Rain production in convective clouds as simulated in an axisymmetric model with detailed microphysics. Part II: Effects of varying drops and ice initiation

Abstract

This paper presents an evaluation of the relative importance of the warm versus cold processes in convective clouds and their relative contribution to the development of rain. For this purpose, an axisymmetrical 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 conditions are used, leading to the formation of convective clouds of medium depth, with relatively strong updrafts. For these specific conditions, the effects of the different microphysical processes on the production of rain are tested by varying the cloud condensation nuclei (CCN ) spectra and the spectra of the nucleated drops. The role of ice crystal concentrations and drop freezing is also reviewed. The simulations showed that 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 increasing CCN concentration (or with the "continentality" of the clouds). For the same dynamics and liquid water content maritime clouds produce more rain with higher intensities than continental clouds. Reducing the ice nuclei concentrations generally produces less rain, especially near the cloud center. In moderate continental clouds, changing the 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 change in the total amount of precipitation. Self-freezing of drops plays only a minor role in rain production because freezing due to interactions of supercooled drops with ice crystals takes precedent. In the simulated clouds snow is inefficiently produced, especially in maritime ones. The Bergeron-Findeisen mechanism plays only a minor role in the depletion of supercooled water during the developing and mature stages of the cloud because of the presence of very low ice crystal concentrations as compared to that of the drops. During the dissipation stage of the clouds, however, the Bergeron-Findeisen mechanism helps to accelerate the glaciation.