Some effects of cloud-aerosol interaction on cloud microphysics structure and precipitation formation: numerical experiments with a spectral microphysics cloud ensemble model

A spectral microphysics Hebrew University Cloud Model (HUCM) is used to eva luate some effects of cloud-aerosol interaction on mixed-phase cloud microp hysics and aerosol particle size distribution in the region of the Eastern Mediterranean coastal circulation. In case of a high concentration of aeros ol particles (APs), the rate of warm rain formation is several times lower, a significant fraction of droplets ascends above the freezing level. These drops produce a large amount of comparably small graupel particles and ice crystals. The warm rain from these clouds is less intense as compared to c louds with low drop concentration. At the same rime, melted rain from cloud s with high droplet concentration is more intense than from low drop concen tration clouds. Melted rain can take place downwind at a distance of severa l tens of kilometers from the convective zone. It is shown that APs enterin g clouds above the cloud base influence the evolution of the drop size spec trum and the rate of rain formation. The chemical composition of APs influe nces the concentration of nucleated droplets and, therefore, changes accumu lated rain significantly (in our experiments these changes are of 25-30%). Clouds in a coastal circulation influence significantly the concentration a nd size distribution of APs, First, they decrease the concentration of larg est APs by nucleation scavenging. In our experiments, about 40% of APs were nucleated within clouds. The remaining APs are transported to middle level s by cloud updrafts and then enter the land at the levels of 3 to 7 km. In our experiments, the concentration of small APs increased several times at these levels. The cut off APs spectrum with an increased concentration of s mall APs remains downwind of the convective zone for several of tens and ev en hundreds of kilometers. The schemes of drop nucleation (based on the dep endence of nucleated drop concentration on supersaturation in a certain pow er) and autoconversion (based on the Kessler formula) are unsuitable for an adequate description of cloud-aerosol interaction. The Kessler formula pre dicts an incorrect tendency in the rate of raindrop formation while increas ing APs' concentration. Prediction errors concerning the rate of raindrop f ormation can easily result in a 10-fold increase. Ir indicates that the spe ctral (bin) microphysics scheme (or parameterizations based on the bin sche mes) can be used for an adequate description of cloud-aerosol interaction. (C) 1999 Published by Elsevier Science B.V. All rights reserved.