Relationships Between Radar-Derived Thermodynamic Variables and Tornadogenesis

Abstract

A methodology for obtaining pressure perturbations and buoyancy in thunderstorms observed with Doppler radar is described and applied to two tornadic thunderstorms. Extracted thermodynamic information is combined with kinematic analyses to study observed severe storm processes. The intensification of tornado parental circulations (mesocyclones) during tornadogenesis is found to be associated with deepening pressure deficits in lower storm levels. Upward directed perturbation pressure forces in the vicinity of the mesocyclone are reduced and can be reversed as the low-level vorticity amplifies. The sudden formation of concentrated rear downdrafts, commonly observed in tornadic thunderstorms, apparently stems from the vertical pressure gradient reversal. Reduced upward pressure forces decrease the storm's ability to lift low-level negatively buoyant air at the base of the updraft. Further, the restructured pressure forces create a flux of air parcels into the mesocyclone from higher levels on the storms' rear. In the final stages, downdrafts fill the mesocyclone, and updrafts in neighboring regions weaken. Qualitative examination of retrieved buoyancy distributions suggests that horizontal vorticity created by buoyancy gradients in inflow regions is not essential for mesocyclone intensification or for tornadogenesis.