Storm-Scale Downdraft Evolution in a High-Precipitation Supercell Thunderstorm

Numerous observational studies have shown that the strength and position of the rear-flank downdraft (RFD) in supercell thunderstorms can vary substantially during the mesocyclone life cycle. Moreover, rapidly evolving buoyancy fields have been observed at the surface underneath the RFD, especially around tornadoes, and are related to variations in RFD formation and composition. Numerical simulations and data assimilation experiments have shown that supercell and tornado longevity are highly sensitive to the low-level buoyancy field. Therefore, it is important to understand the storm-scale structure, behavior, and forcing mechanisms for the RFD in different types of supercell thunderstorms over a prolonged period of time.

On 29 May 2004, the observational systems deployed as part of the Thunderstorm Electrification and Lightning Experiment (TELEX), including two mobile C-band Shared Mobile Teaching and Research Radars (SMART-R’s), observed an extremely long-lived, high-precipitation tornadic supercell near Geary, OK. The radars captured multiple mesocyclogenesis cycles during a period of dual-Doppler radar coverage spanning 90 minutes, with storm-topping volumetric scans every 3 minutes. The temporal and vertical coverage of the storm, in addition to the below average cycling frequency (70 minutes), makes this an ideal case to compare the structure and kinematic behavior of the RFD over an entire mesocyclone life cycle.

Backward trajectories are used to elucidate specific downdraft flow regimes and bulk trajectory behavior over a deep layer during multiple RFD surges. The downdraft structure fundamentally changed after the mesocyclone transitioned from a two-cell vortex to a single-cell vortex preceding the onset of the occlusion stage. The RFD structure will be related to the structure and evolution of the mesocyclone and storm-scale rotation as well as the environment. Additionally, storm-scale, near-surface kinematic boundaries will also be shown in the context of the downdraft evolution. The results will be compared to the findings of previous studies as well as a few other storm-scale cases observed by the SMARTRs.