Some Effects of Surface Heating and Topography on the Regional Severe Storm Environment. Part I: Three-Dimensional Simulations

Abstract

Model experiments have isolated several effects of surface heating and topography which may act in concert to focus the potential for severe thunderstorms in certain areas downstream of dry elevated terrain. These effects are examined primarily in results from 12 h and 36 h three-dimensional model simulations of the 9?11 April 1979 (SESAME I) case, but are also found in simulations of the 9?10 May 1979 (SESAME IV) case. The experiments are performed using the Pennsylvania State University mesoscale model, which includes a sophisticated boundary-layer component with horizontally varying surface characteristics and cloud effects on the surface radiation budget. Comparisons of simulations with and without surface fluxes of heat and moisture confirm that differential surface heating, topography, and differential advection may combine to produce a stabilizing effect. In certain synoptic situations, air strongly heated over the arid Mexican plateau rides over moist potentially cooler air downstream, thereby forming a strong restraining inversion or ?lid? over Texas. This elevated mixed layer inversion prevents the occurrence of thunderstorms over a large area; instead, storms are focused in the region where a southeasterly low-level flow underruns the northern and western boundary of the inversion. Model trajectories also support the hypothesis that this lateral boundary of the lid, a baroclinic zone approximately between 800 and 500 mb, is formed by confluence between the Mexican airstream and a subsided polar airstream which has traversed the upper-level trough approaching from the western United States. Model experiments also reveal that the strength of the low-level flow toward the severe storm region may itself be enhanced by effects related to surface heating and topography. First, the lee trough which forms with flow aloft across the southern Rockies and Mexican plateau intensifies as a result of daytime surface heating. This effect, which is related to differential heating and differential mixing of momentum, is significant in that the low-level flow east of the lee trough from the Gulf of Mexico toward the severe storm region is also strengthened by several meters per second. Secondly, transverse circulations associated with jet streaks, which were crucial in inducing low-level flow in both the SESAME I and IV cases, were also found to intensify in the presence of surface heating.