Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 10 1 2010 10.5194/acpd-10-1221-2010 http://www.atmos-chem-phys-discuss.net/10/1221/2010/ http://www.atmos-chem-phys-discuss.net/10/1221/2010/acpd-10-1221-2010.html http://www.atmos-chem-phys-discuss.net/10/1221/2010/acpd-10-1221-2010.pdf 1221 1259 2010-01-18 Testing remote sensing on artificial observations: impact of drizzle and 3-D cloud structure on effective radius retrievals T. Zinner tobias.zinner@lmu.de G. Wind S. Platnick A. S. Ackerman Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Germany NASA Goddard Space Flight Center, Greenbelt, USA NASA Goddard Institute for Space Studies, New York, USA now at: Meteorological Institute, Ludwig-Maximilians-Universität, München, Germany Remote sensing of cloud effective particle size with passive sensors like the Moderate Resolution Imaging Spectroradiometer (MODIS) is an important tool for cloud microphysical studies. As a measure of the radiatively relevant droplet size, effective radius can be retrieved with different combinations of visible through shortwave and midwave infrared channels. In practice, retrieved effective radii from these combinations can be quite different. This difference is perhaps indicative of different penetration depths and path lengths for the spectral reflectances used. In addition, operational liquid water cloud retrievals are based on the assumption of a relatively narrow distribution of droplet sizes; the role of larger precipitation particles in these distributions is neglected. Therefore, possible explanations for the discrepancy in some MODIS spectral size retrievals could include 3-D radiative transport effects, including sub-pixel cloud inhomogeneity, and/or the impact of drizzle formation. <br><br> The possible factors of influence are isolated and investigated in detail by the use of simulated cloud scenes and synthetic satellite data: marine boundary layer cloud scenes from large eddy simulations (LES) with detailed microphysics are combined with Monte Carlo radiative transfer calculations that explicitly account for the detailed droplet size distributions as well as 3-D radiative transfer to simulate MODIS observations. The operational MODIS optical thickness and effective radius retrieval algorithm is applied to these and the results are compared to the given LES microphysics. <br><br> We investigate two types of marine cloud situations each with and without drizzle from LES simulations: (1) a typical daytime stratocumulus deck at two times in the diurnal cycle and (2) one scene with scattered cumulus. Only small impact of drizzle formation on the retrieved domain average and on the differences between the three effective radius retrievals is noticed for both cloud scene types for different reasons. For the presumably typical overcast stratocumulus scenes, the optical thickness (8 to 9) is large enough to mask the drizzle rain rates at cloud bottom (up to 0.05 mm/h). 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