ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-9-1419-2009 Physical interpretation of the spectral radiative signature in the transition zone between cloud-free and cloudy regions Chiu J. C. 1 Marshak A. 2 Knyazikhin Y. 3 Pilewski P. 4 Wiscombe W. J. 2 5 University of Maryland Baltimore County, Baltimore, MD, USA NASA/Goddard Space Flight Center, Greenbelt, MD, USA Boston University, Boston, MA, USA University of Colorado at Boulder, Boulder, CO, USA Brookhaven National Laboratory, New York, NY, USA 23 02 2009 9 4 1419 1430 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/9/1419/2009/acp-9-1419-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/9/1419/2009/acp-9-1419-2009.pdf

One-second-resolution zenith radiance measurements from the Atmospheric Radiation Measurement program's new shortwave spectrometer (SWS) provide a unique opportunity to analyze the transition zone between cloudy and cloud-free air, which has considerable bearing on the aerosol indirect effect. In the transition zone, we find a remarkable linear relationship between the sum and difference of radiances at 870 and 1640 nm wavelengths. The intercept of the relationship is determined primarily by aerosol properties, and the slope by cloud properties. We then show that this linearity can be predicted from simple theoretical considerations and furthermore that it supports the hypothesis of inhomogeneous mixing, whereby optical depth increases as a cloud is approached but the effective drop size remains unchanged.

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