Vertical distributions of cumulus cloud microphysical properties and their effects on shortwave radiative forcing

This study investigates vertical profiles of droplet effective radius in shallow convective clouds using large eddy simulations. Results show that the in-cloud droplet effective radius increases with height for polluted and intermediate aerosol conditions. Under a clean condition, the in-cloud effective radius has high variability at each level. The differences in droplet effective radius profiles between clean and polluted aerosol conditions derived in this paper are distinct. Cloud top droplet effective radii of a population of clouds at different stages of development were used to construct a profile. On average, the constructed effective radius profile is in agreement with the in-cloud profile for the polluted and intermediate cases. This suggests that the cloud top droplet effective radius measured from satellites can be used to represent the in-cloud droplet effective radius at the same height for cumulus clouds that have negligible precipitation. However, for the clean case where drizzle develops, the constructed effective radius profile cannot be used to represent the in-cloud profile because they are both highly variable due to droplet collision-coalescence and sedimentation. Results indicate that droplet effective radius profiles of drizzling cumulus clouds are complicated, hence difficult to be described. Variability of droplet effective radius at each height is found to be associated with mixing for the polluted and intermediate cases. Droplet effective radius becomes smaller and its variability becomes larger in regions with stronger mixing.

Vertical profiles of droplet effective radius play an important role in atmospheric radiative transfer. This paper uses the atmospheric radiative transfer model SBDART to study the effects of vertical distributions of microphysical properties on radiative transfer, and to compare the shortwave radiative forcings from a vertically inhomogeneous cloud and a vertically homogeneous cloud. For a constant liquid water path, when droplet effective radius of the vertically homogeneous cloud is about 83% of the cloud-top effective radius in a vertically inhomogeneous cloud, the two clouds have similar radiatve forcing. Results in this study are consistent with previous studies for stratified clouds.