Effects of Cloud Shape on Scattered Solar Radiation

Abstract

Calculations are presented which compare the effects on directional reflectance and relative radiance of changes in microphysical structure and geometric shape for scattered solar radiation in terrestrial water clouds. The effect of changes in microphysical structure was relatively small and was 5.5% in directional reflectance for cloud optical depth of 60 and solar zenith of 23.0°. In contrast, effects of changes in geometric shape from a semi-infinite layer to a cube of optical depth 60 were 35%. Narrow turrets growing above a base cloud are shown to be darker than the base cloud with a reduction in relative radiance of 34.5% for a vertical sun. Directional reflectances for turret clouds are smaller than for clouds with flat tops due to the presence of additional edges. It appears that the interactions between the turret and the base cloud represent a small effect compared to the addition of more edges and surface area of the turret. Cumulus clouds in nature contain many more surface features which, although less sharp than in this model, undoubtedly contribute to a reduced directional reflectance.