Lee mixing and nocturnal structure over gentle topography

Abstract

ur study analyzes measurements from a network of sonic anemometers over gentle topography to investigate the possibility of nocturnal lee-generated turbulence. Although the valley side slopes are less than 6° and extend only about 12 m above the floor of the valley, the turbulence can be enhanced in the lee of one of the slopes. Significant lee turbulence develops downwind from an abrupt slope transition between a mini-plateau and the modest valley side-slope. Lee-generated turbulence is not observed on the opposite slope where the slope magnitude gradually decreases with height. With intermediate 1-m wind speeds of roughly 2?4 m s?1, the cold pool is advected up the downwind slope. This downwind displacement of the cold pool and warming by lee-generated turbulence, when it occurs, leads to significant horizontal asymmetry of the temperature across the valley. For stronger winds, the cold pool is eliminated by mixing. For weak winds, a more traditional cold pool forms centered on the valley floor with limited or no lee-generated turbulence. While the impact of the gentle topography is modest compared to more dramatic terrain, less organized gentle topography covers a large fraction of the Earth?s surface. However, with gentle topography, various relationships show substantial scatter and the generalization of results from an individual network is probably not possible.