Turbulent Erosion of Persistent Cold-Air Pools: Numerical Simulations

Abstract

igh-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, wavelike 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 (Fr)] characterizes the onset and amplification of turbulent mixing and the time of cold-air pool removal. When Fr > 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; that is, a strong inversion generates larger heat fluxes once turbulent erosion is underway.