Mesoscale Simulations of Dynamical Factors Discriminating Between a Tornado Outbreak and Non-Event Over the Southeast US

摘要

We found a self-maintaining, low-level instability originated in the lee of the Sierra Madre Mountains, propagated eastward along the Gulf Coast states and over the Carolina Piedmont at the time of the 1988 Raleigh tornado outbreak. This instability was characterized by a surface trough, low-level potential vorticity (PV) maximum, mid-level jet and a warm airmass located to its southeast. These features created an environment favorable to deep convection and the release of latent heat that helped maintain the low-level trough and PV as it propagated across the Gulf Coast. The STJ was crucial to this 3-day process. It transported stratospheric PV-rich air to the south then downward to the mid-levels over the Mexican plateau. Its associated ageostrophic circulation created upper-level divergence (mass removal from the air column) that helped to maintain deep convection and the surface trough. It phased with the PJ creating upper-level divergence and ascent over the Piedmont. We developed a predictive tornado index based on: upper-level divergence, airmass buoyancy and the low-level tilting effects associated with shear and thermal gradients. We found values greater then 15 corresponded to tornadic activity early in the period (over TX, AR, LA and MS) and over central NC. Additionally, the event case was compared to a synoptically similar case where severe weather was forecasted but did not develop over the Piedmont. In the non-event case, there was no STJ, mid-level jet, low-level PV or surface instability over the southeast US.