The Simulation of a Convective Cloud in a 3-D Model With Explicit Microphysics. Part I: Model Description and Sensitivity Experiments

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  • 1 Cooperative Institute for Mesoscale Meteorological Studies at the University of Oklahoma and National Severe Storms Laboratory, Norman, Oklahoma
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Abstract

A three-dimensional nonhydrostatic anelastic numerical model of a convective cloud with an explicit description of microphysical processes has been developed. Two distribution functions are considered in the model—one for cloud condensation nuclei (19 categories from 0.0076 to 7.6 microns) and another for cloud droplets and raindrops (30 categories on a logarithmic scale from 4 to 3250 microns). The prognostic kinetic equations for these distribution functions enable the calculation of the aerosol and drop spectra starting from activation and culminating in rain formation. The warm rain microphysical processes studied include nucleation, condensation/evaporation, coalescence, breakup and sedimentation. Analysis of supersaturation evolution with time shows that it does not experience growth with the onset of coalescence and correlates well with values derived from quasi-steady assumptions. Sensitivity tests of the accuracy of the supersaturation calculations and the effect of the salt factor in the condensation calculations are also presented.

To demonstrate the model's potential the results of a numerical experiment showing the evolution of a multicellular cloud are described briefly. They reveal such interesting features as vortex formation through merger of updrafts from cells of different intensity, interaction of the cloud with a stable environment, and repeated reentrainment into the cloud of the same air masses that circulate along spiral trajectories near cloud boundaries.

Abstract

A three-dimensional nonhydrostatic anelastic numerical model of a convective cloud with an explicit description of microphysical processes has been developed. Two distribution functions are considered in the model—one for cloud condensation nuclei (19 categories from 0.0076 to 7.6 microns) and another for cloud droplets and raindrops (30 categories on a logarithmic scale from 4 to 3250 microns). The prognostic kinetic equations for these distribution functions enable the calculation of the aerosol and drop spectra starting from activation and culminating in rain formation. The warm rain microphysical processes studied include nucleation, condensation/evaporation, coalescence, breakup and sedimentation. Analysis of supersaturation evolution with time shows that it does not experience growth with the onset of coalescence and correlates well with values derived from quasi-steady assumptions. Sensitivity tests of the accuracy of the supersaturation calculations and the effect of the salt factor in the condensation calculations are also presented.

To demonstrate the model's potential the results of a numerical experiment showing the evolution of a multicellular cloud are described briefly. They reveal such interesting features as vortex formation through merger of updrafts from cells of different intensity, interaction of the cloud with a stable environment, and repeated reentrainment into the cloud of the same air masses that circulate along spiral trajectories near cloud boundaries.

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