Interactions Between a Thermal Boundary and Deep Convection During the 27 April 2011 Superoutbreak

The 27 April 2011 superoutbreak can be separated into three distinct events: an extensive early morning quasi-linear convective system (QLCS), a shorter midday QLCS confined to northern Alabama, and numerous afternoon tornadic supercells. The midday QLCS, in particular, produced a significant cold pool and associated thermal boundary over northern AL and northeast Mississippi. This thermal boundary produced clouds, initiated deep convection, and provided a duct for gravity waves which also contributed to convective initiation (see poster by Todd Murphy). These clouds and showers persisted throughout the afternoon hours and reinforced the cool air, while a capping inversion in the air mass south of the boundary experienced warmer temperatures via solar heating that also reinforced the thermal boundary. By mid-afternoon this boundary started to propagate northward as indicated by NOAA, FAA, DOD, university, and general public network surface observation stations located over Alabama and Mississippi. The cool side of this boundary was characterized by a shallow near surface stable layer that extended only a few hundred meters AGL with an elevated unstable layer of 2500 J/kg. This boundary was well sampled by UAH and NWS/DoD facilities including WSR-88D radars at Hytop, AL and Columbus, MS; the Advanced Radar for Meteorological and Operational Research (ARMOR) C-band dual polarimetric radar, the Mobile Alabama X-band (MAX) dual polarimetric radar, the Mobile Integrated Profiling System (MIPS) (which includes a 915 MHz Doppler wind profiler and microwave profiling radiometer), Mobile Meteorological Measurement Vehicle (M3V), and a balloon sounding acquired at 2120 UTC.

Numerous storms formed on or near this thermal boundary including the Cullman EF-4, Rainesville EF-5, Jackson County EF-4, Smithville EF-5 and, Hackleburg EF-5. All of these storms were measured by one or more of the above radars. Using these radar data, time vs height measurements of maximum Z and rotational velocity can be related to each storm's distance from the boundary. Both the Cullman and Hackleburg storms crossed from the warm side of the boundary to the cool side of the boundary during their lifetimes. A significant change in the LCLs of these can be observed by surface measurements and photography. The EF-5 Hackleburg supercell appears to be the most significantly influenced by this boundary. This storm formed in eastern Mississippi and traversed north Alabama before dissipating in south central Tennessee. It produced a tornado that remained on the ground for approximately 212 km. Surface data indicate that this storm formed just south of the boundary. The storm's first damage report is colocated near the location the storm appeared to have crossed over to the cool side of the boundary. In addition, single and dual-doppler analyses from MAX, ARMOR, and KHTX are used to relate storm structure and strength with proximity to the boundary. Finally, VAD analyses from each radar demonstrates the additional backing of the wind at the surface on the cool side of the boundary. This backing of the surface wind led to an increase of SRH by as much as 400 m2/s2 at the surface on the cool side of the boundary.