Buoyancy Flux Modeling for Cloudy Boundary Layers

Abstract

The modeling of the buoyancy flux in a partly cloudy atmospheric boundary layer is complicated by its dependence on the correlation between clouds and vertical velocity elements, which can vary significantly with the underlying layer dynamics (e.g., stratocumulus versus shallow cumulus). The buoyancy flux is sometimes modeled in terms of higher-order statistics, such as vertical velocity skewness or saturation variance, to try to capture some of these dynamical effects. In this work an approximate expression for the buoyancy flux is formulated solely in terms of the liquid potential temperature and total water profiles and their respective flux profiles. The predictions compare favorably with the results of an extensive set of large-eddy simulations (LES), including simulations of stratocumulus, shallow cumulus, and transitional behavior in between. This formulation is combined with previous results on the relation between cloud-top entrainment rate and circulation structure to predict the behavior of quasi-steady cumulus-coupled boundary layers as a function of a basic set of physical input parameters. These predictions also compare favorably with LES results.