Convective Meteorology (Mesoscale Dynamics)

Impacts of a Storm Merger on the 24 May 2011 El Reno, Oklahoma Tornadic Supercell

Dr. Robin Tanamachi
Research Scientist, OU-CIMMS

05 December 2014, 2:00 PM

National Weather Center, Room 5600
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

On 24 May 2011, as part of an Oklahoma severe weather outbreak, a tornadic supercell (the El Reno, Oklahoma storm) produced an EF-3 and EF-5 tornadoes in series. The transition (“handoff”) between the two tornadoes occurred as the El Reno storm merged with a weaker, ancillary storm. To examine the impacts of the merger on the dynamics of the storms, a series of three-dimensional cloud-scale analyses are created by assimilating 1-min volumetric observations from the National Weather Radar Testbed Phased Array Radar into a numerical cloud model using the Local Ensemble Transform Kalman Filter technique. We objectively identify the El Reno storm, its updrafts, and vortices in the analyzed fields, and examine the changes in these objects before, during, and after the merger.

It is found that the merger did not cause the handoff, which preceded the updraft merger by a few minutes. Instead, tornado handoff likely resulted from midlevel mesocyclone occlusion, in which the midlevel mesocyclone split, and a portion was shed rearward with respect to storm motion. During the merger process, which lasted approximately 10 minutes, the midlevel mesocylone and updraft structure in the El Reno storm became relatively disorganized. New updraft pulses formed above colliding outflow boundaries between the two storms, tilting an environmental vortex line and generating an additional midlevel vortex that later merged with the El Reno storm’s midlevel mesocyclone. Once the merger process was complete, the El Reno storm and its mesocyclone re-intensified rapidly, as access to buoyant inflow sector air was restored.

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