Time scale for cold-air pool breakup by turbulent erosion

Turbulent erosion has been proposed as a major mechanism for removing wintertime cold-air pools (CAPs) from basins and valleys. The time scales involved in this erosion process, which are of great interest for winter weather forecasting, have not been studied systematically in the past. In this short contribution, a semi-analytical model is developed to estimate the time required for the dissipation of cold air pools from above by downward micro-scale turbulent erosion for different wind speeds aloft, different static stabilities inside the cold pool, and for different basin cross sections. The calculations show that micro-scale turbulent erosion is a rather slow process and that the erosion rate decreases rapidly with time as static stability increases in the capping inversion at the top of the cold pool. The rate of erosion is determined mainly by wind speed above the CAP and the temperature inversion strength inside the CAP; it is less sensitive to the shape of the topography. Shallow CAPs of a few tens of meters in depth with a weak inversion may be removed in a matter of hours if winds aloft are sufŽ ciently strong to initiate and maintain turbulent mixing. It is unlikely, however, that deeper CAPs with a moderate to strong inversion can be destroyed by micro-scale turbulent erosion unless combined with other regional and synoptic-scale processes that produce larger-scale turbulent mixing.