Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 8 2 2008 10.5194/acpd-8-6379-2008 http://www.atmos-chem-phys-discuss.net/8/6379/2008/ http://www.atmos-chem-phys-discuss.net/8/6379/2008/acpd-8-6379-2008.html http://www.atmos-chem-phys-discuss.net/8/6379/2008/acpd-8-6379-2008.pdf 6379 6407 2008-03-28 How small is a small cloud? I. Koren Ilan.Koren@weizmann.ac.il L. Oreopoulos G. Feingold L. A. Remer O. Altaratz Department of Environmental Sciences Weizmann Institute, Rehovot 76100, Israel Joint Center for Earth Systems Technology, University of Maryland, Baltimore County, Baltimore, MD, USA Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, MD, USA NOAA Earth System Research Laboratory, Boulder, CO, USA The interplay between clouds and aerosols and their contribution to the radiation budget is one of the largest uncertainties of climate change. Most work to date has separated cloudy and cloud-free areas in order to evaluate the individual radiative forcing of aerosols, clouds, and aerosol effects on clouds. Here we examine the size distribution and the optical properties of small, sparse cumulus clouds and the associated optical properties of what is considered a cloud-free atmosphere within the cloud field. We show that any separation between clouds and cloud free atmosphere will incur errors in the calculated radiative forcing. The nature of small cumulus cloud size distributions suggests that at any resolution, a significant fraction of the clouds are missed, and their optical properties are relegated to the apparent cloud-free optical properties. At the same time, the cloudy portion incorporates significant contribution from non-cloudy pixels. 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