Physical interpretation of the spectral radiative signature in the transition zone between cloud-free and cloudy regions

Chiu, J. C., Marshak, A., Knyazikhin, Y., Pilewskie, P. and Wiscombe, W. J. (2009) Physical interpretation of the spectral radiative signature in the transition zone between cloud-free and cloudy regions. Atmospheric Chemistry and Physics, 9 (4). pp. 1419-1430. ISSN 1680-7324 doi: 10.5194/acp-9-1419-2009

Abstract/Summary

One-second-resolution zenith radiance measure- ments from the Atmospheric Radiation Measurement pro- gram’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 re- markable 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 prop- erties, and the slope by cloud properties. We then show that this linearity can be predicted from simple theoretical con- siderations 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 un- changed.