A LAGRANGIAN PERSPECTIVE OF MESOCYCLONES AND DOWNDRAFTS IN OBSERVED AND NUMERICALLY SIMULATED SUPERCELLS

On 29 May 2004, a high-precipitation supercell thunderstorm developed in western Oklahoma and produced tornadoes during almost every distinct mesocyclone cycle over a six-hour period. The storm was exceptional in its size, lightning activity, and the duration of the parent mesocyclone lifecycles. Fortunately, the TELEX field project was in position to collect one of the best storm-scale radar datasets for a tornadic supercell with respect to the length of record, temporal resolution, and spatial coverage. The primary goal of this study was to explore the storm-scale structure of the mesocyclones, downdrafts, and low-level boundaries as the storm passed near the city of Geary, OK. Due to a lack of available tools, the secondary goal of this study was to develop methods for elucidating Lagrangian flow behavior and highlighting the most influential flow characteristics. A trajectory mapping framework was explored and developed whereby three-dimensional trajectory behavior is mapped out in two-dimensional space, representing either a horizontal or vertical plane of reference. The framework proved adept at highlighting past or future behavior, such as prior horizontal location that reveals regions with common source regions or future attributes air parcels that eventually flow into the mesocyclone. An idealized numerical simulation was used to explore the methodology and to show the lack of sensitivity in the patterns to spatial and temporal data limitations associated with radar-based wind analyses. After applying the trajectory mapping framework to the radar analyses, it was found that the exceptionally large and deep mesocyclone was responsible for organizing the storm-scale downdrafts throughout its lifecycle. As the midlevel circulation grew stronger, easterly cyclonic flow opposed the environmental westerly momentum and setup a deep convergence zone associated with the rear-flank downdraft on the north side of the circulation. Near the surface, the outflow from the RFD surges was consistently demarked by secondary rear-flank gust fronts on the western and southern sides of the circulation. Throughout the lifecycle of the mesocyclone, there was a strong correlation between the vertical structure of the mesocyclone and the location of the occlusion downdraft. When the circulation strength decreased with height, air parcels descended near the axis of rotation. However, when the gradient was negligible or increasing with height, air parcels descended on the outside of the circulation and reinforced outflow from the RFD, eventually helping initiate the occlusion process. Finally, mesocyclone source regions were mapped out in time and space and suggested that air parcels over a shallow layer from the southern forward flank and inflow regions were reaching the mesocyclone during the mature stage of the circulation. Trajectory-based estimates of mesocyclone inflow depth and volume both increased with time as the circulation strengthened but then abruptly decreased as the inflow was cutoff by an eastward shift in the RFD. An idealized simulation was used to explore the robustness of the mesocyclone source regions and generally found similar behavior, supporting the radar-based analysis. Furthermore, it was found that the simulated mesocyclones first drew in air from the baroclinic zone in the forward flank region but eventually expanded into the inflow region as the circulations gained strength