RAIN PRODUCTION IN CONVECTIVE CLOUDS AS SIMULATED IN AN AXISYMMETRICAL MODEL WITH DETAILED MICROPHYSICS .2. EFFECTS OF VARYING DROPS AND ICE INITIATION

This paper presents an evaluation of the relative importance of the wa rm versus cold processes in convective clouds acid their relative cont ribution to the development of rain. For this purpose, an axisymmetric al model of a cold convective cloud with detailed microphysics is used . Five different types of clouds having characteristics from maritime to extreme continental are simulated. Identical initial renditions are used, leading to the formation of convective clouds of medium depth, with relatively strong updrafts. For these specific conditions, the ef fects of the different microphysical processes on the production of ra in are tested by varying the cloud condensation nuclei (CCN) spectra a nd the spectra of the nucleated drops. The role of ice crystal concent rations and drop freezing is also reviewed. The simulations showed tha t maritime clouds are efficient rain producers. In these clouds, large graupel mass contents develop by the freezing of large drops through their interaction with ice crystals. Rain efficiency decreases with in creasing CCN concentration (or with the ''continentality'' of the clou ds). For the same dynamics and liquid water content maritime clouds pr oduce more rain with higher intensities than continental clouds. Reduc ing the ice nuclei concentrations generally produces less rain, especi ally near the cloud center. In moderate continental clouds, changing t he concentration of ice crystals by a few orders of magnitude results in a change in the spatial distribution of the rain but only a small c hange in the total amount of precipitation. Self-freezing of drops pla ys only a minor role in rain production because freezing due to intera ctions of supercooled drops with ice crystals takes precedent. In the simulated clouds snow is inefficiently produced, especially in maritim e ones. The Bergeron-Findeisen mechanism plays only a minor role in th e depletion of supercooled water during the developing and mature stag es of the cloud because of the presence of very low ice crystal concen trations as compared to that of the drops. During the dissipation stag e of the clouds, however, the Begeron-Findeisen mechanism helps to acc elerate the glaciation.