Three-Dimensional Simulations of Cumulus Congestus Clouds on GATE Day 261

Abstract

Schlesinger's (1978) three-dimensional cumulus model is applied to showering congestus clouds on day 261 of GATE. Adjustments are made to the microphysical and turbulent parameterizations, the former to be consistent with coalescence growth of warm rain. A cylindrical initial perturbation, in approximate agreement with GATE observations, was run with a characteristic thermodynamic sounding and four different wind profiles. The four wind profiles were 1) the observed three-dimensional flow, 2) uniplanar winds, 3) unidirectional winds, and 4) zero synoptic flow. A fifth run was made with the observed three-dimensional wind and a slightly moistened destabilized sounding characteristic of a cumulonimbus environment. Model results are compared with each other and with observations to analyze the effects of varying shear and altered sounding. Relationships between shear, mesovortices and dynamic entrainment are examined, as well as the model clouds? impact on the environment as a function of shear. The simulations appear to resemble reality in many important aspects. Altostratus layers observed on day 261 are found to be a by-product of convection in three-dimensional shear. Rapid erosion of cloud base to 3.6 km is related to the ambient thermal structure, with wind shear and initial perturbation playing a secondary role. Some of the apparent conflict regarding lateral versus cloud-top entrainment is clarified, as well as some factors governing convective downdraft structure and intensity. Finally, recommendations are made for further observations and model improvements.