Cloud droplet/particle sizes obtained from emitted radiances

摘要

Abstract: Global scale satellite observations of cloud properties are a key to unraveling the physical processes that govern the role clouds play in the climate system. The retrieval of cloud particle size from satellite imagery data is of particular interest. Cloud emissivities and transmissivities depend substantially on the constituent single particle scattering and absorbing properties which in turn depend on particle size. Two clouds with the same ice water content but different particle sizes may have substantially different optical depths. This study presents a retrieval scheme for obtaining cloud droplet/particle size from satellite measurements at infrared wavelengths. The scheme employs Mie theory and the Eddington approximation and uses the advanced very high resolution radiometer (AVHRR) data at 3.7, 11, and 12 $mu@m. It also relies on the nonlinear relationships between 3.7, 11, and 12 $mu@m radiances observed for single- layered semitransparent cloud systems. Theory indicates that the nonlinear relationships disappear when the spherical particle radius reaches about 60 $mu@m. An automated retrieval procedure has been developed. Some retrieved results are presented. !2