightness of a dark pixel next to an ice edge results in uncertainties of up to 90 and 30 % in retrievals of t and effective radius r_eff, respectively. With the help of ΔL, an estimate of the distance to the ice edge is given, where the retrieval uncertainties due to 3-D radiative effects are negligible.
• Emde, C., Barlakas, V., Cornet, C., Evans, F., Korkin, S, Ota, Y, Labonnote, L. C., Lyapustin, A., Macke, A., Mayer, B., and Wendisch, M., IPRT polarized radiative transfer model intercomparison project - Phase A, J. Quant. Spectr. & Rad. Trans., 164, 8-36, doi:10.1016/j.jqsrt.2015.05.007, 2015.
  The polarization state of electromagnetic radiation scattered by atmospheric particles such as aerosols, cloud droplets, or ice crystals contains much more information about the optical and microphysical properties than the total intensity alone. For this reason an increasing number of polarimetric observations are performed from space, from the ground and from aircraft. Polarized radiative transfer models are required to interpret and analyse these measurements and to develop retrieval algorithms exploiting polarimetric observations. In the last years a large number of new codes have been developed, mostly for specific applications. Benchmark results are available for specific cases, but not for more sophisticated scenarios including polarized surface reflection and multi-layer atmospheres. The International Polarized Radiative Transfer (IPRT) working group of the International Radiation Commission (IRC) has initiated a model intercomparison project in order to fill this gap. This paper presents the results of the first phase A of the IPRT project which includes ten test cases, from simple setups with only one layer and Rayleigh scattering to rather sophisticated setups with a cloud embedded in a standard atmosphere above an ocean surface. All scenarios in the first phase A of the intercomparison project are for a one-dimensional plane-parallel model geometry. The commonly established benchmark results are available at the IPRT website (http://www.meteo.physik.uni-muenchen.de/iprt).
• Rösch, C., Wissenbach, D. K, von Bergen, M., Franck, U., Wendisch, M., and Schlink, U., The lasting effect of limonene-induced particle formation on air quality in a genuine indoor environment, Environ. Sci. Pollut. Res., 22, 14209, doi:10.1007/s11356-015-4663-8, 2015.
  Atmospheric ozone-terpene reactions, which form secondary organic aerosol (SOA) particles, can affect indoor air quality when outdoor air mixes with indoor air during ventilation. This study, conducted in Leipzig, Germany, focused on limonene-induced particle formation in a genuine indoor environment (24 m³). Particle number, limonene and ozone concentrations were monitored during the whole experimental period. After manual ventilation for 30 min, during which indoor ozone levels reached up to 22.7 ppb, limonene was introduced into the room at concentrations of approximately 180 to 250 μg m^-3. We observed strong particle formation and growth within a diameter range of 9 to 50 nm under real-room conditions. Larger particles with diameters above 100 nm were less affected by limonene introduction. The total particle number concentrations (TPNCs) after limonene introduction clearly exceed outdoor values by a factor of 4.5 to 41 reaching maximum concentrations of up to 267,000 particles cm^-3. The formation strength was influenced by background particles, which attenuated the formation of new SOA with increasing concentration, and by ozone levels, an increase of which by 10 ppb will result in a six times higher TPNC. This study emphasizes indoor environments to be preferred locations for particle formation and growth after ventilation events. As a consequence, SOA formation can produce significantly higher amounts of particles than transported by ventilation into the indoor air.
• Schmeissner, T., R. A. Shaw, J. Ditas, F. Stratmann, M. Wendisch, and H. Siebert, Turbulent Mixing in Shallow Trade Wind Cumuli: Dependence on Cloud Life Cycle, J. Atmos. Sci., 72, 1447-1465, doi:10.1175/JAS-D-14-0230.1, 2015.
  Helicopter-borne observations of the impact of turbulent mixing and cloud microphysical properties in shallow trade wind cumuli are presented. The measurements were collected during the Cloud, Aerosol, Radiation and Turbulence in the Trade Wind Regime over Barbados (CARRIBA) project. Basic meteorological parameters (3D wind vector, air temperature, and relative humidity), cloud condensation nuclei concentrations, and cloud microphysical parameters (droplet number, size distribution, and liquid water content) are measured by the Airborne Cloud Turbulence Observation System (ACTOS), which is fixed by a 160-m-long rope underneath a helicopter flying with a true airspeed of approximately 20 m s^-1. Clouds at different evolutionary stages were sampled. A total of 300 clouds are classified into actively growing, decelerated, and dissolving clouds. The mixing process of these cloud categories is investigated by correlating the cloud droplet number concentration and cubed droplet mean volume diameter. A significant tendency to more inhomogeneous mixing with increasing cloud lifetime is observed. Furthermore, the mixing process and its effects on droplet number concentration, droplet size, and cloud liquid water content are statistically evaluated. It is found that, in dissolving clouds, liquid water content and droplet number concentration are decreased by about 50% compared to actively growing clouds. Conversely, the droplet size remains almost constant, which can be attributed to the existence of a humid shell around the cloud that prevents cloud droplets from rapid evaporation after entrainment of premoistened air. Moreover, signs of secondary activation are found, which results in a more difficult interpretation of observed mixing diagrams.
• Müller, S., Hoor, P., Berkes, F., Bozem, H., Klingebiel, M., Reutter, P., Smit, H. G. J., Wendisch, M., Spichtinger, P. and Borrmann, S., In situ detection of stratosphere-troposphere exchange of cirrus particles in the midlatitudes, Geophys. Res. Lett., 42, 949-955, doi:10.1002/2014GL062556, 2015.
  Airborne trace gas, microphysical, and radiation measurements were performed during the AIRcraft TOwed Sensor Shuttle - Inhomogeneous Cirrus Experiment over northern Germany in 2013. Based on high-precision nitrous oxide (N₂O) and carbon monoxide (CO) in situ data, stratospheric air could be identified, which contained cirrus cloud particles. Consistent with the stratospheric N₂O data, backward trajectories indicate that the sampled air masses crossed the dynamical tropopause in the last 3 h before the measurement. These air masses contained cirrus particles, which were formed during slow ascent in the troposphere and subsequently mixed with stratospheric air. From the CO-N₂O correlation the irreversibility of this transport is deduced. To our knowledge, this is the first in situ detection of cirrus particles mixed with stratospheric air in the midlatitudes.
• Klingebiel, M., A. de Lozar, S. Molleker, R. Weigel, A. Roth, L. Schmidt, J. Meyer, A. Ehrlich, R. Neuber, M. Wendisch, and S. Borrmann, Arctic low-level boundary layer clouds: in situ measurements and simulations of mono- and bimodal supercooled droplet size distributions at the top layer of liquid phase clouds, Atmos. Chem. Phys., 15, 617-631, doi:10.5194/acp-15-617-2015, 2015.
  Aircraft borne optical in situ size distribution measurements were performed within Arctic boundary layer clouds with a special emphasis on the cloud top layer during the VERtical Distribution of Ice in Arctic clouds (VERDI) campaign in April and May 2012. An instrumented Basler BT-67 research aircraft operated out of Inuvik over the Mackenzie River delta and the Beaufort Sea in the Northwest Territories of Canada. Besides the cloud particle and hydrometeor size spectrometers the aircraft was equipped with instrumentation for aerosol, radiation and other parameters. Inside the cloud, droplet size distributions with monomodal shapes were observed for predominantly liquid-phase Arctic stratocumulus. With increasing altitude inside the cloud the droplet mean diameters grew from 10 to 20 μm. In the upper transition zone (i.e., adjacent to the cloud-free air aloft) changes from monomodal to bimodal droplet size distributions (Mode 1 with 20 μm and Mode 2 with 10 μm diameter) were observed. It is shown that droplets of both modes co-exist in the same (small) air volume and the bimodal shape of the measured size distributions cannot be explained as an observational artifact caused by accumulating data point populations from different air volumes. The formation of the second size mode can be explained by (a) entrainment and activation/condensation of fresh aerosol particles, or (b) by differential evaporation processes occurring with cloud droplets engulfed in different eddies. Activation of entrained particles seemed a viable possibility as a layer of dry Arctic enhanced background aerosol (which was detected directly above the stratus cloud) might form a second mode of small cloud droplets. However, theoretical considerations and model calculations (adopting direct numerical simulation, DNS) revealed that, instead, turbulent mixing and evaporation of larger droplets are the most likely reasons for the formation of the second droplet size mode in the uppermost region of the clouds.

• Li, L., Z. Li, K. Li, L. Blarel and M. Wendisch, A method to calculate Stokes parameters and angle of polarization of skylight from polarized CIMEL sun/sky radiometers, J. Quant. Spectrosc. Radiat. Transfer, 149, 334-346, doi:10.1016/j.jqsrt.2014.09.003, 2014.
  The polarized CIMEL sun/sky radiometers have been routinely operated within the Sun/sky-radiometer Observation NETwork (SONET) in China and some sites of the AErosol RObotic NETwork (AERONET) around the world. However, the polarization measurements are not yet widely used due to in a certain degree the lack of Stokes parameters derived directly from these polarization measurements. Meanwhile, it have been shown that retrievals of several microphysical properties of aerosol particles can be significantly improved by using degree of linear polarization (DoLP) measurements of polarized CIMEL sun/sky radiometers (CE318-DP). The Stokes parameters Q and U, as well as angle of polarization (AoP) contain additional information about linear polarization and its orientation. A method to calculate Stokes parameters Q, U, and AoP from CE318-DP polarized skylight measurements is introduced in this study. A new polarized almucantar geometry based on CE318-DP is measured to illustrate abundant variation features of these parameters. The polarization parameters calculated in this study are consistent with previous results of DoLP and I, and also comparable to vector radiative transfer simulations.
• Brückner, M., B. Pospichal, A. Macke, and M. Wendisch, A new multispectral cloud retrieval method for ship-based solar transmissivity measurements, J. Geophys. Res. Atmos., 119, 11,338-11,354, doi:10.1002/2014JD021775, 2014.
  Within the German Leibniz-network OCEANET project, ship-based lidar and microwave remote sensing as well as spectral zenith radiance observations with the COmpact RAdiation measurements System (CORAS) were performed. During three cruises latitudes between 50°N and 50°S were covered. A new spectral retrieval method to derive the cloud optical thickness t and the droplet effective radius r_eff using CORAS measurements is developed. The method matches CORAS measurements of ratios of spectral transmissivity at six wavelengths with modeled transmissivities. This retrieval is fast and accurate and thus suitable for operational purposes. The new approach circumvents ambiguities of existing cloud retrievals and reduces the influence of measurement uncertainties. It is applied to homogenous and heterogeneous liquid water and cirrus clouds. In boundary layer liquid water clouds, the retrieved effective radius was more variable, whereas in the cirrus it was rather constant. Furthermore, the liquid water path LWP was derived and compared to data from a microwave radiometer. The new retrieval tends to overestimate LWP for thick liquid water clouds but slightly underestimate LWP for thin clouds. The presented method cannot be applied to mixed-phase clouds. The maximum retrieval of t and r_eff for liquid water clouds is 80 in t and 30 μm in r_eff, respectively; for cirrus clouds the limitations of the retrieval are 10 in t and 60 μm inr_eff.
• Werner, F., F. Ditas, H. Siebert, M. Simmel, B. Wehner, P. Pilewskie, T. Schmeissner, R. A. Shaw, S. Hartmann, H. Wex, G. C. Roberts, and M. Wendisch, Twomey effect observed from collocated microphysical and remote sensing measurements over shallow cumulus, J. Geophys. Res. Atmos., 119, 1534-1545, doi:10.1002/2013JD020131, 2014.
  Clear experimental evidence of the Twomey effect for shallow trade wind cumuli near Barbados is presented. Effective droplet radius (r_eff) and cloud optical thickness (t), retrieved from helicopter-borne spectral cloud-reflected radiance measurements, and spectral cloud reflectivity (y_t) are correlated with collocated in situ observations of the number concentration of aerosol particles from the subcloud layer (N). N denotes the concentration of particles larger than 80 nm in diameter and represents particles in the activation mode. In situ cloud microphysical and aerosol parameters were sampled by the Airborne Cloud Turbulence Observation System (ACTOS). Spectral cloud-reflected radiance data were collected by the Spectral Modular Airborne Radiation measurement sysTem (SMART-HELIOS). With increasing N a shift in the probability density functions of t and y_t toward larger values is observed, while the mean values and observed ranges of retrieved r_eff decrease. The relative susceptibilities (RS) of r_eff, t, and y_t to N are derived for bins of constant liquid water path. The resulting values of RS are in the range of 0.35 for r_eff and t, and 0.27 for y_t. These results are close to the maximum susceptibility possible from theory. Overall, the shallow cumuli sampled near Barbados show characteristics of homogeneous, plane-parallel clouds. Comparisons of RS derived from in situ measured r_eff and from a microphysical parcel model are in close agreement.
• Henning, S., Dieckmann, K., Ignatius, K., Schäfer, M., Zedler, P., Harris, E., Sinha, B., van Pinxteren, D., Mertes, S., Birmili, W., Merkel, M., Wu, Z., Wiedensohler, A., Wex, H., Herrmann, H., and Stratmann, F., Influence of cloud processing on CCN activation behaviour in the Thuringian Forest, Germany during HCCT-2010, Atmos. Chem. Phys., 14, 7859-7868, doi:10.5194/acp-14-7859-2014, 2014.
  Within the framework of the "Hill Cap Cloud Thuringia 2010" (HCCT-2010) international cloud experiment, the influence of cloud processing on the activation properties of ambient aerosol particles was investigated. Particles were probed upwind and downwind of an orographic cap cloud on Mt Schmücke, which is part of a large mountain ridge in Thuringia, Germany. The activation properties of the particles were investigated by means of size-segregated cloud condensation nuclei (CCN) measurements at 3 to 4 different supersaturations. The observed CCN spectra together with the total particle spectra were used to calculate the hygroscopicity parameter κ for the upwind and downwind stations. The upwind and downwind critical diameters and κ values were then compared for defined cloud events (FCE) and non-cloud events (NCE). Cloud processing was found to increase the hygroscopicity of the aerosol particles significantly, with an average increase in κ of 50%. Mass spectrometry analysis and isotopic analysis of the particles suggest that the observed increase in the hygroscopicity of the cloud-processed particles is due to an enrichment of sulfate and possibly also nitrate in the particle phase.
• Baumgardner, D., Newton, R., Krämer, M., Meyer, J., Beyer, A., Wendisch, M. and Vochezer, P, The Cloud Particle Spectrometer with Polarization Detection (CPSPD): A next generation open-path cloud probe for distinguishing liquid cloud droplets from ice crystals, Atmos. Res., 142, 2-14, doi:10.1016/j.atmosres.2013.12.010, 2014.
  The differentiation of small water droplets and ice crystals by in situ measurements, in the size range < 50 μm, remains a challenge and the lack of such measurements is an obstacle to progress in understanding ice formation in clouds. A new microphysical instrument, the Cloud Particle Spectrometer with Polarization Detection (CPSPD), has been developed that measures light intensity scattered (in forward and backward directions) by individual cloud particles that pass through a focused laser beam and derives their size and thermodynamic phase (liquid or ice) in the optical diameter range from 2 to 50 μm. The optical equivalent diameter is derived from the light scattered in the forward direction. The change in polarization state of the incident light, caused by interaction with the cloud particle, is determined from the polarized components of the backscattered light. The CPSPD, along with several other cloud microphysical probes, has been flown on the University of North Dakota Citation aircraft in mixed phase clouds. It has also been deployed and operated at the Zugspitze research station studying mountain clouds. The preliminary results show that liquid cloud droplets can be distinguished from ice crystals and that the ice fraction can be estimated; an important parameter for better understanding of cloud processes, particularly that of glaciation.
• Fricke, C., Ehrlich, A., Jäkel, E., Bohn, B., Wirth, M., and Wendisch, M., Influence of local surface albedo variability and ice crystal shape on passive remote sensing of thin cirrus, Atmos. Chem. Phys., 14, 1943-1958, doi:10.5194/acp-14-1943-2014, 2014.
  Airborne measurements of solar spectral radiance reflected by cirrus are performed with the HALO-Solar Radiation (HALO-SR) instrument onboard the High Altitude and Long Range Research Aircraft (HALO) in November 2010. The data are used to quantify the influence of surface albedo variability on the retrieval of cirrus optical thickness and crystal effective radius. The applied retrieval of cirrus optical properties is based on a standard two-wavelength approach utilizing measured and simulated reflected radiance in the visible and near-infrared spectral region. Frequency distributions of the surface albedos from Moderate resolution Imaging Spectroradiometer (MODIS) satellite observations are used to compile surface-albedo-dependent lookup tables of reflected radiance. For each assumed surface albedo the cirrus optical thickness and effective crystal radius are retrieved as a function of the assumed surface albedo. The results for the cirrus optical thickness are compared to measurements from the High Spectral Resolution Lidar (HSRL). The uncertainty in cirrus optical thickness due to local variability of surface albedo in the specific case study investigated here is below 0.1 and thus less than that caused by the measurement uncertainty of both instruments. It is concluded that for the retrieval of cirrus optical thickness the surface albedo variability is negligible. However, for the retrieval of crystal effective radius, the surface albedo variability is of major importance, introducing uncertainties up to 50%. Furthermore, the influence of the bidirectional reflectance distribution function (BRDF) on the retrieval of crystal effective radius was investigated and quantified with uncertainties below 10%, which ranges below the uncertainty caused by the surface albedo variability. The comparison with the independent lidar data allowed for investigation of the role of the crystal shape in the retrieval. It is found that if assuming aggregate ice crystals, the HSRL observations fit best with the retrieved optical thickness from HALO-SR.
• Otto, S., Analytical properties of the radiance in atmospheric radiative transfer theory, J. Quant. Spectrosc. Radiat. Transfer, 133, 329-350, doi:10.1016/j.jqsrt.2013.08.018, 2014.
  It is demonstrated mathematically strictly that state density functions, as the radiance (specific intensity), exist to describe certain state properties of transported photons on microscopic and the state of the radiation field on macroscopic scale, which have independent physical meanings. Analytical properties as boundedness, continuity, differentiability and integrability of these functions to describe the photon transport are discussed. It is shown that the density functions may be derived based on the assumption of photons as real particles of non-zero and finite size, independently of usual electrodynamics, and certain historically postulated functional relationships between them were proved, that is, these functions can be derived mathematically strictly and consistently within the framework of the theory of the phenomenological radiative transfer if one takes the theory seriously by assuming real photons as particles. In this sense these functions may be treated as fundamental physical quantities within the scope of this theory, if one considers the possibility of the existence of photons.

• Wendisch, M., Yang, P. and Ehrlich, A., Amplified climate changes in the Arctic: Role of clouds and atmospheric radiation., Sitzungsberichte der Sächsischen Akademie der Wissenschaften zu Leipzig, Mathematisch-Naturwissenschaftliche Klasse, 132 (3), 1-34, S. Hirzel Verlag, Stuttgart/Leipzig, 2013.
  A presentation given at the regular plenary session of the Academy of Sciences of Saxony in Leipzig (Germany) on October 12, 2012, is thoroughly summarized. Additional aspects important to the theme but not covered in the talk have been added to complete the text. The characteristic conditions and processes leading to the so-called Arctic amplification are outlined. The phenomenon of Arctic amplification originally comprises an enhanced variability and amplified increase of the near-surface air temperature in the Arctic in comparison to the average near-surface warming at lower latitudes. Observations and simulations show the magnitude of the observed Arctic near-surface air temperature increase is more than double the air temperature increase at lower latitudes. To illustrate the phenomenon of Arctic amplification, several examples of observed Arctic near-surface air temperature increases are presented. In general, Arctic amplification also implies serious Arctic climate changes other than near-surface air temperature, such as the dramatic summer melting of Arctic Sea ice and the Greenland ice sheet, and the decrease of snow cover and surface albedo of the Greenland ice sheet. Numerous reasons for the Arctic climate changes are discussed; the direct and indirect surface albedo feedback and the related increase of near-surface water vapor and cloudiness, meridional heat and water vapor transports in the atmosphere and ocean, and increased soot amounts in both the atmosphere and snow/ice surfaces. The special role of low-level clouds under Arctic conditions (low Sun, polar day and night, high surface albedo) for the self-enforcing amplification processes is described. In particular, the impact of ice in Arctic mixed-phase clouds on the cloud radiative forcing is investigated. Methods of ice detection in mixed-phase Arctic clouds are presented along with verification examples.
• Siebert, H., Beals, M., Bethke, J., Bierwirth, E., Conrath, T., Dieckmann, K., Ditas, F., Ehrlich, A., Farrell, D., Hartmann, S., Izaguirre, M. A., Katzwinkel, J., Nuijens, L., Roberts, G., Schäfer, M., Shaw, R. A., Schmeissner, T., Serikov, I., Stevens, B., Stratmann, F., Wehner, B., Wendisch, M., Werner, F., and Wex, H., The fine-scale structure of the trade wind cumuli over Barbados - an introduction to the CARRIBA project, Atmos. Chem. Phys., 13, 10061-10077, doi:10.5194/acp-13-10061-2013, 2013.
  The CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over BArbados) project, focused on high resolution and collocated measurements of thermodynamic, turbulent, microphysical, and radiative properties of trade wind cumuli over Barbados, is introduced. The project is based on two one-month field campaigns in November 2010 (climatic wet season) and April 2011 (climatic dry season). Observations are based on helicopter-borne and ground-based measurements in an area of 100 km² off the coast of Barbados. CARRIBA is accompanied by long-term observations at the Barbados Cloud Observatory located at the East coast of Barbados since early in 2010 and which provides a longer-term context for the CARRIBA measurements. The deployed instrumentation and sampling strategy are presented together with a classification of the meteorological conditions. The two campaigns were influenced by different air masses advected from the Caribbean area, the Atlantic Ocean, and the African continent which led to distinct aerosol conditions. Pristine conditions with low aerosol particle number concentrations of ~100 cm³ were alternating with periods influenced by Saharan dust or aerosol from biomass burning resulting in comparably high number concentrations of ~ 500 cm³. The biomass burning aerosol was originating from both the Caribbean area and Africa. The shallow cumulus clouds responded to the different aerosol conditions with a wide range of mean droplet sizes and number concentrations. Two days with different aerosol and cloud microphysical properties but almost identical meteorological conditions have been analyzed in detail. The differences in the droplet number concentration and droplet sizes appear not to show any significant change for turbulent cloud mixing, but the relative roles of droplet inertia and sedimentation in initiating coalescence, as well as the cloud reflectivity, do change substantially.
• Schäfer, M., Bierwirth, E., Ehrlich, A., Heyner, F., and Wendisch, M., Retrieval of cirrus optical thickness and assessment of ice crystal shape from ground-based imaging spectrometry, Atmos. Meas. Tech., 6, 1855-1868, doi:10.5194/amt-6-1855-2013, 2013.
  A ground-based hyperspectral imaging spectrometer (AisaEAGLE, manufactured by Specim Ltd., Finland) is applied to measure downward spectral radiance fields with high spatial (1024 spatial pixels within 36.7° field of view), spectral (488 spectral pixels, 400-970 nm, 1.25 nm full width at half maximum), and temporal (4-30 Hz) resolution. The calibration, measurement and data evaluation procedures are introduced. A new method is presented to retrieve the cirrus optical thickness (tci) using the spectral radiance data collected by AisaEAGLE. The data were collected during the Cloud Aerosol Radiation and tuRbulence of trade wInd cumuli over BArbados (CARRIBA) project in 2011. The spatial inhomogeneity of the investigated cirrus is characterised by the standard deviation of the retrieved t_ci as well as the width of its frequency distribution. By comparing measured and simulated downward solar spectral radiance as a function of scattering angle, some evidence of the prevailing cirrus ice crystal shape can be obtained and subsequently used to substantiate the retrieval of t_ci. The sensitivity of the retrieval method with respect to surface albedo, effective radius (r_eff), cloud height and ice crystal shape is quantified. An enhanced sensitivity of the retrieved t_ci is found with respect to the surface albedo (up to 30%) and ice crystal shape (up to 90%). The sensitivity with regard to the effective ice crystal radius (= 5%) and the cloud height (= 0.5%) is rather small and can be neglected.
• Werner F., H. Siebert, P. Pilewskie, T. Schmeissner, R. A. Shaw, and M. Wendisch, New airborne retrieval approach for trade wind cumulus properties under overlying cirrus, J. Geophys. Res. Atmos., 118, 3634-3649, doi:10.1002/jgrd.50334, 2013.
  A new retrieval method is presented to derive the optical thickness t and effective droplet radius r_eff of shallow cumulus in the presence of overlying thin cirrus. This new approach allows for a retrieval without a priori knowledge of the microphysical and optical properties of the overlying cirrus. The retrieval is applied to helicopter-borne solar spectral reflectivity measurements gathered by the Spectral Modular Airborne Radiation measurements sysTem (SMART-HELIOS) above trade wind cumuli near Barbados. Collocated microphysical cumulus properties (liquid water content, effective droplet radius, droplet number concentration) were measured by in situ instruments installed on the Airborne Cloud Turbulence Observation System (ACTOS). Cloud inhomogeneities lead to an underestimation of retrieved t of up to 114%, while r_eff is biased by up to 27%. Moreover, misrepresentation of the overlying cirrus may cause an overestimation of the classically retrieved cumulus r_eff of up to 50% and an underestimation of tof up to 6%. The new retrieval, effectively correcting for the influence of overlying cirrus, enables reliable estimates of t of the cumuli for optically thin, overcast cirrus conditions and reduces the retrieval error for r_eff of the cumuli by almost 50%. Agreement between in situ measured and retrieved r_eff is in the range of ±1 μm. The retrieval can also reproduce the wide range of in situ measured mean r_eff (7-18 μm), which is a result of different aerosol load and cloud top heights on the different flight days. The observed t ranges between 5 and 36.
• Bierwirth, E., Ehrlich, A., Wendisch, M., Gayet, J.-F., Gourbeyre, C., Dupuy, R., Herber, A., Neuber, R., and Lampert, A., Optical thickness and effective radius of Arctic boundary-layer clouds retrieved from airborne nadir and imaging spectrometry, Atmos. Meas. Tech., 6, 1189-1200, doi:10.5194/amt-6-1189-2013, 2013.
  Arctic boundary-layer clouds in the vicinity of Svalbard (78° N, 15° E) were observed with airborne remote sensing and in situ methods. The cloud optical thickness and the droplet effective radius are retrieved from spectral radiance data from the nadir spot (1.5°, 350-2100 nm) and from a nadir-centred image (40°, 400-1000 nm). Two approaches are used for the nadir retrieval, combining the signal from either two or five wavelengths. Two wavelengths are found to be sufficient for an accurate retrieval of the cloud optical thickness, while the retrieval of droplet effective radius is more sensitive to the number of wavelengths. Even with the comparison to in-situ data, it is not possible to definitely answer the question which method is better. This is due to unavoidable time delays between the in-situ measurements and the remote-sensing observations, and to the scarcity of vertical in-situ profiles within the cloud.
• Jäkel, E., Walter, J., and Wendisch, M., Thermodynamic phase retrieval of convective clouds: impact of sensor viewing geometry and vertical distribution of cloud properties, Atmos. Meas. Tech., 6, 539-547, doi:10.5194/amt-6-539-2013, 2013.
  The sensitivity of passive remote sensing measurements to retrieve microphysical parameters of convective clouds, in particular their thermodynamic phase, is investigated by three-dimensional (3-D) radiative transfer simulations. The effects of different viewing geometries and vertical distributions of the cloud microphysical properties are investigated. Measurement examples of spectral solar radiance reflected by cloud sides (passive) in the near-infrared (NIR) spectral range are performed together with collocated lidar observations (active). The retrieval method to distinguish the cloud thermodynamic phase (liquid water or ice) exploits different slopes of cloud side reflectivity spectra of water and ice clouds in the NIR. The concurrent depolarization backscattering lidar provides geometry information about the cloud distance and height as well as the depolarization.
• Jäkel, E., Wendisch, M., and Mayer, B., Influence of spatial heterogeneity of local surface albedo on the area-averaged surface albedo retrieved from airborne irradiance measurements, Atmos. Meas. Tech., 6, 527-537, doi:10.5194/amt-6-527-2013, 2013.
  Spectral airborne upward and downward irradiance measurements are used to derive the area-averaged surface albedo. Real surfaces are not homogeneous in their reflectivity. Therefore, this work studies the effects of the heterogeneity of surface reflectivity on the area-averaged surface albedo to quantify how well aircraft measurements can resolve the small-scale variability of the local surface albedo. For that purpose spatially heterogeneous surface albedo maps were input into a 3-dimensional (3-D) Monte Carlo radiative transfer model to simulate 3-D irradiance fields. The calculated up- and downward irradiances in altitudes between 0.1 and 5 km are used to derive the area-averaged surface albedo using an iterative retrieval method that removes the effects due to atmospheric scattering and absorption within the layer beneath the considered level. For the case of adjacent land and sea surfaces, parametrizations are presented which quantify the horizontal distance from the coastline that is required to reduce surface heterogeneity effects on the area-averaged surface albedo to a given limit. The parametrization which is a function of altitude, aerosol optical depth, single scattering albedo, and the ratio of local land and sea albedo was applied for airborne spectral measurements. In addition, the deviation between area-averaged and local surface albedo is determined for more complex surface albedo maps. For moderate aerosol conditions (optical depth less than 0.4) and a wavelength range between 400 and 1000 nm, the altitude and the heterogeneity of the surface albedo are the dominant factors determining the mean deviation between local and area-averaged surface albedo. A parametrization of the mean deviation is applied to an albedo map that was derived from a Landsat image of an area in East Anglia (UK). Parametrization and direct comparison of local and area-averaged surface albedo show similar mean deviations (20% vs. 25%) over land.

• A. Ehrlich, E. Bierwirth, M. Wendisch, A. Herber, and J.-F. Gayet, Airborne hyperspectral observations of surface and cloud directional reflectivity using a commercial digital camera, Atmos. Chem. Phys., 12, 3493-3510, doi:10.5194/acp-12-3493-2012, 2012.
  Spectral radiance measurements by a digital single-lens reflex camera were used to derive the directional reflectivity of clouds and different surfaces in the Arctic. The camera has been calibrated radiometrically and spectrally to provide accurate radiance measurements with high angular resolution. A comparison with spectral radiance measurements with the Spectral Modular Airborne Radiation measurement sysTem (SMART-Albedometer) showed an agreement within the uncertainties of both instruments (6% for both). The directional reflectivity in terms of the hemispherical directional reflectance factor (HDRF) was obtained for sea ice, ice-free ocean and clouds. The sea ice, with an albedo of ρ = 0.96 (at 530 nm wavelength), showed an almost isotropic HDRF, while sun glint was observed for the ocean HDRF (ρ = 0.12). For the cloud observations with ρ = 0.62, the cloudbow - a backscatter feature typically for scattering by liquid water droplets - was covered by the camera. For measurements above heterogeneous stratocumulus clouds, the required number of images to obtain a mean HDRF that clearly exhibits the cloudbow has been estimated at about 50 images (10 min flight time). A representation of the HDRF as a function of the scattering angle only reduces the image number to about 10 (2 min flight time).
  The measured cloud and ocean HDRF have been compared to radiative transfer simulations. The ocean HDRF simulated with the observed surface wind speed of 9 m s^-1 agreed best with the measurements. For the cloud HDRF, the best agreement was obtained by a broad and weak cloudbow simulated with a cloud droplet effective radius of R_eff = 4 μm. This value agrees with the particle sizes derived from in situ measurements and retrieved from the spectral radiance of the SMART-Albedometer.
• D. Baumgardner, L. Avallone, A. Bansemer, S. Borrmann, P. Brown, U. Bundke, P. Y. Chuang, D. Cziczo, P. Field, M. Gallagher, J.-F. Gayet, A. Heymsfield, A. Korolev, M. Krämer, G. McFarquhar, S. Mertes, O. Möhler, S. Lance, P. Lawson, M. Petters, K. Pratt, G. Roberts, D. Rogers, O. Stetzer, J. Stith, W. Strapp, C. Twohy, and M. Wendisch, Workshop summary: In situ, airborne instrumentation: Addressing and solving measurement problems in ice clouds, Bull. Amer. Meteorol. Soc., 93, ES29-ES34, doi:10.1175/BAMS-D-11-00123.1, 2012.
• D. Müller, K.-H. Lee, J. Gasteiger, M. Tesche, B. Weinzierl, K. Kandler, T. Müller, C. Toledano, S. Otto, D. Althausen, and A. Ansmann, Comparison of optical and microphysical properties of pure Saharan mineral dust observed with AERONET Sun photometer, Raman lidar, and in situ instruments during SAMUM 2006, J. Geophys. Res., 117, D07211, doi:10.1029/2011JD016825, 2012.
  The Saharan Mineral Dust Experiment (SAMUM) 2006, Morocco, aimed at the characterization of optical, physical, and radiative properties of Saharan dust. AERONET Sun photometer, several lidars (Raman and high-spectral-resolution instruments), and airborne and ground-based in situ instruments provided us with a comprehensive set of data on particle-shape dependent and particle-shape independent dust properties. We compare 4 measurement days in detail, and we carry out a statistical analysis for some of the inferred data products for the complete measurement period. Particle size distributions and complex refractive indices inferred from the Sun photometer observations and measured in situ aboard a research aircraft show systematic differences. We find differences in the wavelength-dependence of single-scattering albedo, compared to light-scattering computations that use data from SOAP (spectral optical absorption photometer). AERONET data products of particle size distribution, complex refractive index, and axis ratios were used to compute particle extinction-to-backscatter (lidar) ratios and linear particle depolarization ratios. We find differences for these parameters to lidar measurements of lidar ratio and particle depolarization ratio. Differences particularly exist at 355 nm, which may be the result of differences of the wavelength-dependent complex refractive index that is inferred by the methods employed in this field campaign. We discuss various error sources that may lead to the observed differences.

• S. Bauer, E. Bierwirth, M. Esselborn, A. Petzold, A. Macke, T. Trautmann, and M. Wendisch, Airborne spectral radiation measurements to derive solar radiative forcing of Saharan dust mixed with biomass burning smoke particles, Tellus B, 63, 742-750, doi:10.1111/j.1600-0889.2011.00567.x, 2011.
  Airborne measurements of upward solar spectral irradiances were performed during the second Saharan Mineral dUst experiMent (SAMUM-2) campaign based on the Cape Verde Islands. Additionally, airborne high resolution lidar measurements of vertical profiles of particle extinction coefficients were collected in parallel to the radiation data. Aerosol layers of Saharan dust, partly mixed with biomass-burning smoke, were probed. With corresponding radiative transfer simulations the single scattering albedo and the asymmetry parameter of the aerosol particles were derived although with high uncertainty. The broad-band aerosol solar radiative forcing at the top of atmosphere was calculated and examined as a function of the aerosol types. However, due to uncertainties in both the measurements and the calculations the chemical composition cannot be identified. In addition, a mostly measurement-based method to derive the broad-band aerosol solar radiative forcing was used. This approach revealed clear differences of broad-band net irradiances as a function of the aerosol optical depth. The data were used to identify different aerosol types from different origins. Higher portions of biomass-burning smoke lead to larger broad-band net irradiances.
• S. Otto, T. Trautmann, and M. Wendisch, On realistic shape and size equivalence of spheroidal Saharan mineral dust particles applied in solar and thermal radiative transfer calculations, Atmos. Chem. Phys., 11, 4469-4490, doi:10.5194/acp-11-4469-2011, 2011.
  Realistic size equivalence and shape of Saharan mineral dust particles are derived from in-situ particle, lidar and sun photometer measurements during SAMUM-1 in Morocco (19 May 2006), dealing with measured size- and altitude-resolved axis ratio distributions of assumed spheroidal model particles. The data were applied in optical property, radiative effect, forcing and heating effect simulations to quantify the realistic impact of particle non-sphericity. It turned out that volume-to-surface equivalent spheroids with prolate shape are most realistic: particle non-sphericity only slightly affects single scattering albedo and asymmetry parameter but may enhance extinction coefficient by up to 10 %. At the bottom of the atmosphere (BOA) the Saharan mineral dust always leads to a loss of solar radiation, while the sign of the forcing at the top of the atmosphere (TOA) depends on surface albedo: solar cooling/warming over a mean ocean/land surface. In the thermal spectral range the dust inhibits the emission of radiation to space and warms the BOA. The most realistic case of particle non-sphericity causes changes of total (solar plus thermal) forcing by 55/5 % at the TOA over ocean/land and 15 % at the BOA over both land and ocean and enhances total radiative heating within the dust plume by up to 20 %. Large dust particles significantly contribute to all the radiative effects reported. They strongly enhance the absorbing properties and forward scattering in the solar and increase predominantly, e.g., the total TOA forcing of the dust over land.
• S. Otto, and M. Meringer, Positively homogeneous functions in atmospheric radiative transfer theory, J. Math. Anal. Appl., 376, 588-601, doi:10.1016/j.jmaa.2010.10.068, 2011.
  Positively homogeneous functions are applied to describe absorption and scattering processes within the framework of atmospheric radiative transfer theory. Solid angle integrations are understood as surface integrations over arbitrary solid angle surfaces which are defined beyond the common picture of atmospheric radiative transfer.

• I. Tegen, E. Bierwirth, B. Heinold, J. Helmert, and M. Wendisch, Effect of measured surface albedo on modeled Saharan dust solar radiative forcing, J. Geophys. Res., 115, D24312, doi:10.1029/2009JD013764, 2010.
  The clear-sky solar radiative forcing of Saharan dust is computed for a case study during the SAharan Mineral DUst ExperiMent (SAMUM) in May 2006. Size-resolved dust concentrations simulated with a regional model and spectrally resolved surface albedo measurements were used as input for a 1D radiative transfer model to study the dependence of the dust radiative forcing at solar wavelengths on surface albedo and particle optical properties. Within the considered parameter range the surface albedo can have a larger impact on the solar radiative forcing of dust at the top of atmosphere (TOA) than the variations of optical properties. At the location of Ouarzazate in Morocco different measured surface albedo values lead to differences in instantaneous solar TOA solar forcing of up to 15 W m⁻² for identical dust properties. This highlights the importance of using an accurate characterization of surface albedo values for estimating solar dust forcing over land. In the regional average over the Sahara using either the standard model values or satellite-based surface albedos leads to differences in the order of 9 W m⁻² in the instantaneous solar forcing at TOA, and 5 W m⁻² for the diurnal mean TOA forcing.
• K. S. Schmidt, P. Pilewskie, B. Mayer, M. Wendisch, B. Kindel, S. Platnick, M. D. King, G. Wind, G. T. Arnold, L. Tian, G. Heymsfiel and H. Kalesse, Apparent absorption of solar spectral irradiance in heterogeneous ice clouds, J. Geophys. Res., 115, D00J22, doi:10.1029/2009JD013124, 2010.
  Coordinated flight legs of two aircraft above and below extended ice clouds played an important role in the Tropical Composition, Cloud and Climate Coupling Experiment (Costa Rica, 2007). The Solar Spectral Flux Radiometer measured up- and downward irradiance on the high-altitude (ER-2) and the low-altitude (DC-8) aircraft, which allowed deriving apparent absorption on a point-by-point basis along the flight track. Apparent absorption is the vertical divergence of irradiance, calculated from the difference of net flux at the top and bottom of a cloud. While this is the only practical method of deriving absorption from aircraft radiation measurements, it differs from true absorption when horizontal flux divergence is nonzero. Differences between true and apparent absorption are inevitable in any inhomogeneous atmosphere, especially clouds. We show, for the first time, the spectral shape of measured apparent absorption and compare with results from a three-dimensional radiative transfer model. The model cloud field is created from optical thickness and effective radius retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS) Airborne Simulator and from reflectivity profiles from the Cloud Radar System, both on board the ER-2. Although the spectral shape is reproduced by the model calculations, the measured apparent absorption in the visible spectral range is higher than the model results along extended parts of the flight leg. This is possibly due to a net loss of photons into neighboring cirrus-free areas that are not contained within the model domain.
• F. Henrich, H. Siebert, E. Jäkel, R. A. Shaw and M. Wendisch, Collocated Measurements of Boundary-Layer Cloud Microphysical and Radiative Properties - A Feasibility Study, J. Geophys. Res., 115, D24214, doi:10.1029/2010JD013930, 2010.
  First data from collocated, helicopter{based measurements of boundary-layer cloud microphysical properties (effective droplet radius R_eff, droplet number concentration) and spectral radiative quantities (cloud optical thickness t, cloud top albedo Ρ, reflectivity R) are presented. The in situ measurements of the microphysical cloud properties were collected by the Airborne Cloud Turbulence Observation System (ACTOS) attached to a helicopter by a 145 m long rope. Cloud spectral reflectivity was derived from radiances measured by grating spectrometers combined with downward-looking optical inlets installed underneath the helicopter. Correlations between cloud microphysics and reflected radiation are presented for two cloud cases with different optical and geometrical thicknesses. On the basis of common retrieval techniques t and R_eff are derived using a radiative transfer model. The results of the retrieval are compared to the collocated in situ measurements and data from the Moderate-Resolution Imaging Spectroradiometer (MODIS). Within the limitations of the relatively small data set, the feasibility of closely collocated microphysics and radiation data and their benefits was demonstrated.
• C. Voigt, U. Schumann, T. Jurkat, D. Schäuble, H. Schlager, A. Petzold, J.-F. Gayet, M. Krämer, J. Schneider, S. Borrmann, J. Schmale, P. Jessberger, T. Hamburger, M. Lichtenstern, M. Scheibe, C. Gourbeyre, J. Meyer, M. Kübbeler, W. Frey, H. Kalesse, T. Butler, M. G. Lawrence, F. Holzäpfel, F. Arnold, M. Wendisch, A. Döpelheuer, K. Gottschaldt, R. Baumann, M. Zöger, I. Sölch, M. Rautenhaus, and A. Dörnbrack, In-situ observations of young contrails - overview and selected results from the CONCERT campaign, Atmos. Chem. Phys., 10, 9039-9056, doi:10.5194/acp-10-9039-2010, 2010.
  Lineshaped contrails were detected with the research aircraft Falcon during the CONCERT - CONtrail and Cirrus ExpeRimenT - campaign in October/November 2008. The Falcon was equipped with a set of instruments to measure the particle size distribution, shape, extinction and chemical composition as well as trace gas mixing ratios of sulfur dioxide (SO2), reactive nitrogen and halogen species (NO, NOy, HNO3, HONO, HCl), ozone (O3) and carbon monoxide (CO). During 12 mission flights over Europe, numerous contrails, cirrus clouds and a volcanic aerosol layer were probed at altitudes between 8.5 and 11.6 km and at temperatures above 213 K. 22 contrails from 11 different aircraft were observed near and below ice saturation. The observed NO mixing ratios, ice crystal and soot number densities are compared to a process based contrail model. On 19 November 2008 the contrail from a CRJ-2 aircraft was penetrated in 10.1 km altitude at a temperature of 221 K. The contrail had mean ice crystal number densities of 125 cm⁻³ with effective radii r_eff of 2.6 μm. The presence of particles with r>50 μm in the less than 2 min old contrail suggests that natural cirrus crystals were entrained in the contrail. Mean HONO/NO (HONO/NOy) ratios of 0.037 (0.024) and the fuel sulfur conversion efficiency to H2SO4 of 2.9 % observed in the CRJ-2 contrail are in the range of previous measurements in the gaseous aircraft exhaust. On 31 October 2010 aviation NO emissions could have contributed by more than 40% to the regional scale NO levels in the mid-latitude lowest stratosphere. The CONCERT observations help to better quantify the climate impact from contrails and will be used to investigate the chemical processing of trace gases on contrails.
• E. Bierwirth, M. Wendisch, E. Jäkel, A. Ehrlich, K. S. Schmidt, H. Stark, P. Pilewskie, M. Esselborn, G. P. Gobbi, R. Ferrare, T. Müller, and A. Clarke, A new method to retrieve the aerosol layer absorption coefficient from airborne flux density and actinic radiation measurements, J. Geophys. Res., 115, D14211, doi:10.1029/2009JD013636, 2010.
  A new method is presented to derive the mean value of the spectral absorption coefficient of an aerosol layer from combined airborne measurements of spectral net irradiance and actinic flux density. While the method is based on a theoretical relationship of radiative-transfer theory, it is applied to atmospheric radiation measurements for the first time. The data have been collected with the Spectral Modular Airborne Radiation Measurement System (SMART)-Albedometer, the Solar Spectral Flux Radiometer (SSFR), and the Actinic Flux Spectroradiometer (AFSR) during four field campaigns between 2002 and 2008 (SAMUM, INSPECTRO, ARCTAS/ARCPAC). The retrieval algorithm is tested in a series of radiative-transfer model runs and then applied to measurement cases with different aerosol species and loading. The method is shown to be a feasible approach to obtain the mean aerosol absorption coefficient across a given accesible altitude range. The results indicate that the method is viable whenever the difference of the net irradiance at top and bottom of a layer is equal to or higher than the measurement uncertainty for net irradiance. This can be achieved by a high optical depth or a low single-scattering albedo within the layer.
• A. Lampert, C. Ritter, A. Hoffmann, J.-F. Gayet, G. Mioche, A. Ehrlich, A. Dörnbrack, M. Wendisch, and M. Shiobara, Lidar characterization of the Arctic atmosphere during ASTAR 2007: four cases studies of boundary layer, mixed-phase and multi-layer clouds, Atmos. Chem. Phys., 10, 2847-2866, doi:10.5194/acp-10-2847-2010, 2010.
  During the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR), which was conducted in Svalbard in March and April 2007, tropospheric Arctic clouds were observed with two ground-based backscatter lidar systems (micro pulse lidar and Raman lidar) and with an airborne elastic lidar. In the time period of the ASTAR 2007 campaign, an increase in low-level cloud cover (cloud tops below 2.5 km) from 51% to 65% was observed above Ny-Ålesund. Four different case studies of lidar cloud observations are analyzed: With the ground-based Raman lidar, a layer of spherical particles was observed at an altitude of 2 km after the dissolution of a cloud. The layer probably consisted of small hydrated aerosol (radius of 280 nm) with a high number concentration (around 300 cm-3) at low temperatures (-30 °C). Observations of a boundary layer mixed-phase cloud by airborne lidar and concurrent airborne in situ and spectral solar radiation sensors revealed the localized process of total glaciation at the boundary of different air masses. In the free troposphere, a cloud composed of various ice layers with very different optical properties was detected by the Raman lidar, suggesting large differences of ice crystal size, shape and habit. Further, a mixed-phase double layer cloud was observed by airborne lidar in the free troposphere. Local orography influenced the evolution of this cloud. The four case studies revealed relations of cloud properties and specific atmospheric conditions, which we plan to use as the base for numerical simulations of these clouds.

• W. Frey, H. Eichler, M. de Reus, R. Maser, M. Wendisch, and S. Borrmann, A new airborne tandem platform for collocated measurements of microphysical cloud and radiation properties, Atmos. Meas. Tech., 2, 147-158, doi:10.5194/amt-2-147-2009, 2009.
  A new airborne tandem measurement platform for cloud-radiation interaction studies is introduced in this paper. It consists of a Learjet 35A research aircraft and the AIRcraft TOwed Sensor Shuttle (AIRTOSS), which is an instrumented drag-body towed by the Learjet. Currently, the AIRTOSS is instrumented with a Cloud Imaging Probe (CIP) for measuring cloud microphysical properties and an Inertial Navigation System (INS) for measurements of flight attitudes. The cable dragging AIRTOSS can be as long as four kilometres. Thus, truly collocated measurements in two altitudes above, in, and below clouds can be obtained. Results from first test flights with Learjet and AIRTOSS are reported here. The flights were performed from Hohn Airport, Germany. Specific manoeuvres were flown to test the aerodynamic behaviour of the drag-body and to investigate the suitability of AIRTOSS for high-precision irradiance measurements which require a stable flight attitude of AIRTOSS. The flight attitude data show that AIRTOSS is sensitive to several flight manoeuvres such as turns, altitude and airspeed changes, and also to changes of towing cable length. The effects of these manoeuvres on the attitude angles of AIRTOSS have been quantified. Maximum roll angle deviations were observed during turns. Even small changes in heading can lead to high roll angles (one degree change in heading causes a change in roll angle of about eight degrees). The pitch angle varies during climb or dive periods, extending or retracting of towing cable, acceleration or deceleration, and even when flying at too low or too high true airspeed depending on altitude. Values of pitch angle between -5° (dive) and 8° (climb and retracting towing cable) have been observed. While change in attitude is not problematic for cloud particle property measurements it is for radiation measurements. Here, the deviation from the horizontal should be no more than 3° to avoid large errors. When keeping the above mentioned flight parameters constant, sufficiently stable flight conditions can be maintained to perform high-quality irradiance measurements with AIRTOSS in future experiments. During this test campaign also observations of cloud microphysical data as for example droplet number concentrations and size distributions with the AIRTOSS in stratocumulus clouds were performed to prove the compliance with scientific needs. Simultaneous spectral cloud radiation measurements have been made. The measurements of internal operational data of AIRTOSS as well as the first atmospheric data demonstrate the suitability of this tandem platform for detailed cloud microphysics and radiation interaction studies.
• A. Ehrlich, M. Wendisch, E. Bierwirth, J.-F. Gayet, G. Mioche, A. Lampert, B. Mayer, Evidence of ice crystals at cloud top of Arctic boundary-layer mixed-phase clouds derived from airborne remote sensing, Atmos. Chem. Phys., 9, 9401-9416, doi:doi:10.5194/acp-9-9401-2009, 2009.
  The vertical distribution of ice crystals in Arctic boundary-layer mixed-phase (ABM) clouds was investigated by airborne remote-sensing and in situ measurements during the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign in March and April 2007. Information on the spectral absorption of solar radiation by ice and liquid water cloud particles is derived from airborne measurements of solar spectral radiation reflected by these clouds. It is shown by calculation of the vertical weighting function of the measurements that the observed absorption of solar radiation is dominated by the upper cloud layers (50% within 200 m from cloud top). This vertical weighting function is shifted even closer to cloud top for wavelengths where absorption by ice dominates. On this basis an indicator of the vertical distribution of ice crystals in ABM clouds is designed.
  Applying in situ measured microphysical properties, the cloud-top reflectivity was calculated by radiative transfer simulations and compared to the measurements. It is found that ice crystals near cloud top (mixed-phase cloud top layer) are necessary to reproduce the measurements at wavelengths where absorption by ice dominates. The observation of backscatter glories on the cloud top generally indicating liquid water droplets does not contradict the postulated presence of ice crystals. Radiative transfer simulations reproduce the observed glories even if the cloud top layer is of mixed-phase character.
• H. Eichler, A. Ehrlich, M. Wendisch, G., Mioche, J.-F. Gayet, M. Wirth, C. Emde, A. Minikin, Influence of ice crystal shape on retrieval of cirrus optical thickness and effective radius– A case study, J. Geophys. Res., 114, D19203, doi:10.1029/2009JD012215, 2009.
  Airborne measurements of spectral upwelling radiances (350-2200 nm) reflected by cirrus using the Spectral Modular Airborne Radiation measurement sysTem (SMART)-Albedometer were made over land and water surfaces. Based on these data, cloud optical thickness t and effective radius R_eff of the observed cirrus were retrieved. By using different crystal shape assumptions (hexagonal plates, solid and hollow columns, rough aggregates, planar and spatial rosettes, ice spheres, and a mixture of particle habits) in the retrieval, the influence of crystal shape on the retrieved t and R_eff was evaluated. With relative differences of up to 70%, the influence of particle habit on t is larger than on R_eff (up to 20% differences). Retrieved t values agreed with values derived from concurrent lidar measurements within the measurement uncertainties.
• J.-F. Gayet, G. Mioche, A. Dörnbrack, A. Ehrlich, A. Lampert, and M. Wendisch, Microphysical and optical properties of Arctic mixed-phase clouds. The 9 April 2007 case study., Atmos. Chem. Phys., 9, 6581-6595, doi:10.5194/acp-9-6581-2009, 2009.
  A. Lampert, A. Ehrlich, A. Dörnbrack, O. Jourdan, J.-F. Gayet, G. Mioche, V. Shcherbakov, C. Ritter, M. Wendisch, Microphysical and radiative characterization of a subvisible midlevel Arctic ice cloud by airborne observations – a case study, Atmos. Chem. Phys., 9, 2647-2661, doi:10.5194/acp-9-2647-2009, 2009.
  During the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign, which was conducted in March and April 2007, an optically thin ice cloud was observed south of Svalbard at around 3 km altitude. The microphysical and radiative properties of this particular subvisible midlevel cloud were investigated with complementary remote sensing and in situ instruments. Collocated airborne lidar remote sensing and spectral solar radiation measurements were performed at a flight altitude of 2300 m below the cloud base. Under almost stationary atmospheric conditions, the same subvisible midlevel cloud was probed with various in situ sensors roughly 30 min later. From individual ice crystal samples detected with the Cloud Particle Imager and the ensemble of particles measured with the Polar Nephelometer, microphysical properties were retrieved with a bi-modal inversion algorithm. The best agreement with the measurements was obtained for small ice spheres and deeply rough hexagonal ice crystals. Furthermore, the single-scattering albedo, the scattering phase function as well as the volume extinction coefficient and the effective diameter of the crystal population were determined. A lidar ratio of 21(±6) sr was deduced by three independent methods. These parameters in conjunction with the cloud optical thickness obtained from the lidar measurements were used to compute spectral and broadband radiances and irradiances with a radiative transfer code. The simulated results agreed with the observed spectral downwelling radiance within the range given by the measurement uncertainty. Furthermore, the broadband radiative simulations estimated a net (solar plus thermal infrared) radiative forcing of the subvisible midlevel ice cloud of −0.4 W m⁻² (−3.2 W m⁻² in the solar and +2.8 W m⁻² in the thermal infrared wavelength range).
• E. Bierwirth, M. Wendisch, A. Ehrlich, B. Heese, M. Tesche, D. Althausen, A. Schladitz, D. Müller, S. Otto, T. Trautmann, T. Dinter, W. von Hoyningen-Huene, W., R. Kahn, Spectral surface albedo over Morocco and its impact on radiative forcing of Saharan dust, Tellus B, 61, 252-269, doi:10.1111/j.1600-0889.2008.00395.x, 2009.
  In May-June 2006, airborne and ground-based solar (0.3-2.2 um) and thermal infrared (4-42 um) radiation measurements have been performed in Morocco within the Saharan Mineral Dust Experiment (SAMUM). Upwelling and downwelling solar irradiances have been measured using the Spectral Modular Airborne Radiation Measurement System (SMART)-Albedometer. With these data, the areal spectral surface albedo for typical surface types in southeastern Morocco was derived from airborne measurements for the first time. The results are compared to the surface albedo retrieved from collocated satellite measurements, and partly considerable deviations are observed. Using measured surface and atmospheric properties, the spectral and broad-band dust radiative forcing at top-of-atmosphere (TOA) and at the surface has been estimated. The impact of the surface albedo on the solar radiative forcing of Saharan dust is quantified. In the SAMUM case of 19 May 2006, TOA solar radiative forcing varies by 12 W m^-2 per 0.1 surface-albedo change. For the thermal infrared component, values of up to +22 W m^-2 were derived. The net (solar plus thermal infrared) TOA radiative forcing varies between -19 and +24 W m^-2 for a broad-band solar surface albedo of 0.0 and 0.32, respectively. Over the bright surface of southeastern Morocco, the Saharan dust always has a net warming effect.
• S. Otto, E. Bierwirth, B. Weinzierl, K. Kandler, M. Esselborn, M. Tesche, A. Schladitz, M. Wendisch, and T. Trautmann, Solar radiative effects of a Saharan dust plume observed during SAMUM assuming spheroidal model particles, Tellus B, 61, 270-296, doi:10.1111/j.1600-0889.2008.00389.x, 2009.
  The solar optical properties of Saharan mineral dust observed during the Saharan Mineral Dust Experiment (SAMUM) were explored based on measured size-number distributions and chemical composition. The size-resolved complex refractive index of the dust was derived with real parts of 1.51-1.55 and imaginary parts of 0.0008-0.006 at 550 nm wavelength. At this spectral range a single scattering albedo omega_o and an asymmetry parameter g of about 0.8 were derived. These values were largely determined by the presence of coarse particles. Backscatter coefficients and lidar ratios calculated with Mie theory (spherical particles) were not found to be in agreement with independently measured lidar data. Obviously the measured Saharan mineral dust particles were of non-spherical shape. With the help of these lidar and sun photometer measurements the particle shape as well as the spherical equivalence were estimated. It turned out that volume equivalent oblate spheroids with an effective axis ratio of 1:1.6 matched these data best. This aspect ratio was also confirmed by independent single particle analyses using a scanning electron microscope. In order to perform the non-spherical computations, a database of single particle optical properties was assembled for oblate and prolate spheroidal particles. These data were also the basis for simulating the non-sphericity effects on the dust optical properties: omega_o is influenced by up to a magnitude of only 1% and g is diminished by up to 4% assuming volume equivalent oblate spheroids with an axis ratio of 1:1.6 instead of spheres. Changes in the extinction optical depth are within 3.5%. Non-spherical particles affect the downwelling radiative transfer close to the bottom of the atmosphere, however, they significantly enhance the backscattering towards the top of the atmosphere: Compared to Mie theory the particle non-sphericity leads to forced cooling of the Earth-atmosphere system in the solar spectral range for both dust over ocean and desert.

• A. Ehrlich, E. Bierwirth, M. Wendisch, J.-F. Gayet, G. Mioche, A. Lampert, J. Heintzenberg, Cloud phase identification of arctic boundary-layer clouds from airborne spectral reflection measurements: Test of three approaches, Atmos. Chem. Phys., 8, 7493-7505, doi:10.5194/acp-8-7493-2008, 2008.
  Arctic boundary-layer clouds were investigated with remote sensing and in situ instruments during the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign in March and April 2007. The clouds formed in a cold air outbreak over the open Greenland Sea. Beside the predominant mixed-phase clouds pure liquid water and ice clouds were observed. Utilizing measurements of solar radiation reflected by the clouds three methods to retrieve the thermodynamic phase of the cloud are introduced and compared. Two ice indices I_S and I_P were obtained by analyzing the spectral pattern of the cloud top reflectance in the near infrared (1500-1800 nm wavelength) spectral range which is characterized by ice and water absorption. While I_S analyzes the spectral slope of the reflectance in this wavelength range, IS utilizes a principle component analysis (PCA) of the spectral reflectance. A third ice index I_A is based on the different side scattering of spherical liquid water particles and nonspherical ice crystals which was recorded in simultaneous measurements of spectral cloud albedo and reflectance. Radiative transfer simulations show that I_S, I_P and I_A range between 5 to 80, 0 to 8 and 1 to 1.25 respectively with lowest values indicating pure liquid water clouds and highest values pure ice clouds. The spectral slope ice index I_S and the PCA ice index I_P are found to be strongly sensitive to the effective diameter of the ice crystals present in the cloud. Therefore, the identification of mixed-phase clouds requires a priori knowledge of the ice crystal dimension. The reflectance-albedo ice index I_A is mainly dominated by the uppermost cloud layer (t<1.5). Therefore, typical boundary-layer mixed-phase clouds with a liquid cloud top layer will be identified as pure liquid water clouds. All three methods were applied to measurements above a cloud field observed during ASTAR 2007. The comparison with independent in situ microphysical measurements shows the ability of the three approaches to identify the ice phase in Arctic boundary-layer clouds.
• M. Wendisch, O. Hellmuth, A. Ansmann, J. Heintzenberg, R. Engelmann, D. Althausen, H. Eichler, D. Müller, M. Hu, Y. Zhang, J. Mao, Radiative and dynamic effects of absorbing aerosol particles over the Pearl River Delta, China, Atmos. Environ., 42, 6405-6416, doi:10.1016/j.atmosenv.2008.02.033, 2008.
  Results are reported from a ground-based measurement campaign conducted in a highly polluted region in southeast of China in October-November 2004. The experiment focused on absorbing aerosol particles and their effects on the solar radiation field and local meteorology. A Raman lidar in conjunction with Sun photometer data measured profiles of particle extinction; ground-based in situ data of aerosol optical properties were collected by nephelometer and absorption photometer. Exceptionally high values of aerosol optical depth of up to 1.5 were observed.
  The measurements were input to a radiative transfer model, which simulated high solar radiative forcing values for the aerosol particles of up to -160 W m(-2) at the ground (daily average) for the observed particle single-scattering albedo of 0.85. Maximum solar heating rates of 7-8 K day(-1) were simulated at the top of the aerosol layer.
  The radiative simulations were used to drive a dynamic model of the planetary boundary layer (PBL). With this model the temporal course of the height of the PBL was simulated and compared with respective lidar data. The results show that the height of the PBL is significantly decreased due to the warming of the aerosol particles at the top of the PBL. In this way, the stabilizing effect of absorbing aerosol particles within the PBL was confirmed by a combination of experimental and modeling means.
• H. Eichler, Y.F. Chenga, W. Birmili, A. Nowak, A. Wiedensohler, E. Brüggemann, T. Gnauk, H. Herrmann, D. Althausen, A. Ansmann, R. Engelmann, M. Tesche, M. Wendisch, Y.H. Zhang, M. Hu, S. Liu, L.M. Zeng, Hygroscopic properties and extinction of aerosol particles at ambient relative humidity in South-Eastern China, Atmos. Env., 42, 6321-6334, doi:10.1016/j.atmosenv.2008.05.007, 2008.
  During the ‘‘Program of Regional Integrated Experiments of Air Quality over Pearl River Delta 2004 (PRIDE-PRD2004)’’ hygroscopic properties of particles in the diameter range 22nm to 10 µm were determined. For that purpose, a Humidifying Differential Mobility Particle Sizer (H-DMPS) and a Micro-Orifice Uniform Deposition Impactor (MOUDI) were operated. The derived size-dependent particle hygroscopic growth factors were interpolated to ambient relative humidity (RH) and used to calculate the particle number size distributions (PNSDs) at ambient conditions. A comparison between the modeled particle extinction coefficients (s_ext;Mie) and those observed with a Raman lidar was made. It is shown that the particle extinction coefficient (s_ext) at ambient RH can be properly estimated with Mie-model calculations based on the in situ physico-chemical measurements of dry and humidified PNSD and chemical composition.
  A. Ehrlich, M. Wendisch, E. Bierwirth, A. Herber, A. Schwarzenböck, Ice crystal shape effects on solar radiative properties of Arctic mixed-phase clouds - Dependence on microphysical properties, Atmos. Res., 88, 266-276, doi:10.1016/j.atmosres.2007.11.018, 2008.
  Based on 1-year cloud measurements with radar and microwave radiometer broadband solar radiative transfer simulations were performed to quantify the impact of different ice crystal shapes of Arctic mixed-phase clouds on their radiative properties (reflectance, transmittance and absorptance). The ice crystal shape effects were investigated as a function of microphysical cloud properties (ice volume fraction f_iM< I and D_M^W of ice/water particle number size distributions, NSDs). The required NSDs were statistically derived from radar data. The NSD was composed of a liquid and a solid mode defined by LWC_, D_{M^W (}water mode) and IWC, D_M^I (ice mode). It was found that the ratio of D_M^I and D_M^W determines the magnitude of the shape effect. For mixed-phase clouds with D_M^I =27 µm a significant shape effect was obtained. The shape effect was almost insensitive with regard to the solar zenith angle, but highly sensitive to the ice volume fraction of the mixed-phase cloud. For mixed-phase clouds containing small ice crystals (D_M^I =27 µm) and high ice volume fractions (f_i>0.5) crystal shape is crucial. The largest shape effects were observed assuming aggregates and columns. If the IWC was conserved the shape effect reaches values up to 0.23 in cloud reflectance and transmittance. If the ice mode NSD was kept constant only a small shape effect was quantified (≤0.04).

• K.S. Schmidt, P. Pilewskie, S. Platnick, G. Wind, P. Yang, M. Wendisch, Comparing irradiance fields derived from Moderate Resolution Imaging Spectroradiometer airborne simulator cirrus cloud retrievals with solar spectral flux radiometer measurements, J. Geophys. Res., 112, D24206, doi:10.1029/2007JD008711, 2007.
  During the Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment, the Moderate Resolution Imaging Spectroradiometer (MODIS) airborne simulator (MAS) and the solar spectral flux radiometer (SSFR) operated on the same aircraft, the NASA ER-2. While MAS provided two-dimensional horizontal fields of cloud optical thickness and effective ice particle radius, the SSFR measured spectral irradiance in the visible to near-infrared wavelength range (0.3-1.7 µm). The MAS retrievals, along with vertical profiles from a combined radar/lidar system on board the same aircraft were used to construct three- dimensional cloud fields, which were input into Monte Carlo radiative transfer models. The simulated field of spectral albedo (ratio of reflected upwelling to incident downwelling irradiance) was compared with the SSFR measurements. For two cases, the relative importance of spatial cloud heterogeneities, various approximations of the single scattering parameters, vertical structure, cirrus optical thickness, and ice crystal effective radius was studied.
• S. Otto, M. de Reus, T. Trautmann, A. Thomas, M. Wendisch, S. Borrmann, Atmospheric radiative effects of an in situ measured Saharan dust plume and the role of large particles, Atmos. Chem. Phys., 18, 4887-4903, doi:10.5194/acp-7-4887-2007, 2007.
  This work will present aerosol size distributions measured in a Saharan dust plume between 0.9 and 12 km altitude during the ACE-2 campaign 1997. The distributions contain a significant fraction of large particles of diameters from 4 to 30 µm. Radiative transfer calculations have been performed using these data as input. Shortwave, longwave as well as total atmospheric radiative effects (AREs) of the dust plume are investigated over ocean and desert within the scope of sensitivity studies considering varied input parameters like solar zenith angle, scaled total dust optical depth, tropospheric standard aerosol profiles and particle complex refractive index. The results indicate that the large particle fraction has a predominant impact on the optical properties of the dust. A single scattering albedo of &omega=0.75-0.96 at 550 nm was simulated in the entire dust column as well as 0.76 within the Saharan dust layer at similar to 4 km altitude indicating enhanced absorption. The measured dust leads to cooling over the ocean but warming over the desert due to differences in their spectral surface albedo and surface temperature. The large particles absorb strongly and they contribute at least 20% to the ARE in the dusty atmosphere.
  From the measured size distributions modal parameters of a bimodal lognormal column volume size distribution were deduced, resulting in a coarse median diameter of similar to 9 µm and a column single scattering albedo of 0.78 at 550 nm. A sensitivity study demonstrates that variabilities in the modal parameters can cause completely different AREs and emphasises the warming effect of the large mineral dust particles.
• J. Helmert, B. Heinold, I. Tegen, O. Hellmuth, M. Wendisch, On the direct and semidirect effects of Saharan dust over Europe: A modeling study, J. Geophys. Res., 112, D13208, doi:10.1029/2006JD007444, 2007.
  On the basis of a new regional dust model system, the sensitivity of radiative forcing to dust aerosol properties and the impact on atmospheric dynamics were investigated. Uncertainties in optical properties were related to uncertainties in the complex spectral refractive index of mineral dust. The climatological-based distribution of desert-type aerosol in the radiation scheme of the nonhydrostatic regional model LM was replaced by dust optical properties from spectral refractive indices, derived from in situ measurements, remote sensing, bulk measurements, and laboratory experiments, employing Mie theory. The model computes changes in the solar and terrestrial irradiance from a spatially and temporally varying atmospheric dust load for five size classes. A model study of a Saharan dust outbreak in October 2001 was carried out when large amounts of Saharan dust were transported to Europe. The dust optical thickness computed from the simulation results in values of about 0.5 in large regions of the Saharan desert but can be larger than 5.0 near large dust sources (for example, Bodele depression). During the dust outbreak, the aerosol in the southern Sahara causes a daytime reduction in 2-m temperature of 3 K in average with differences of 10% depending on used dust optical properties. The simulations indicated that the large variability in radiative properties due to different mixture of clay aggregates in Saharan dust can lead in regional average to differences of up to 48% in net forcing efficiency at top of the atmosphere.
• E. Jäkel, P. N. den Outer, R. B. Tax, P. C. Görts, and H. A. J. M. Reinen, Improving solar ultraviolet irradiance measurements by applying a temperature correction method for Teflon diffusers, Appl. Opt., 46, 4222-4227, doi:10.1364/AO.46.004222, 2007.
  To establish trends in surface ultraviolet radiation levels, accurate and stable long-term measurements are required. The accuracy level of today's measurements has become high enough to notice even smaller effects that influence instrument sensitivity. Laboratory measurements of the sensitivity of the entrance optics have shown a decrease of as much as 0.07-0.1%/deg temperature increase. Since the entrance optics can heat to greater than 45 °C in Dutch summers, corrections are necessary. A method is developed to estimate the entrance optics temperatures from pyranometer measurements and meteorological data. The method enables us to correct historic data records for which temperature information is not available. The temperature retrieval method has an uncertainty of less than 2.5 °C, resulting in a 0.3% uncertainty in the correction to be performed. The temperature correction improves the agreement between modeled and measured doses and instrument intercomparison as performed within the Quality Assurance of Spectral Ultraviolet Measurements in Europe project. The retrieval method is easily transferable to other instruments.
• K.S. Schmidt, V. Venema, F.  Di Giuseppe, R. Scheirer, M. Wendisch, and P. Pilewskie, Reproducing cloud microphysical and irradiance measurements using three 3D cloud generators,  Quat. J. of Royal Met. Soc. , 133, 765-780, doi:10.1002/qj.53, 2007.
  Using three cloud generators, three-dimensional (3D) cloud fields are reproduced from microphysical cloud data measured in situ by aircraft. The generated cloud fields are used as input to a 3D radiative transfer model to calculate the corresponding fields of downward and upward irradiance, which are then compared with airborne and ground-based radiation measurements. One overcast stratocumulus scene and one broken cumulus scene were selected from the European INSPECTRO field experiment, which was held in Norwich, UK, in September 2002. With these data, the characteristics of the three different cloud reproduction techniques are assessed. Besides vertical profiles and histograms of measured and modelled liquid water content and irradiance, the horizontal structure of these quantities is examined in terms of power spectra and autocorrelation lengths. 3D radiative transfer calculations are compared with the independent pixel approximation, and their differences with respect to domain-averaged quantities and 3D fields are interpreted.
• E. Jäkel, M. Wendisch, M. Blumthaler, R. Schmitt, A. R. Webb, A CCD Spectroradiometer for Ultraviolet Actinic Radiation Measurements, J. Atmos. Ocean. Technol., 24, 449-462, doi:10.1175/JTECH1979.1, 2007.
  A new spectroradiometer for spectral measurements of ultraviolet (UV) atmospheric radiation (290–400 nm) using a charge coupled device (CCD) as a detector is introduced. The instrument development is motivated by the need for measurements with (a) high accuracy in the UV-B spectral range (290–315 nm) for photochemistry applications and (b) high temporal resolution in quickly changing atmospheric conditions such as partial cloud cover. The new CCD instrument is mainly intended for airborne use. It allows fast data collection (
  300 ms time resolution for each spectrum) with improved sensitivity in the UV spectral range. The instrumental setup and its characterization in terms of stray light, dark current, noise, and detection limits are described and compared to a spectroradiometer with a photodiode array (PDA) detector. The new CCD spectroradiometer has a one order of magnitude greater sensitivity than the PDA-based spectroradiometer. However, the stray light of the CCD instrument is wavelength dependent, which requires a more complicated data evaluation procedure than the PDA instrument. Comparison with other UV spectroradiometers (a PDA spectroradiometer and two ground-based double monochromators) shows the advantages of the CCD system for UV-B measurements of actinic flux densities and photolysis frequencies of ozone and nitrogen dioxide, and the improved performance compared to PDA spectroradiometers.
• M. Wendisch, P. Yang, P. Pilewskie, Effects of ice crystal habit on thermal infrared radiative properties and forcing of cirrus, J. Geophys. Res. , 112, D08201, doi:10.1029/2006JD007899, 2007.
  The impact of assumed ice crystal morphology on thermal infrared (IR) radiative properties of subtropical cirrus is quantified. In particular, the crystal-shape-dependent profiles of downwelling and upwelling thermal IR (broadband and spectral) irradiances and the radiative forcing of cirrus (at the top and bottom of the atmosphere) are investigated. For this purpose, airborne measurements of ice crystal size distribution (in terms of ice crystal maximum dimension) from the CRYSTAL-FACE campaign and a recently published library of thermal IR optical properties of nonspherical ice crystal habits are implemented into radiative transfer simulations. Two cirrus cases are studied in detail: (1) a high (cold) cirrus with small optical thickness (≈1 at 10.8 mm wavelength) and (2) a lower (warmer) cirrus of relatively large optical thickness (≈7). The relative effects of ice crystal shape on thermal IR irradiance are substantial for the high, optically thin cirrus (up to 70%). Spectrally, the largest effects of ice crystal shape are identified in the atmospheric window spanning from 8 to 12 mm wavelengths, especially for the upwelling irradiance above the cirrus. For the low cirrus of large optical thickness the thermal IR irradiance is only slightly sensitive to ice crystal habit (less than 15–20%). Within the major gas absorption bands the thermal IR radiation is essentially insensitive to ice crystal shape. Furthermore, it is concluded that the thermal IR radiative forcing at the top of the atmosphere contains significant ice crystal shape dependence for the high cirrus case.

• H. Siebert, K. Lehmann, M. Wendisch, Observations of Small-Scale Turbulence and Energy Dissipation Rates in the Cloudy Boundary Layer, J. Atmosph. Scienc., 63, 1451-1466, doi:10.1175/JAS3687.1, 2006.
• E. Jäkel, M. Wendisch, B. Lefer, Parameterization of ozone photolysis frequency in the lower troposphere using data from photodiode array detector spectrometers, J. Atmos. Chem., 54, 67-87, doi:10.1007/s10874-006-9014-1, 2006.
  Spectroradiometers using photodiode array detectors (PDAs) are increasingly applied for airborne and ground-based atmospheric measurements of spectral actinic flux densities due to their high time resolution (less than one second). However they have limited sensitivity of ultraviolet (UV) radiation for wavelengths less than about 305 nm. This results in uncertainties of ozone photolysis frequencies derived from spectral actinic flux density measurements using PDA spectrometers. To overcome this limitation a parameterization method is introduced which extrapolates the data towards the wavelength range of limited sensitivity of the PDA spectrometers (less than about 305 nm). The parameterization is based on radiative transfer simulations and is valid for measurements in the lower troposphere. The components of the suggested parameterization are the lower threshold wavelength of the PDA spectrometer, the slant ozone column (ratio of the total ozone column and the cosine of the solar zenith angle), and the ambient temperature. Tests of the parameterization with simulated actinic flux density spectra have revealed an uncertainty of the derived ozone photolysis frequency of +/- 5%. Field comparisons of the parameterization results with independent measurements of the ozone photolysis frequency were within +/- 10% for solar zenith angles less than 70 degrees. Finally the parameterization was applied to airborne measurements to emphasize the advantage of high time resolution of PDA spectrometers to study ozone photolysis frequency fields in inhomogeneous cloud condtitions.
• M. Wendisch, D. Müller, I. Mattis, A. Ansmann, Potential of lidar backscatter data to estimate solar aerosol radiative forcing, Appl. Opt., 45, 770-783, doi:10.1364/AO.45.000770, 2006.
  The potential to estimate solar aerosol radiative forcing (SARF) in cloudless conditions from backscatter data measured by widespread standard lidar has been investigated. For this purpose 132 days of sophisticated ground-based Raman lidar observations (profiles of particle extinction and backscatter coefficients at 532 nm wavelength) collected during two campaigns [the European Aerosol Research Lidar Network (EARLINET) and the Indian Ocean Experiment (INDOEX)] were analyzed. Particle extinction profiles were used as input for radiative transfer simulations with which to calculate the SARF, which then was plotted as a function of the column (i.e., height-integrated) particle backscatter coefficient (beta(c)) A close correlation between the SARF and beta(c) was found. SARF-beta(c) parameterizations in the form of polynomial fits were derived that exhibit an estimated uncertainty of +/-(10-30)%. These parameterizations can be utilized to analyze data of upcoming lidar satellite missions and for other purposes. The EARLINET-based parameterizations can be applied to lidar measurements at mostly continental, highly industrialized sites with limited maritime influence (Europe, North America), whereas the INDOEX parameterizations rather can be employed in polluted maritime locations, e.g., coastal regions of south and east Asia.

2005

• E. Jäkel, M. Wendisch, A. Kniffka, T. Trautmann, Airborne system for fast measurements of upwelling and downwelling spectral actinic flux densities, Appl. Opt., 44, 434-444, doi:10.1364/AO.44.000434, 2005.
  An airborne system for fast measurements of spectral actinic flux densities in the wavelength range 305-700 nm is introduced. The system is called the Actinic Flux Density Meter (AFDM). The AFDM utilizes the diode array technique and measures downwelling and upwelling spectral actinic flux densities separately with a time resolution of less than 1 s. For airborne measurements this means a spatial resolution of similar to60 m, assuming an average aircraft velocity of 60 m/s. Thus the AFDM resolves fast changes in the actinic radiation field, which are of special importance for conditions of inhomogeneous clouds or surface reflection. Laboratory characterization measurements of the AFDM are presented, and a method to correct the nonideal angular response of the optical inlets is introduced. Furthermore, exemplar field data sampled simultaneously with spectral irradiance measurements are shown. The horizontal variability of the measured spectra of actinic flux density is quantified, and profile measurements for overcast situations are presented. Finally, the effects of clouds on the spectral actinic flux density are discussed.
• M. Wendisch, P. Pilewskie P, J. Pommier, S. Howard, P. Yang, A.J. Heymsfield, C.G. Schmitt, D. Baumgardner, B. Mayer, Impact of cirrus crystal shape on solar spectral irradiance: A case study for subtropical cirrus, J. Geophys. Res., 110, doi:10.1029/2004JD005294, 2005.
  Profiles of in situ measurements of ice crystal size distribution of subtropical cirrus were used to calculate solar spectral irradiances above and below the clouds. Spheres and nonspherical ice crystal habits (columns, hollows, plates, bullets, and aggregates) were assumed in the calculations. The simulation results were compared to irradiance measurements from the NASA Solar Spectral Flux Radiometer. The microphysical and radiation data were collected by three aircraft during CRYSTAL- FACE. Two cirrus cases ( optical thickness of about 1 and 7) from two mission dates ( 26 and 23 July 2002) were investigated in detail. The measured downwelling and upwelling irradiance spectra above the cirrus could mostly be reproduced by the radiation model to within +/-5 - 10% for most ice crystal habits. Below the cirrus the simulations disagreed with the measured irradiances due to surface albedo variability along the flight track, and nonoptimal colocation between the microphysical and irradiance measurements. The impact of shape characteristics of the crystals was important for the reflected irradiances above the optically thin cirrus, especially for small solar zenith angles, because in this case single- scattering dominated the solar radiation field. For the cirrus of moderate optical thickness the enhanced multiple scattering tended to diminish particular shape features caused by nonspherical single- scattering. Within the ice absorption bands the shape- related differences in the absorption characteristics of the individual nonspherical ice crystals were amplified if multiple scattering prevailed. Furthermore, it was found that below the cloud the shape sensitivity of the downwelling irradiance spectra is larger compared to the nonsphericity effects on reflected irradiances above the cirrus. Finally, it was shown that the calculated cirrus solar radiative forcing could vary by as much as 26% depending on the ice crystal habit.

2004

• M. Wendisch, P. Pilewski, E. Jäkel, S. Schmidt, J. Pommier, S. Howard, H.H. Jonson, H. Guan, M. Schröder, B. Mayer, Airborne measurements of areal spectral surface albedo over different sea and land surfaces, J. Geophys. Res., 109, doi:10.1029/2003JD004392, 2004.
  Airborne measurements of the ratio of spectral upward and downward irradiances (so-called spectral albedo) are used to derive the areal spectral surface albedo in the wavelength range from 330 to 1670 nm. The data were collected over different sea and land surfaces in cloudless atmospheric conditions during three field campaigns. Measurements from the Albedometer (developed at IfT) and the NASA Solar Spectral Flux Radiometer (SSFR) are employed. Spectral radiative transfer calculations show that atmospheric scattering and absorption within the layer beneath the flight level considerably contribute to the airborne albedo measurements reported here, even for low flight altitudes (0.2-0.5 km). To remove this atmospheric masking, a nonlinear extrapolation of the airborne albedo measurements to the ground is performed. The nonlinearity is due to the vertically inhomogeneous distribution of the particle microphysical properties. This fact underlines the importance of aerosol profile measurements for the proper correction of atmospheric masking. Examples of the extrapolated areal spectral surface albedos are discussed in terms of their solar zenith angle dependence, their small-scale, and general variability. Finally, typical areal spectral surface albedos for different sea and land surfaces, as derived from the three measurement campaigns, are supplied in parameterized form for use in radiative transfer applications.

2003

• M. Wendisch, B. Mayer, Vertical distribution of spectral solar irradiance in the cloudless sky: A case study, Geophys. Res. Lett. , 30, doi:10.1029/2002GL016529, 2003.
  First airborne measurements of spectral solar down- and upwelling irradiances with a high spectral resolution (2-3 nm) are presented. The data were gathered in cloudless, hazy conditions over sea and land using a new instrument (called albedometer), which is equipped with a unique sensor head leveling technique. The downwelling irradiances and their spectral slope increase with altitude, illustrating the impact of Rayleigh scattering and boundary layer aerosols. The upwelling spectra reveal typical reflection features of the underlying surface (e.g., vegetation step around 700 nm). The measurements are compared with radiative transfer model results based on simultaneous airborne aerosol observations. The downwelling irradiances mostly agree to +/-10% which is within the experimental and modeling uncertainties. The upwelling irradiances are highly sensitive to uncertainties in the spectral surface albedo. Reasonable agreement between measurements and simulations required us to determine the surface albedo from flights at low altitude, rather than adopting literature values.

2001

• M. Wendisch, D. Müller, D. Schell, J. Heintzenberg, An Airborne Spectral Albedometer with Active Horizontal Stabilization, J. Atmos. Ocean. Technol., 18, 1856-1866, 2001. doi.org/10.1175/1520-0426(2001)018<1856:AASAWA>2.0.CO;2
  An airborne albedometer including a low-cost, precise, and fast sensor head horizontal stabilization system was developed to measure spectral down- and upward irradiances between 400- and 1000-nm wavelength. It is installed on a small research aircraft (type Partenavia P68-B), but it can easily be mounted on other aircraft as well. The stabilization unit keeps the two radiation sensor heads (up- and downward looking) of the albedometer in a horizontal position during the flight with an accuracy of better than +/-0.2 degrees over a range of pitch and roll angles of +/-6 degrees. The system works properly for angular velocities up to 3 degrees s(-1) with a response time of the horizontal adjustment of 43 ms. Thus it can be applied even under turbulent atmospheric conditions. The limitations of the stabilization have been determined by laboratory and in-flight performance tests. As a result it is found that the new horizontal stabilization system ensures that misalignment-related uncertainties of the measured irradiances are less than 1% for solar zenith angles up to 70 degrees. This does not include uncertainties due to deviations from the ideal cosine response of the optical inlet system and measurement errors resulting from absolute calibration problems. An example of downward spectral irradiances measured under cloudless conditions above and within a distinct boundary layer with enhanced aerosol particle concentrations shows the potential of the new instrument for improved radiative budget measurements in the atmosphere.