Preliminary Observations of Changes in Supercell RFD Buoyancy across Significant Topography in Northeastern Alabama during VORTEX-SE

The buoyancy of a supercell rear-flank downdraft (RFD) relative to its environment has been shown in past studies to be a potentially critical factor in the potential for tornadogenesis with a supercell. The buoyancy deficit of the RFD is often related to the planetary boundary layer (PBL) water vapor content of the ambient environment through the potential for sub-cloud evaporation and melting of hydrometers in the RFD, with higher PBL humidity and lower lifted condensation level (LCL) heights leading to more favorable (less negatively buoyant) RFDs for tornadogenesis.

One of the primary goals of the Verification of the Origins of Rotation in Tornadoes Experiment – Southeast (VORTEX-SE) field campaign has been to assess the role of topography in the evolution of tornadic storms. Understanding how internal buoyancy characteristics may or may not change as storms traverse regions of significant topography is a primary research problem within this broader goal. It is hypothesized that a significant increase in elevation may lead to lower LCL heights relative to the ground level, leading to potentially less sub-cloud evaporational cooling and less negatively buoyant RFDs more favorable for tornadogenesis. Exploratory investigation of past surface observations preceding tornado events in this region have shown a propensity for lower cloud base heights over the adjacent Tennessee Valley than over the plateaus. Major uncertainties exist, however, in how meaningful these RFD buoyancy changes may be and the temporal and spatial scales necessary for relevant RFD buoyancy changes to evolve.

In working toward addressing the role of topography in the behavior of tornadic storms, several VORTEX-SE deployments were conducted in northeastern Alabama. This region contains numerous significant topographic features. The most prominent topographic features are a pair of plateaus that extend from the Cumberland Plateau system: Sand Mountain and Lookout Mountain. These plateaus are of particular research interest as they represent a qualitative maximum in tornado activity relative to surrounding areas. The combination of a relative apparent maximum in tornado activity and large (~125 km x 40 km) spatial scale of these plateaus, along with relatively open space for mobile radar siting across the Sand Mountain plateau, made the plateaus leading candidates for studying how tornadic storms and their near-storm environments may be augmented by topography. As part of these deployments, surface observations both atop the Sand Mountain plateau and within the adjacent Tennessee Valley have been gathered from the RFDs of two tornadic supercells, with one dataset gathered on 22 April 2017 during IOP 3.5b of VORTEX-SE year 2 and another dataset gathered on 19 March 2018 during IOP 3 of VORTEX-SE year 3.

This presentation compares the surface observational datasets between the valley and plateau observations during the passage of the RFDs of each supercell. Preliminary analysis of these datasets suggest that buoyancy deficits may meaningfully change within supercell RFDs as they move from the valley to the higher elevations atop the plateaus. While these datasets are extremely limited, representing only point observations in two cases, they serve as motivation for continued investigation of how topography may impact the internal thermodynamic processes of tornadic thunderstorms.