High-Resolution Observations of a Tornado-Producing Quasi-Linear Convective System

There is an abundance of literature regarding the evolution of tornadic and nontornadic mesovortices associated with quasi-linear convective systems (QLCSs). Much of the present body of work involves Doppler radar observations and numerical simulations to estimate the properties of the gust fronts and cold pools of tornadic QLCSs. Though these studies have greatly expanded our understanding of QLCS mesovortices and associated tornadoes, they also present various limitations to such understanding. For instance, tornadic vortices occur well below the scale of most numerical output, and Doppler radar data contains limitations in near-surface data and vertical profiles, owing to known limitations such as curvature effects, beam elevation, and the time necessary to complete each successive plan-position indicator (PPI) elevation scan.

On 11 April 2013, a tornadic QLCS impacted the greater Huntsville, Alabama metropolitan area. In addition to complex internal processes inherent to QLCS tornado formation, the genesis and evolution of the tornadoes in Huntsville appeared to be greatly impacted by processes external to the parent QLCS, including wave interactions and impacts from Huntsville Mountain, a significant terrain feature with an elevation rise of approximately 235 meters from the surrounding terrain. The first tornadoes produced by this QLCS were in western Alabama and associated with an initial discrete supercell that merged with the QLCS and formed a broader mesoscale vortex (MV). The MV gradually weakened as it moved into the Huntsville area. As it weakened, a new mesovortex formed to the south and produced two EF1-rated tornadoes across Redstone Arsenal and the south side of the city of Huntsville. Huntsville Mountain served as a nearly perfect division between the two tornado tracks, with the end point of the first tornado and the genesis point of the second tornado having only an approximately 1-meter difference in elevation.

This highly dynamic QLCS evolution fortuitously took place across a domain saturated with a plethora of in-situ and ground-based remote sensing equipment. This equipment includes the Weather Surveillance Radar-88 Doppler (WSR-88D) located at Hytop, Alabama (KHTX), the Advanced Radar for Meteorological and Operational Research (ARMOR) located at Huntsville International Airport, the Mobile Alabama X-band radar (MAX), which is owned and operated by the University of Alabama in Huntsville (UAH) and was deployed to New Market, Alabama, the Mobile Integrated Profiling System (MIPS), located at UAH, UAH's Mobile Meteorological Measurement Vehicle (M3V), an S-band Doppler radar located on Redstone Arsenal (RSA radar), a series of ten observation towers on RSA that vary from 2 meters to 107 meters AGL and report 15-minute mean observations, a 2-meter tower on RSA that records one-minute resolution data (dcp02), the KHSV (Huntsville) and KDCU (Decatur) ASOS stations, and a 5-second resolution surface observation site located on the UAH campus. The locations of many of these platforms were also highly fortuitous, with KDCU located northwest of the path of the initial MV, KHSV located directly in the path of the initial MV, UAH/MIPS located between the decayed MV and the new mesovortex, and dcp_02 on RSA located only 1.15 km south-southeast of the tornadogenesis point of the first EF1 tornado produced by the new mesovortex. Additionally, the tornadogenesis point of the first tornado was only 1.5 km northwest of RSA radar, 12 km east of ARMOR, 40 km southwest of MAX, and 65 km southwest of KHTX, giving a good variability of ranges and orientations between the tornadogenesis point and the various radar platforms, including multiple-Doppler synthesis in several domains.

This presentation will provide a preliminary summary of the observations from the above platforms. We investigate the surface characteristics of the passage of the gust front at the different locations along the line relative to the decaying MV and the intensifying mesovortex. We utilize the MIPS platform, which includes a 915-MHz Doppler wind profiler, a vertically-pointed X-band radar (XPR), and a multi-channel profiling radiometer (MPR), in conjunction with the UAH surface observing station to profile a unique gust front and QLCS structure north of the intensifying tornadic mesovortex. Furthermore, we utilize single-Doppler and dual-Doppler techniques to document the evolution of the RSA/Huntsville tornadic mesovortex and the associated gust front, with emphases on tornadogenesis as well as the evolution of the mesovortex and the gust front as it interacts with Huntsville Mountain. We utilize these observations to formulate a preliminary conceptual model of the evolution of this QLCS and compare this model to past observational and numerical studies of QLCSs and QLCS tornadoes. Special emphasis is placed on comparing the evolution of the tornadic mesovortex and gust front as they cross Huntsville Mountain to past studies, both numerical and observational, of tornado and gust front interactions with terrain to compare the behavior of this system to previous observed and modeled phenomena.