Date of Award

May 2024

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Atmospheric Sciences

First Advisor

Mark Askelson

Abstract

Thunderstorm downdrafts have been determined to be important factors in the near-surface dynamics of tornadogenesis. Tornadogenesis could be assisted by a downdraft and outflow that is cold enough to either tilt existing horizontal vortex lines or baroclinically generate vorticity at the surface. However, the downdraft should not be so cold that it would cut off the updraft of a thunderstorm with overly negatively buoyant air or by limiting the stretching of the updraft. The presence of a capping feature, or a layer of decreased lapse rate, can influence downdrafts by preventing transport of air from aloft, with associated properties, to the surface. The thermodynamic properties of these capping features as well as the presence of a downward directed perturbation pressure gradient force were investigated to determine their impacts on downdrafts and downward transport of associated properties. Using both a 1.5-Dimensional Downdraft Model and Cloud Model 1, simulations were performed to examine impacts of a wide range of cap properties. The depth, temperature gradient, and interaction between these two properties were found to have statistically significant impacts on downward transport of air. In addition, downdrafts are assisted by the perturbation pressure gradient force (across the cap), as it partially offsets positive buoyancy experienced by downdrafts inside stable layers.

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