Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 9 4 2009 10.5194/acp-9-1419-2009 http://www.atmos-chem-phys.net/9/1419/2009/ http://www.atmos-chem-phys.net/9/1419/2009/acp-9-1419-2009.html http://www.atmos-chem-phys.net/9/1419/2009/acp-9-1419-2009.pdf 1419 1430 2009-02-23 Physical interpretation of the spectral radiative signature in the transition zone between cloud-free and cloudy regions J. C. Chiu christine.chiu@nasa.gov A. Marshak Y. Knyazikhin P. Pilewski W. J. Wiscombe 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 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. 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