SEMINAR SERIES and COLLOQUIA

A high-precipitation supercell thunderstorm developed along a dry-line in western Oklahoma on 29 May 2004 and propagated across the state producing several tornadoes along its path. The storm was observed for about two hours by two mobile C-band Doppler radars that were deployed along a ~40 km baseline oriented nearly parallel to the storm motion. Volumetric sector-scan data, collected every 3 minutes between 0011 – 0052 UTC from the mobile radars were used to retrieve the three-dimensional airflow through the storm prior to and following the second tornadic circulation that developed near Geary, OK.

Initially, the storm exhibited some multicellular characteristics, with an elongated updraft zone containing several distinct maxima. A midlevel mesocyclone was associated with the primary updraft core while a separate lower-level mesocyclone straddled the interface between the northern part of the updraft zone and a weak convective downdraft feeding the rear-flank gust-front. With time, the primary updraft intensified with peak values approaching the thermodynamic theoretical maximum (~ 90 m s-1) as the entire updraft zone rotated about the low-level circulation. Following the updraft pulse, a surge occurred in the rear-flank downdraft with downdraft speeds increasing from ~5 ms-1 to around 20 ms-1 one kilometer above the surface. Backward trajectories from the rear-flank downdraft indicate multiple source regions of the air in the downdraft column, consistent with an entraining plume model. Air in the lower part of the rear-flank downdraft originated in the environmental boundary layer along the forward flank of the storm. Hence, the downdraft near the surface would likely not have been very cold.

The downdraft surge further intensified the low-level convergence and helped orient the updraft zone such that more of the inflow had positive streamwise vorticity. The low-level circulation responded accordingly, with vorticity values nearly doubling and the scale of the circulation apparently decreasing. The changes in the low-level circulation correspond well to the time of the Geary tornado. The rotation of the updraft zone around the low-level circulation created a fortuitous juxtaposition with the mid-level mesocyclone associated with the primary updraft as apparently the low level and mid-level circulations merged to form a deep, strong column of vertical vorticity. With time, the hook echo wrapped around the vorticity column and the circulation became cut-off from the primary updraft.

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