Evolution of Raindrops in an Axisymmetric Cumulus Model

[摘要] A numerical simulation study on the evolution of raindrops in a maritime warm cumulus is presented by taking interaction of dynamical and micropohysical processes into consideration. The model is two-dimensional with axisymmetric feature of deep moist convection. Microphysical processes considered in the model are condensation of water vapour on condensation nuclei, diffusional growth of droplets, stochastic coalescence, evaporation, sedimentation, and drop breakup.In Part I, the effect of the way of treatment of water drops on the aspect of precipitation as well as on the overall behavior of a model precipitating convective cloud with detailed microphysical processes is investigated. Characteristic features of the parameterized microphysics model used Berry's and Kessler's parameterizations are also examined with emphasis on the comparison with the detailed microphysics model.The result shows that the way of treatment of water drops has a significant effect on the evolutional process of raindrops as well as on the dynamical aspect of the cloud. A smaller number of size-groups of water drops considerably distorts physically meaningful features of various aspects of clouds, leading to an exceedingly earlier onset of rainfall and a spuriously high rainfall efficiency through the life time. This is due to a numerical spreading of the droplet spectrum.The parameterized microphysics model in combination with the Marshall-Palmer distribution for raindrop sizes also fairly distorts the result and gives an exceedingly high rainfall efficiency as welll as a spuriously earlier onset of precipitation. The effect is caused by overestimation of mean terminal velocities of raindrops, particularly in the developing stage of the cloud.The present results are compared with those by other researchers.

[学科分类] 大气科学