Evaluation of subsiding cloud shells in sheared environments and associated impacts on entrainment/detrainment

Shallow cumulus convection plays a critical role in the climate system, through radiative feedbacks and the redistribution of heat and moisture. All current shallow convection parameterization techniques (EDMF, buoyancy-sorting, bulk-plume, CLUBB, episodic mixing) include some assumption or closure regarding entrainment frequency and magnitude, but entrainment is oftentimes treated as a tuning exercise. The existence of a subsiding shell, shown in observations and large-eddy simulations (LES), is a significant deviation from the standard bulk-plume model that assumes distinct cloud and environmental properties. The shell is driven by evaporative cooling at the cloud edge, and the air laterally entrained into the updraft is more humid than what would be traditionally expected in the absence of a shell. Turbulence near the cloud edge is also altered by the presence of significant negative buoyancy and downward mass transport. Direct entrainment estimates are obtained from high-resolution (10 m) LES in conjunction with a 3-D cloud identification algorithm, which allows for the analysis of entrainment and detrainment processes at the cloud scale. The variability of shell properties is assessed through sensitivity experiments using various wind profiles. The behavior of entrainment and detrainment on upshear, downshear, and cross-shear transects of the clouds are discussed, along with physical mechanisms governing the geometry of subsiding shells and humidity halos.