ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-9535-2010 Testing remote sensing on artificial observations: impact of drizzle and 3-D cloud structure on effective radius retrievals Zinner T. 1 4 Wind G. 2 Platnick S. 2 Ackerman A. S. 3 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: Ludwig-Maximilians-Universität, München, Germany 08 10 2010 10 19 9535 9549 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/10/9535/2010/acp-10-9535-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/9535/2010/acp-10-9535-2010.pdf

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> For three cloud cases 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 our, presumably typical, overcast stratocumulus scenes with an optical thickness of 8 to 9 and rain rates at cloud bottom up to 0.05 mm/h clear drizzle impact on the retrievals can be excluded. The cumulus scene does not show much drizzle sensitivity either despite extended drizzle areas being directly visible from above (locally >1 mm/h), which is mainly due to technical characteristics of the standard retrieval approach. 3-D effects, on the other hand, produce large discrepancies between the 1.6 and 2.1 μm channel observations compared to 3.7 μm retrievals in the latter case. A general sensitivity of MODIS particle size data to drizzle formation is not corroborated by our case studies.

 References Ackerman, A. S., Kirkpatrick, M. P., Stevens, D. E., and Toown, O. B.: The impact of humidity above stratiform clouds on indirect aerosol climate forcing, Nature, 432, 1014–1017, doi:10.1038/nature03174, 2004. Ackerman, A. S., Toon, O. B., Stevens, D. E., and Coakley, Jr., J. A.: Enhancement of cloud cover and suppression of nocturnal drizzle in stratocumulus polluted by haze. Geophys. Res. Lett., 30, 1381, doi:10.1029/2002GL016634, 2003. Ackerman, S. A., Strabala, K. I., Menzel, W. P., Frey, R. A., Moeller, C. C., and Gumley, L. E.: Discriminating clear-sky from clouds with MODIS. J. Geophys Res.-Atmos., 103(D24), 32141–32157, 1998. Cahalan, R. F., Ridgway, W., Wiscombe, W. J., Gollmer, S., and Harshvardhan:, Independent pixel and Monte Carlo estimates of stratocumulus albedo, J. Atmos. Sci., 51, 3776–3790, 1994. Chang, F.-L. and Li, Z.: Retrieving vertical profiles of water-cloud droplet effective radius: Algorithm modification and preliminary application. J. Geophys. Res., 108, D24, 4763, doi:10.1029/2003JD003906, 2003. Chen R., Wood, R., Li, Z., Ferraro, R., and Chang, F.-L.: Studying the vertical variation of cloud droplet effective radius using ship and space-borne remote sensing data, J. Geophys. Res., 113, D00A02, doi:10.1029/2007JD009596, 2008. Hansen, J. E. and Travis, T. D.: Light scattering in planetary atmosphere, Space Sci. Rev., 16, 527–610, 1974. Kato, S., Hinkelman, L. M., and Cheng, A.: Estimate of satellite-derived cloud optical thickness and effective radius errors and their effect on computed domain-averaged irradiances, J. Geophys. Res., 111, D17201, doi:10.1029/2005JD006668, 2006. King, M.: Determination of the Scaled Optical Thickness of Clouds from Reflected Solar Radiation Measurements. J. Atmos. Sci, 44.13, 1734–1751, 1987. Lebsock, M. D., Stephens, G. L., and Kummerow, C.: Multisensor satellite observations of aerosol effects on warm clouds, J. Geophys. Res., 113, D15205, doi:10.1029/2008JD009876, 2008. Leon, D. C., Wang, Z., and Liu, D., Climatology of drizzle in marine boundary layer clouds based on 1 year of data from CloudSat and CALIPSO, J. Geophys. Res., 113, DOOA14, doi:10.1029/2008JD009835, 2008. Loeb, N. G., Varnai, T., and Winker, D. M.: Influence of subpixelscale cloud-top structure of reflectances from overcast stratiform cloud layers, J. Atmos. Sci., 55, 2960–2973, 1998. Marshak, A., Platnick, S. E., Varnai, T., Wen, G., and Cahalan, R. F.: Impact of 3-D radiative effects on satellite retrievals of cloud droplet sizes, J. Geophys. Res., 111, DO9207, doi:10.1029/2005JD006686, 2006. Mayer, B.: Radiative transfer in the cloudy atmosphere, Eur. Phys. J. Conferences, 1, 75–99, 2009. Mayer, B., and Kylling, A.: Technical note: The libRadtran software package for radiative transfer calculations: description and examples of use, Atmos. Chem. Phys., 5, 1855–1877, doi:10.5194/acp-5-1855-2005, 2005. Menzel, W. P., Frey, R., Zhang, H., Wylie, D., Moeller, C., Holz, R., Maddux, B., Baum, B. A., Strabala, K., and Gumley, L.: MODIS global cloud-top pressure and amount estimation: algorithm description and results, J. Appl. Meteor. Clim., 47, 1175–1198, 2008. Minnis, P., Kratz, D. P., Coakley, J. A. Jr., King, M. D., Garber, D., Heck, P., Mayor, S., Young, D. F., and Arduini, R.: Cloud Optical Property Retrieval (Subsystem 4.3). "Clouds and the Earth's Radiant Energy System (CERES) Algorithm Theoretical Basis Document, Volume III: Cloud Analyses and Radiance Inversions (Subsystem 4)", edited by: CERES Science Team, NASA RP 1376 Vol. 3, 135–176, 1995. Minnis, P., Arduini, R. F., Young, D. F., Ayers, J. K., Albracht, B. A., Sharon, T., Stevens, B.: An examination of the impact of drizzle drops on satellite retrieved effective particle sizes. Proceedings of the 14th International Conference on Clouds and Precipitation, Bologna, Italy, July 2004. Nakajima, T. and King, K. D.: Determination of the Optical Thickness and Effective Particle Radius of Clouds from Reflected Solar Radiation Measurements. Part 1: Theory, J. Atmos. Sci., 47, 1878–1893, 1990. Platnick, S.: Vertical photon transport in cloud remote sensing problems. J. Geophys. Res., 105(D18), 22919–22935, doi:10.1029/2000JD900333, 2000. Platnick, S., Li, J. Y., King, M. D., Gerber, H., and Hobbs, P. V: A solar reflectance method for retrieving the optical thickness and droplet size of liquid water clouds over snow and ice surfaces, J. Geophys. Res., 106(D14), 15185–15199, 2001. Platnick, S., King, M. D., Ackerman, S. A., Menzel, W. P., Baum, B. A., Riedi, J. C., and Frey, R. A.: The MODIS cloud products: Algorithms and examples from Terra, IEEE Trans. Geosci. Remote Sens., 41(2), 459-473, 2003. Roebeling, R., Feijt, A., and Stammes, P.: Cloud property retrievals for climate monitoring: Implications of differences between Spinning Enhanced Visible and Infrared Imager (SEVIRI) on METEOSAT-8 and Advanced Very High Resolution Radiometer (AVHRR) on NOAA-17, J. Geophys. Res., 111, doi:10.1029/2005JD006 990, 2006. Stevens, B., Vali, G., Comstock, K., Wood, R., van Zanten, M. C., Austin, P. H., Bretherton, C. S., and Lenschow, d. H.: Pockets of open cells and drizzle in marine stratocumulus, Bull. Am. Meteor. Soc., 51–57, 2005. Vant-Hull, B., Marshak, A., Remer, L., and Li, Z.: The effects of scattering angle and cumulus cloud geometry on satellite retrievals of cloud drop effective radius, Geosci. Remote S., 45(4), 1039–1045, 2007. Varnai, T. and Marshak A.: Statistical analysis of the uncertainties in cloud optical depth retrievals caused by three-dimensional radiative effects, J. Atmos. Sci., 58, 1540–1548, 2001. Varnai, T. and Marshak, A.: View angle dependence of cloud optical thickness retrieved by MODIS. J. Geophys. Res., 112, D06203, doi:10.1029/2005JD006912, 2007. Wood, R., Comstock, K. K., Bretherton, C. S., Cornish, C., Tomlinson, J., Collins, D. R., and Fairall, C.: Open cellular structure in marine stratocumulus sheets, J. Geophys. Res., 113, D12207, doi:10.1029/2007JD009371, 2008. vanZanten, M. C., Stevens, B. B., Nuijens, L., Siebesma, A. P., Ackerman, A., Burnet, F., Cheng, A., Couvreux, F., Jiang, H., Khairoutdinov, M., Kogan, Y., Lewellen, D. C., Mechem, D., Nakamura, K., Noda, A., Shipway, B. J., Slawinska, J., Wang, S., and Wyszogrodzki, A.: Controls on precipitation and cloudiness in simulations of trade-wind cumulus as observed during RICO, submitted, J. Adv. Model. Earth Syst., 2010. Zinner, T. and Mayer, B.: Remote sensing of stratocumulus clouds: Uncertainties and biases due to inhomogeneity, J. Geophys. Res., 111, D14209, doi:10.1029/2005JD006955, 2006.