Numerical Simulation of Warm Rain Development in an Axisymmetric Cloud Model

Abstract

An axisymmetric warm cloud model including 36 classes of dropletsfrom 1 to 4096 m is developed. A reference spectrum is prescribedfor the formation of droplets around condensation nuclei. Other microphysicalprocesses incorporated in the model are condensation/evaporation,stochastic coalescence, sedimentation and drop breakup. Accuratecomputation for the condensation/evaporation process and for thestochastic coalescence equation can be achieved by the methods usedin this study. The differences in the life cycle and the precipitationprocess between maritime and continental cumuli with cloud tops around3 km are investigated in this model by taking into account only thedifferences in microphysical processes. The production of large dropsin the continental cloud is confined to the cloud top region anda distinct bimodal distribution in the drop spectrum is formed asthe large drops fall into the lower parts of the cloud. The failingrain has little effect on the cloud life cycle because the amountis very small. Cloud decay is mainly caused by mixing and evaporationin the cloud top and in levels just above the cloud base. In themaritime cloud, large drops are produced throughout the depth ofthe cloud. The drop spectrum also has a bimodal distribution andthis bimodal distribution is produced by the coalescence process.There is also a period of intensive rain. The evaporation of someof this rainwater in the subcloud layer is the main reason for productionof a downdraft, which in turn caused the decay of the cloud.A cloudmodel with parameterized microphysical processes is also used toexamine the adequacy of the parameterization scheme. Rain formedin the parameterized cloud earlier than in the maritime cloud, butits rainfall pattern and life span are close to those of the maritimecloud. On the other hand, the continental cloud and the parameterizedcloud both exhibit a second updraft pulsation. However, the overallstructure and important factors such as the cloud efficiency of theparameterized cloud are different from either the continental orthe maritime clouds.