Investigation of a cell merger accompanying cyclic tornado production

Numerous right-flank cell mergers were noted in the 24 May 2011 El Reno, Oklahoma tornadic supercell. One particular merger coincided with the demise of the EF-3 Lookeba, Oklahoma tornado and the genesis of the long-tracked, EF-5 El Reno – Piedmont – Guthrie tornado. Our objective is to investigate the effects, if any, of the merger on the tornado-producing low-level mesocyclone. To simulate the merger and tornado handoff, we assimilate volumetric National Weather Radar Testbed Phased Array Radar (NWRT PAR) reflectivity and Doppler velocity data, which has 1-min temporal resolution, into a numerical cloud model. This study is novel in that NWRT PAR data with high temporal resolution are assimilated to retrieve vertical velocity, similarly frequent (1 min) surface thermodynamic observations are used for verification of the resulting analyses at low levels, and an object identification technique is used to delimit the mesocylone.

The merger event did not appear to impact significantly tornado production by the El Reno storm. In the analyses, outflow from the merging cell collided with the rear flank gust front of the El Reno storm, causing uplift above and vortex stretching along the resulting wind shift line. However, it does not appear that this enhanced vorticity fed directly into the low-level mesocyclone of the El Reno storm. The interval between the EF-3 and EF-5 tornadoes appears to have resulted mainly from commonly observed, internal, occlusion processes in the El Reno supercell.