A Radar-based Analysis of the Mesoscale Convective Vortex that Crossed N AL on the Morning of 27 April 2011

Early in the morning of 27 April 2011, an expansive QLCS crossed into Alabama, and by 1030 UTC (5:30 am local time), a mesoscale convective vortex (MCV) became apparent over Walker County. The MCV remained coherent over the next few hours as it progressed through Marshall and Jackson Counties, and on through extreme northwest Georgia and southeast Tennessee. Over a dozen tornadoes occurred in association with this mesoscale circulation, including EF-2s that impacted the Hanceville area (Cullman County) and the Lake Guntersville State Park (Marshall County). Dual-Doppler sampling was accomplished by the Huntsville NWS 88-D (S-band) and UAHuntsville's Advanced Radar for Meteorological and Operational Research (ARMOR, C-band) radars along a 65 km NE-SW oriented baseline. A true dual-Doppler analysis is completed to retrieve the 3-d wind field and assess the presence of multiple circulation centers supported by initial radar inspection and early NWS damage survey.

Kinematic parameters derived from the dual-Doppler wind field show multiple centers of large magnitude vertical vorticity (up to 10^-2 s^-1) concentrated in the lowest levels of the atmosphere. While not of the same spatial scale as the tornadoes produced, it is noted that many of these vorticity cores line up well with tracks of the MCV-produced tornadoes over Marshall and Jackson Counties. Vertical motion magnitudes were relatively weak (< 10 m s^-1) and peaked below 6 km AGL, consistent with the shallow extent of the reflectivity field (generally below 8 km AGL) and lack of observed lightning. Dual-polarimetric observations from the ARMOR permit an investigation of hydrometeor distribution. Initial results suggest a centrifuge-like separation of targets.

In addition to discussing the radar-derived structures, we will attempt to evaluate the governing dynamics of the MCV feature and assess the possible impact of its passage over more heterogeneous terrain. Of particular interest is the question of why the MCV became such a prolific tornado producer. The entity had produced few tornadoes (an EF-3 in Walker County, and an EF-2 in Cullman County) from its development, but once it reached the Lake Guntersville area, it spawned a flurry of weaker tornadoes, some of which occurred simultaneously.