Physical interpretation of the spectral radiative signature in the transition zone between cloud-free and cloudy regions J. C. Chiu1, A. Marshak2, Y. Knyazikhin3, P. Pilewski4, and W. J. Wiscombe2,5 1University of Maryland Baltimore County, Baltimore, MD, USA 2NASA/Goddard Space Flight Center, Greenbelt, MD, USA 3Boston University, Boston, MA, USA 4University of Colorado at Boulder, Boulder, CO, USA 5Brookhaven National Laboratory, New York, NY, USA
Abstract. 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
Citation: Chiu, J. C., Marshak, A., Knyazikhin, Y., Pilewski, P., and Wiscombe, W. J.: Physical interpretation of the spectral radiative signature in the transition zone between cloud-free and cloudy regions, Atmos. Chem. Phys., 9, 1419-1430, doi:10.5194/acp-9-1419-2009, 2009.