ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-11295-2010 Spectrally-invariant behavior of zenith radiance around cloud edges simulated by radiative transfer Chiu J. C. 1 Marshak A. 2 Knyazikhin Y. 3 Wiscombe W. J. 2 4 Department of Meteorology, University of Reading, Reading, UK Climate and Radiation Branch, NASA/Goddard Space Flight Center, Greenbelt, MD, USA Department of Geography, Boston University, Boston, MA, USA Atmospheric Sciences Division, Brookhaven National Laboratory, New York, NY, USA 30 11 2010 10 22 11295 11303 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/10/11295/2010/acp-10-11295-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/11295/2010/acp-10-11295-2010.pdf

In a previous paper, we discovered a surprising spectrally-invariant relationship in shortwave spectrometer observations taken by the Atmospheric Radiation Measurement (ARM) program. The relationship suggests that the shortwave spectrum near cloud edges can be determined by a linear combination of zenith radiance spectra of the cloudy and clear regions. Here, using radiative transfer simulations, we study the sensitivity of this relationship to the properties of aerosols and clouds, to the underlying surface type, and to the finite field-of-view (FOV) of the spectrometer. Overall, the relationship is mostly sensitive to cloud properties and has little sensitivity to other factors. At visible wavelengths, the relationship primarily depends on cloud optical depth regardless of cloud phase function, thermodynamic phase and drop size. At water-absorbing wavelengths, the slope of the relationship depends primarily on cloud optical depth; the intercept, by contrast, depends primarily on cloud absorbing and scattering properties, suggesting a new retrieval method for cloud drop effective radius. These results suggest that the spectrally-invariant relationship can be used to infer cloud properties near cloud edges even with insufficient or no knowledge about spectral surface albedo and aerosol properties.

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