Mass-Flux Budgets of Shallow Cumulus Clouds

Abstract

The vertical transport by shallow nonprecipitating cumulus clouds of conserved variables, such as the total specific humidity or the liquid water potential temperature, can be well modeled by the mass-flux approach, in which the cloud field is represented by a top-hat distribution of clouds and its environment. The mass-flux budget is computed by conditionally sampling the prognostic vertical velocity equation by means of a large eddy simulation of shallow cumulus clouds. The model initialization is based on observations made during the Barbados Oceanographic and Meteorological Experiment (BOMEX). Several different sampling criteria are applied. The presence of liquid water is used to select clouds, whereas additional criteria are applied to sample cloud updraft, downdraft, and core properties. A comparison between the budgets of the mass flux and the vertical velocity variance show that they appear to be qualitatively similar. The mass flux is driven by buoyancy in the lower part of the cloud layer, whereas turbulent transport is important in generating mass flux in the upper part of the cloud layer. Pressure and subgrid-scale effects typically act to dissipate mass flux. Entrainment and detrainment rates for the vertical velocity equation are presented. They are found to be smaller in comparison to the ones for conserved variables. It is explained that the top-hat structure for the virtual potential temperature is degraded by mixing at the cloud boundaries leading to a subsequent evaporative cooling of cloud droplets that supports the formation of negatively buoyant cloud parcels.