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

Abstract

A spectral microphysics Hebrew University Cloud Model (HUCM) is used to evaluate some effects of cloud–aerosol interaction on mixed-phase cloud microphysics and aerosol particle size distribution in the region of the Eastern Mediterranean coastal circulation. In case of a high concentration of aerosol 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 clouds with low drop concentration. At the same time, melted rain from clouds with high droplet concentration is more intense than from low drop concentration clouds. Melted rain can take place downwind at a distance of several tens of kilometers from the convective zone. It is shown that APs entering clouds above the cloud base influence the evolution of the drop size spectrum and the rate of rain formation. The chemical composition of APs influences the concentration of nucleated droplets and, therefore, changes accumulated rain significantly (in our experiments these changes are of 25–30%). Clouds in a coastal circulation influence significantly the concentration and size distribution of APs. First, they decrease the concentration of largest APs by nucleation scavenging. In our experiments, about 40% of APs were nucleated within clouds. The remaining APs are transported to middle levels 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 small APs remains downwind of the convective zone for several of tens and even hundreds of kilometers. The schemes of drop nucleation (based on the dependence of nucleated drop concentration on supersaturation in a certain power) and autoconversion (based on the Kessler formula) are unsuitable for an adequate description of cloud–aerosol interaction. The Kessler formula predicts an incorrect tendency in the rate of raindrop formation while increasing APs' concentration. Prediction errors concerning the rate of raindrop formation can easily result in a 10-fold increase. It indicates that the spectral (bin) microphysics scheme (or parameterizations based on the bin schemes) can be used for an adequate description of cloud–aerosol interaction.