TURBULENT EROSION OF COLD-AIR POOLS: NUMERICAL SIMULATIONS

High-resolution idealized numerical simulations are used to examine the turbulent removal of cold-air pools commonly observed in mountain valleys and basins. A control simulation with winds aloft increasing from 0.5 to 20 m s-1 over 20 h combined with typical cold-air pool stratification illustrates the interplay over time of: lowering of the top of the cold-air pool; spillover downstream of the valley from the upper reaches of the cold-air pool; wave-like undulations affecting the cold-air pool's depth and stratification across the valley; and smaller temporal and spatial scale Kelvin-Helmholtz waves within the uppermost layers of the cold-air pool. The heat budget within the cold-air pool demonstrates the nearly compensating effects of vertical and horizontal advection combined with turbulent heating of the upper portion of the cold-air pool and cooling in the layers immediately above the cold-air pool. Sensitivities of turbulent mixing in cold-air pools to stratification and upstream terrain are examined. Although the characteristics of the turbulent mixing differ as the stratification and topography are modified, a bulk parameter (the cold-air pool Froude number) characterizes the onset and amplification of turbulent mixing as well as when the cold-air pool is removed. When this Froude number exceeds 1, Kelvin-Helmholtz waves and turbulent heat fluxes commence. Turbulent heat flux and wave activity increase until Fr = 2, after which the cold-air pool breaks down and is removed from the valley. The rate of cold-air pool removal is proportional to its strength, i.e., a stronger cold-air pool is removed faster once turbulent erosion is underway.