Integrated Observations of a Near-Surface Based Supercell Located Behind a Gust Front

During the afternoon hours of 11 April 2013, a complex supercell/quasi-linear convective system (QLCS) interaction occurred across western and northern Alabama. The supercell was responsible for a long-tracked (110 km), large (1200 m wide) tornado rated EF3 on the Enhanced Fujita Scale. After merging with the QLCS, the supercell maintained a quasi-discrete identity and produced four additional tornadoes of EF1 intensity. As the supercell and QLCS approached the Huntsville, Alabama area, the gust front associated with the QLCS propagated ahead of the supercell updraft. The supercell updraft moved directly above the University of Alabama in Huntsville's (UAH's) Advanced Radar for Operational and Meteorological Research (ARMOR) and Mobile Integrated Profiling System (MIPS). The MIPS platform utilizes a 915-MHz Doppler wind profiler, a vertically-pointing X-band profiling radar (XPR), a 12-channel microwave profiling radiometer (MPR), and a lidar ceilometer. Additionally, the updraft passed over several surface stations, with one station, collocated with MIPS at UAH, having 5-second resolution.

In this presentation, we review observations from the above platforms to show that the supercell updraft was still rooted near the surface, despite having been overtaken by the QLCS gust front and cold pool. These observations are compared to a similar analysis made on a different QLCS “control case” that did not contain complex supercell dynamic interactions. The surface observations at the UAH-MIPS location showed that a pressure drop occurred coincident with the temperature drop of 6.2 K associated with the gust front. Additionally, observations from the 915-MHz profiler and the XPR depict the updraft passage, with vertical particle velocity (W) values exceeding 11 m/s. Finally, a velocity-azimuth display (VAD) analysis of divergence from ARMOR within the cold pool shows convergence in the lowest 1 km within the supercell updraft. Convergence is not consistent with past observations of mesohighs associated with cold pools and suggests that the supercell updraft was still rooted near the surface. The VAD analysis of the “control case” QLCS shows divergence in the lowest 1 km of the cold pool in the absence of a supercell. The integration of several instruments has shown that, despite being undercut by a cold pool, a supercell updraft can still be rooted near the surface despite being undercut by an ambient cold pool.