Earth and Atmospheric Sciences, Department of

 

Date of this Version

5-2013

Comments

A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Earth and Atmospheric Sciences, Under the Supervision of Professor Adam Houston. Lincoln, Nebraska May, 2013

Copyright (c) 2013 George Limpert

Abstract

Low-level mesovortices are associated with enhanced surface wind gusts and high-end wind damage in quasi-linear thunderstorms. Although damage associated with mesovortices can approach that of moderately strong tornadoes, skill in forecasting mesovortices is low. The overarching goal of this work is to understand mesovortices and how they develop, to improve the skill in forecasting them. This was done by developing a climatology of mesovortices, which required a tracking algorithm, and numerical simulations were conducted to answer questions that could not be answered through the observational data. A climatology of mesovortices was constructed from 44 events during 2009 and 2010 to examine mesovortex characteristics and origins. Approximately 60% of the mesovortices were cyclonic and were longer-lived and stronger at low levels than the anticyclonic mesovortices. A barotropic mechanism that redistributes ambient low-level vorticity was found to generate numerous cyclonic mesovortices in some events with strong vertical shear over lowest 500 m. The modeling component revealed that vorticity is generated in a region of strong horizontal shear between the ambient low-level winds and flow within the cold pool, and concentrated into mesovortices by releasing shearing instability. Upward tilting of baroclinic vorticity was found to generate mesovortices above the cold pool but did not produce significant vorticity near the surface. Low-level mesovortices developed when the updraft remained near the gust front instead of advecting rearward. Cold air is lifted by the updraft and moves rearward and descends, tilting down vorticity atop the cold pool and generating near-surface mesovortices. The initial downward tilting is large for moderate and strong cold pools where there is substantial horizontal vorticity. Stretching of vertical vorticity intensifies the mesovortices and produces the strongest vortices where negative buoyancy in the cold pool is not too strong. Presented in this paper is an algorithm for the tracking of mesovortices across multiple radar domains, a climatology of bow echo mesovortices, and a series of model simulations to evaluate the stability hypothesis.

Adviser: Adam Houston