Numerical Simulation of Warm Rain Development in an Axisymmetric Cloud Model

Abstract

An axisymmetric warm cloud model including 36 classes of droplets from 1 to 4096 μm is developed. A reference spectrum is prescribed for the formation of droplets around condensation nuclei. Other microphysical processes incorporated in the model are condensation/evaporation, stochastic coalescence, sedimentation and drop breakup. Accurate computation for the condensation/evaporation process and for the stochastic coalescence equation can be achieved by the methods used in this study. The differences in the life cycle and the precipitation process between maritime and continental cumuli with cloud tops around 3 km are investigated in this model by taking into account only the differences in microphysical processes. The production of large drops in the continental cloud is confined to the cloud top region and a distinct bimodal distribution in the drop spectrum is formed as the large drops fall into the lower parts of the cloud. The failing rain has little effect on the cloud life cycle because the amount is very small. Cloud decay is mainly caused by mixing and evaporation in the cloud top and in levels just above the cloud base. In the maritime cloud, large drops are produced throughout the depth of the cloud. The drop spectrum also has a bimodal distribution and this bimodal distribution is produced by the coalescence process. There is also a period of intensive rain. The evaporation of some of this rainwater in the subcloud layer is the main reason for production of a downdraft, which in turn caused the decay of the cloud. A cloud model with parameterized microphysical processes is also used to examine the adequacy of the parameterization scheme. Rain formed in the parameterized cloud earlier than in the maritime cloud, but its rainfall pattern and life span are close to those of the maritime cloud. On the other hand, the continental cloud and the parameterized cloud both exhibit a second updraft pulsation. However, the overall structure and important factors such as the cloud efficiency of the parameterized cloud are different from either the continental or the maritime clouds.