A Numerical Case Study of the Squall Line of 6 May 1975

Abstract

Results of moist and dry fine-mesh (?140 km) numerical simulations of the 6 May 1975 Omaha squall line are presented. The moist fine-mesh simulation reproduced several observable features of the squall system and was then used to study other unobservable features. Differential thermal advection was responsible for the creation of potential instability. Low-level horizontal thermal advection contributed to pressure falls which, in turn, enhanced the convergence into the warm tongue. These processes initiated a band of convection. Results of the dry simulation suggested that the location and orientation of the initial low-level convergence was determined by dry mechanisms, that the convective latent heat release increased the rate of cyclonic-scale occlusion and that the occlusion process was followed by the dissipation of the convection. In addition to the fine-mesh simulations, a meso-mesh (?35 km) simulation was conducted. Because of the increased resolution, this simulation was able to reproduce the narrowing of the convective band from 400 to 100 km. This narrowing and intensification generated large values of low-level convergence and cyclonic vorticity. These processes produced a band of intense upward vertical velocity which, along with the convective transports of heat and moisture, stabilized the static energy profiles.