On the edge of a cloud

Cumulus clouds have since long been one of the greatest challenges in the atmospheric sciences. For a correct representation of clouds in weather and climate models, where they are the largest unknowns, a good understanding of interaction between cloud and environment is of prime importance. In this thesis, this problem is attacked with a combination of detailed numerical simulations and air-plane observations.

While the traditional view states that air inside the cloud goes up, and all the air outside goes down in compensation, it is found here that on average, the air far away from the cloud hardly moves. Most of the compensating downward motion happens in the direct vicinity of the cloud, in a subsiding shell. This shell is shown to be due to cloudy air that evaporates, cools, and therefore sinks. Interaction between cloud and environment only happens through the buffer layer created by the shell.
The shell is especially strong due to the horizontal mixing that is dominant in the cloud-environment interaction: processes that happen at the top of the cloud have little or no influence at lower levels. As a function of time, the cloud is not behaving like a continuous upward moving thermal, but consists of several bubble like pulses. Again, horizontal mixing is important in understanding the physics of these pulses.