Multiscale Evolution of Southeast US Storms and their Environments

Abstract

The evolution of atmospheric features across multiple spatiotemporal scales combine to influence the ability for environments to support severe convection and produce damaging winds, hail, and tornadoes. Determining the nature of these influences can help improve our understanding and prediction of these severe convective hazards. Such advances are especially pertinent for the Southeast US whose storm characteristics – namely the prevalence of storms occurring in high-shear, low-CAPE environments (HSLC), for which predictability is inherently lowered, and socioeconomic vulnerabilities – compound existing forecast uncertainty. This dissertation sets out to reduce these uncertainties by examining how variability across three spatiotemporal scales – climate , synoptic, and storm-scale – contribute to the prevalence and underlying characteristics of Southeast severe convection. On the climate scale, several atmospheric and Gulf of Mexico SST patterns are shown to modulate Southeast storm environments in ways that favor the development of widespread tornado outbreaks. Furthermore, a subset of cool season outbreak patterns contribute to the development of HSLC conditions, suggesting that their associated CAPE deficits have some large-scale origin which may lend increased predictability to their associated tornadoes. On the synoptic scale, storm environments evolving across local sunset are shown to change differently depending on the amount of CAPE and shear present pre-sunset, contributing to subsequent changes in storm mode and tornadogenesis frequency. Lastly on the storm-scale, simulations of low-CAPE supercells occurring during this near-sunset period exhibit storm updrafts which increase in depth, width, and strength in the presence of a rapidly destabilizing background environment. These enhancements are primarily attributed to off-hodograph propagation, which act to increase storm-relative flow beyond what is predicted by the base-state hodograph evolution alone. The sum of these results serves to advance our physical model and prediction of Southeast US severe storms and tornadoes.