Daniel Betten-April 25

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 and was fortunately observed by the TELEX field project.  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.

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 that illuminate source regions for the downdrafts or mesocyclones.  An idealized numerical simulation was used to explore the methodology and to test the sensitivity of 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 mesocyclone was responsible for organizing the storm-scale downdrafts throughout its lifecycle.  As the midlevel circulation grew stronger, easterly cyclonic flow opposed environmental westerly momentum and setup a deep convergence zone associated with the rear-flank downdraft on the north side of the circulation.  Meanwhile, 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.

Finally, mesocyclone source regions were mapped out in time and space.  Lagrangian behavior 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.  Trajectories were also used to estimate the inflow depth of air flowing into the mesocyclone.  Another idealized simulation was used to explore the robustness of the mesocyclone Lagrangian behavior and generally found very similar results, supporting the radar-based analysis.  Furthermore, it was found that the simulated mesocyclones first drew in air positioned along the baroclinic zone in the forward flank region before the source region expanded into the inflow region