Entraining Eddies Associated with the Thermal Circulation in Cumulus Clouds

A 3D, high-resolution simulation of a cumulus congestus is used to illustrate the physical process by which dry air is entrained into a cumulus cloud by eddies near the ascending leading edge of the primary thermal. The eddies are smaller than the radius of the thermal and overturn on a time scale of a few minutes. Air originating near the thermal top is swept into the cloud along with air entrained at the rear of the thermal as the thermal ascends. This combined eddy-driven ascending cloud top air, and the rear-thermal entrained air, deplete the liquid water in the ascending core of the cloud.

Entrainment rates at the resolution of the cloud model (50 m) are calculated at the cloud edge according to the method of Dawe and Austin (2011) and confirm that once the eddies in the simulation are developed, those near the ascending cloud top associated with the primary thermal circulation dominate. Smaller entrainment rates result from laterally entraining eddies below the main thermal circulation, in the turbulent wake.

The total flux of air into the cloud integrated over the active portion of the cloud lifetime is evaluated to determine if the most important entrainment is that near the ascending thermal or that occurring laterally in its wake.

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