Why do descending Shells around Cumulus Clouds exist?

In this study Large Eddy Simulations are used to gain more knowledge of the shell of subsiding air which occurs around cumulus clouds. Firstly, a detailed comparison between observational and numerical results is presented to better validate LES as a tool for studies of such microscale phenomena. It is found that cloud profiles of vertical velocity, humidity and temperature were in good agreement with observations, showing features similar to the observations, including the existence of a shell of descending air around the cloud, which was found consistently around the entire cloud. Secondly, the availability of the complete 3D dataset in LES was exploited to examine the role of lateral mixing in the exchange of cloud and environmental air. To do this, the origin of the subsiding shell, which is able to generate a significant negative mass flux, was examined by looking at the individual terms of the vertical momentum equation. It was found that buoyancy is the driving force for this shell, unlike the pressure gradient force. This proves that evaporative cooling at the cloud edge, generated by lateral mixing of in-cloud and environmental air is the responsible mechanism behind this process, which suggests a significant role for lateral mixing throughout the entire cloud layer. Finally, the role of the shell in the interaction between cloud and environment studied by means of a conceptual model resembling the two-layer model of Asai and Kashara (1967), but in which we have added an extra layer for the subsiding shell.