The role of preexisting thermal boundaries in the maintenance and rotation of deep convection in high-CAPE, low-shear environments

The occurrence of long-lived deep convection and violent (F4-F5) tornadoes in the high-CAPE (>6000 J kg^-1), low-shear (~6 m s^-1 0-6km shear) environment of the 27 May 1997 central Texas event challenges the established paradigm of convective longevity and low-level rotation in high-CAPE, low-shear environments. Observational analysis of this event revealed that storm longevity was the result of both discrete and quasi-continuous cell redevelopment/maintenance near the intersection of the storm-generated gust front and cold front segment located north of the dryline. Because the southernmost extent of the storm gust front was located well south of the coldest air in the outflow, as a result of a horizontal distortion of the gust front by differential low-level winds across the front, cells developing near the intersection of the gust front and front were permitted to grow toward maturity before ingesting this coldest/most-stable outflow air. Additionally, analysis of tornado locations relative to boundary positions corroborated previous observations that tornadoes formed along the storm gust front not the dryline or cold front.

In an attempt to identify (1) the possible mechanisms responsible for the regeneration of updrafts (and therefore storm longevity) along with (2) the possible sources of vertical vorticity in mesocyclones, cloud-scale simulations were performed with the Illinois Collaborative Model for Multiscale Atmospheric Simulations (ICOMMAS). Results indicate that long-lived deep convection is only supported when both the dryline and cold front are initialized. It appears that the enhanced forced ascent along the cold front segment along with the distortion of the storm-generated gust front are necessary for the rapid and systematic development of deep convection, however the distortion of the gust front alone is not sufficient for new cell production near the source of low-level vertical vorticity. Furthermore, consistent with observations, the simulated low-level rotation was found along the southern extent of the distorted gust front and was strongest when both the dryline and cold front were initialized. These results, along with results from Lagrangian vorticity budgets will be presented at the conference.