The Effects of Scattering Angle and Cumulus Cloud Geometry on Satellite Retrievals of Cloud Droplet Effective Radius

The effect of scattering angle on Moderate Resolution Imaging Spectroradiometer (MODIS) retrievals of cloud drop effective radius is studied using ensembles of cumulus clouds with varying sun-satellite scattering geometries. The results are interpreted as shadowing and illumination effects. When 3-D clouds are viewed near the backscatter geometry, well-illuminated cloud surfaces are seen, and the retrievals based on plane-parallel geometry underestimate the effective radius. The reverse is true when the satellite is far from the backscatter position, and the shadowed portions of clouds are observed. The shadowing geometry produces a larger bias than the illuminated geometry. These differences between the shadowed and the illuminated ensembles decrease toward zero as the clouds become shallower. Removing the edge pixels based on 1-km-scale geometry partially reduces biases due to the 3-D effects and surface contamination. Recommendations are provided for reducing the 3-D cloud effects using current satellite retrieval algorithms