The Structure and Evolution of a Numerically Simulated High-Precipitation Supercell Thunderstorm

Abstract

The structure and evolution of a high-precipitation (HP) supercell thunderstorm is investigated using a three-dimensional, nonhydrostatic, cloud-scale numerical model (TASS). The model is initialized with a sounding taken from a mesoscale modeling study of the environment that produced the 28 November 1988 Raleigh tornadic thunderstorm. TASS produces a long-lived convective system that compares favorably with the observed Raleigh tornadic thunderstorm. The simulated storm evolves from a multicell-type storm to a multiple-updraft supercell storm. The storm complex resembles a hybrid multicell-supercell thunderstorm and is consistent with the conceptual model of cool season strong dynamic HP supercells that are characterized by shallow mesocyclones. The origin of rotation in this type of storm is often in the lowest levels. Interactions between various cells in the simulated convective system are responsible for the transition to a supercellular structure. An intense low-level updraft core forms on the southwest flank of the simulated storm and moves over a region that is rich in vertical vorticity. The stretching of this preexisting vertical vorticity in the storm?s lowest levels is the most important vertical vorticity production mechanism during the initial stages of the main updraft?s development. Interactions with an extensive cold pool created by the storm complex are also important in producing vertical vorticity as the main updraft grows. Overall, the development of vorticity associated with the main updraft appears similar to nonsupercellular tornadic storms. However, classic supercell signatures are seen early in the simulation associated with other updrafts (e.g., formation of vortex couplet due to tilting of ambient horizontal vorticity, storm splitting, etc.) and are deemed important. In the storm?s supercell stage, rotation is sustained in the lowest levels of the storm despite large amounts of precipitation located near and within the main mesocyclone. Pulsating downdrafts periodically invigorate the storm and the gust front never occludes, thus allowing the main updraft to persist for a prolonged period of time. The storm?s intensity is also maintained by frequent updraft mergers.