Dependence of Radiative Properties of Arctic Stratus Clouds on Cloud Microstructure

Observations of cloud microstructure during the Beaufort Sea Arctic Stratus Cloud Experiments of June 1980 showed that the drop size distributions typically are nonuniform changing from monomodal near the base to bimodal near the top of the cloud. The observed drop size distributions are used to compute the reflection and transmission of solar radiation by Arctic Stratus Clouds in the visible part of the spectrum. Solutions of the radiative transfer equation using three different vertically uniform drop size distributions closely resembling those observed near the bottom, middle and top of the cloud, respectively, resulted in significant changes in the radiative properties even though the column liquid water content is kept constant. This finding implies that the shortwave optical depth of Arctic Stratus Clouds cannot be related to the column liquid water content (inferred from longwave satellite radiometry) unless realistic height-varying drop size distributions are used. It also implies that in order to make reliable predictions concerning the clouds' effect on the surface heat balance, one needs not only the column li .quid water content but also height profiles of the drop size distributions.