Some Effects of Surface Heating and Topography on the Regional Severe Storm Environment. Part II: Two-Dimensional Idealized Experiments

Abstract

A series of two-dimensional (2-D) numerical experiments has been conducted to examine the effects of differential surface heating on flow over a dry, 2000 km-wide plateau. Two effects, found by Benjamin and Carlson in three-dimensional simulations to be significant in the regional severe storm environment, also occur in these 2-D experiments. These effects are a diurnal variation in the intensity of the lee trough and the development of a low-level inversion downstream as the mixed layer, which developed over the hot plateau, is advected over potentially cooler air. When the plateau is strongly heated and surrounded by lowlands with no surface heating, the leeside pressure trough intensifies by an extra 1?3 mb. Subsequently, the low-level flow ahead of the lee trough also increases by several meters per second when surface heating is allowed. The diurnal modulation of this feature suggests that the low-level moist flow toward regions of potential convection in cases such as those modeled by Benjamin and Carlson will tend to be strongest in the late afternoon and early evening. It is shown that this effect is primarily due to the superposition of a plateau heat low upon the mountain wave circulation. To a lesser extent, differential vertical mixing of momentum between the deep mixed layer and surrounding regions also tends to enhance the lee trough. This differential mixing momentum mechanism is active in the presence of an isolated, deep mixed layer and moderately strong lower tropospheric flow even if there is no elevated terrain. The development of the elevated mixed layer inversion appears to depend more strongly on a horizontal gradient of soil moisture and surface heating than it does upon a gradient of terrain elevation. However, while such an inversion may be produced by differential advection and differential heating in the absence of terrain, it will be stronger and develop more rapidly in a shearing environment if the strongly heated region is also elevated.