Diurnal Boundary-Layer Development over Sloping Terrain

Abstract

A one-dimensional prognostic model of the atmospheric boundary layer coupled to a surface energy budget is described which utilizes a profile formulation (non-local) for exchange coefficients in the daytime convective boundary layer and an exchange coefficient scheme based on local Richardson number for the stable nocturnal boundary layer. This combined local and non-local model is used to simulate the full diurnal cycle of the boundary-layer behavior of days 33?34 of the Wangara experiment. Results of the simulation indicate that these simplified exchange coefficient schemes simulate the mean features of the boundary layer as well as the more complex and computationally expensive higher order closure models. This diurnal boundary-layer formulation is incorporated into a two-dimensional mesoscale model employing a terrain-following coordinate system to examine boundary-layer behavior over sloping terrain similar to that of the Great Plains of the United States. Particular emphasis is placed on the generation of mesoscale pressure gradients in response to the heated and cooled terrain and the development of low-level wind maxima. Model results and an analysis of thermal wind relationships in the transformed equations indicate the development of a mesoscale thermal wind component to the south during the day throughout the convective boundary layer. At night, the thermal wind component is reversed within the nocturnal boundary layer. The mesoscale thermal wind components in conjunction with the frictional stress profile produce low-level jets in the velocity field both day and night which exceed the synoptic geostrophic speeds.