Date of Award

2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Atmospheric and Earth Science

Committee Chair

Kevin R. Knupp

Committee Member

Lawrence D. Carey

Committee Member

John R. Mecikalski

Committee Member

Timothy A. Coleman

Committee Member

Todd A. Murphy

Subject(s)

Severe storms--Alabama., Tornadoes--Alabama.

Abstract

The effects of terrain on tornadoes are poorly understood. Efforts to understand terrain effects on tornadoes have been limited in scope, typically examining a small number of cases with limited observations or idealized numerical simulations. One region where topography has been thought to potentially impact tornado frequency is along the Sand Mountain and Lookout Mountain plateaus in northeastern Alabama. It has been suspected that storms often intensify and produce tornadoes upon reaching these plateaus, particularly Sand Mountain. The role of these plateaus in modifying the near-storm environment and storm evolution became a focus of the VORTEX-SE field campaign beginning in Fall 2016. This dissertation investigates the statistics of tornadoes around Sand Mountain and Lookout Mountain and how the near-storm environment evolves atop the plateaus relative to surrounding areas. Results indicate that Sand Mountain and Lookout Mountain form a local statistical maximum in tornadogenesis reports from 1992-2016. Substantial variation can exist in the boundary layer environmental profiles between the plateaus and the Tennessee Valley. Significant horizontal variability in flow exists over Sand Mountain, particularly along the northwestern edge. Storm-relative helicity (SRH) can be enhanced atop the plateaus, particularly when the upstream mountain Froude number is supportive of parcels moving over the plateaus as opposed to being blocked. In addition, a downslope wind enhancement along the northwestern slope of Sand Mountain can develop, which can serve to further enhance SRH on the northwestern portion of Sand Mountain. Lifting condensation level (LCL) heights are nearly ubiquitously lower relative to ground level atop the plateaus than in the valley. This dissertation describes the evidence supporting these effects, as well as potential caveats to these findings. How these environmental variations may impact storm evolution are described, including an assessment of preliminary observations of supercell rear-flank downdraft buoyancy changes that may be related to the observed LCL height differences. Operational impacts of these results are discussed. The final product is a refined hypothesis of how the plateaus impact the near-storm environment of potentially tornadic storms in northeastern Alabama and suggested future work toward understanding the terrain-related near-storm environmental variations and storm-scale effects.

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