Simulation of a progressive derecho using composite initial conditions

Widespread convective windstorms (derechos) are an occasional occurrence east of the Rocky Mountains, particularly during the late Spring and Summer months. This study focuses on the progressive derecho, which is produced by a single, long-lived mesoscale convective system, often within a relatively benign synoptic-scale environment. Sounding data from twelve progressive derechos that occurred in weakly forced, large-scale environments are composited and objectively analyzed in an attempt to include important large-scale features in the pre-convective environment. This analysis, which agrees favorably with prior observational studies of progressive derecho environments, is used to initialize the Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model Version V (MM5). The model develops an explicity resolved, derecho-producing convective system that resembles observed derechos on many scales. Past, idealized numerical simulations suggest that the strength of long-lived squall lines is optimized from a local balance between the vorticity associated with the environmental shear and the vorticity created at the leading edge of the cold pool. In the present simulation, it is found that the structure and evolution of the system is not entirely consistent with the expectations of this local balance theory. In particular, nearly erect, convective updrafts persist for several hours despite a much "less-than-optimal" condition. In addition, the vorticity balance parameter remains nearly constant as the convective system, as a whole, weakens and tilts upshear. These results suggest that within typical environments of weakly forced progressive derechos, it is not necessary for the low-level ambient vorticity to match the cold pool vorticity to maintain the severity of the convection over long time periods.