On the Role of Entrainment in the Fate of Cumulus Thermals

Mixing is one of the most important processes associated with atmospheric moist convection. It determines the two-way interaction between clouds and their environment, thus having a direct impact on the time evolution of convection. The fractional entrainment rate epsilon-the main parameter related to mixing-is often parameterized in global circulation models as a function of updraft properties, and at the same time has a strong influence on how convection evolves. Within the framework of cumulus thermal vortices in large-eddy simulations of convection, here we first investigate the validity of some of the most common parameterizations of epsilon, and then investigate how relevant epsilon is for the fate of these thermals. We find that 1/R, where R is a measure of the thermal's radius, best parameterizes epsilon, but it explains only about 20% of the total variance. On the other hand, we find that both epsilon and favorable initial conditions-including high initial saturated fraction of the thermals-are key factors that affect the thermals' ascent rate, mean buoyancy, and distance traveled. The lifetimes of thermals, however, seem not to be affected significantly by either epsilon or initial conditions, which supports the view of cumulus convection as a succession of many short-lived thermals. Finally, our results suggest that for the majority of in-cloud cumulus thermals the important role of environmental moisture in the deepening of convection results mainly from providing the initial moisture for the short-lived thermals as they initiate at different altitudes above cloud base, rather than favoring their buoyancy as they rise through it.