A Modeling Study and Scaling Analysis of Orographic Effects on Boundary Layer Shallow Convection

Abstract

Effects of orography on boundary layer shallow convection under various background winds are studied using a two-dimensional model together with scaling analysis. Under a motionless background state the flow response over a heated hill is a vortex pair, with one horizontal vortex over each slope. Additional lifting of the inversion by this vortex pair is estimated by wt/N, with wt being the intensity of the terrain-induced thermal circulation, and N the Brunt–Väisälä frequency, for which a simple expression is given based on heat engine framework. Modification of the CBL top by terrain-induced waves is quantified using linear gravity wave theory in a two-layer atmosphere configuration. These simple estimates of the CBL-top perturbation are found to be consistent with the model results when waves and convection dominate the flow pattern. A convective core is found to occur over high hills when the winds are light, developing in the late afternoon when the CBL depth is high. A scaling is proposed to determine the existence of the convective core in terms of hill slope, background winds, boundary layer depth, and tropospheric stability. The downstream modification of convection by terrain-related processes is also examined. Under light wind conditions, the reversed thermal circulation in the lee and corresponding downward motions may suppress thermal eddies in the vicinity of the lee of a hill.