Date of Award

2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Lin, Yuh-Lang

Abstract

Orographic effects on tornadic supercell development, propagation, and structure are investigated using the Cloud Model 1 with idealized bell-shaped mountains of various heights and geometries and a homogeneous fluid flow with a single sounding. In total, the variations of height and geometry yield 16 terrain configurations. It is found that blocking effects are dominative compared to the terrain-induced environmental heterogeneity downwind of the mountain. The isolated bell-shaped mountains tended to shift the track of the storm towards the left of storm motion; however, when the terrain was elongated the effect tended to be rightward. The terrain blocking effect also enhanced the supercells inflow. This allowed the central region of the storm to exhibit clouds with a greater density of hydrometeors than the NMTN control. Moreover, the enhanced inflow increased the areal extent of the supercells’ precipitation, which strengthened the cold pool and enhanced the storm’s updraft until becoming strong enough to undercut and weaken the storm considerably. Orographic blocking enhanced low-level vertical vorticity directly under the updraft when the storm approached the mountain. A modified NWS Tornado Detection Algorithm is used to investigate supercellular tornadogenesis; it is found that blocking effects are dominative and that elongating the terrain axis approximately parallel to the storm motion produces the strongest enhancement to tornadogenesis. Although the simulated cases with the highest mountains produced the most tornadic thunderstorms it is seen that increasing the terrain height alone is not sufficient to enhance tornadogenesis. Furthermore, an effective Froude number is developed to determine the amount of effective blocking that several terrain configurations exhibit in relation to both the mean winds and the storm relative winds, and although this effective Froude number does order the terrain geometries in an appropriate manner, it alone is not sufficient to concretely determine which orientation is most likely to enhance tornadogenesis in supercell thunderstorms.

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