Clouds and Climate
The Romps Group at the University of California, Berkeley
Cold pools of air, which are generated by the cooling effect from evaporating rain, flow along the surface of the Earth as a gravity current (i.e., a slumping of fluid caused by the fluid being denser than its surroundings). These cold pools play a vital role in initiating new storm clouds by kicking up warmer air at its edge. As the importance of this process has become more widely appreciated in the past several years, efforts have begun to parameterize cold pools in global climate models (GCMs).
To parameterize cold pools in a GCM, we must at least understand some very basic facts about the sizes and lifetimes of cold pools. In this paper, we derive analytical expressions for the evolution of cold pools that then tell us how large a cold pool can become and how long it can live. This theory reveals that cold pools reach a maximum radius of about 10-20 times their initial radius before ceasing to be cold (due to surface heat fluxes). The terminal times, however, are much more variable and depend on the cold pool's initial potential and kinetic energies. The predictions from this theory are validated against large-eddy simulations of cold pools, such as the one animated below.
Figure 5. Column-integrated mass of original air (kg/m2) in the cold pool as it evolves over four hours. At four hours, the animation is reversed to rewind to the beginning.