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MS Defense - Ross Dixon

Ross successfully defended his masters thesis on July 29, 2009.

Forcing Mechanisms For Heavy Precipitation in the Extratropical Transition of Atlantic Hurricanes

Freshwater flooding is the number one inland killer associated with hurricanes that make landfall in the Mid-Atlantic region. Although great improvements in hurricane track forecasting have been made over the past decade, forecasting hurricane intensity change and rainfall has remained problematic. This challenge becomes even more difficult after the storm makes landfall. Over land, storms typically weaken; however, strong nonlinear interactions with mid-latitude systems or forcing from terrain can reintensify the storm or trigger extreme precipitation events. The goal of the work presented here is to better understand the physical processes that lead to heavy precipitation and storm reintensification during the extratropical transition of hurricanes in the Mid-Atlantic region. We use the North American Regional Reanalysis to analyze in detail two landfalling storms: Hurricane Gaston (2004) and Hurricane Ernesto (2006). Both storms presented forecast challenges and both resulted in heavy precipitation, although through different mechanisms. Gaston was shown to create its own baroclinic zone, which led to heavy rainfall and latent heat release which allowed the storm to briefly rejuvenate over land. Ernesto interacted strongly with an upper level trough and jet, which created a secondary circulation that fueled the storm with moisture from the Atlantic. A potential vorticity analysis shows evidence for a case of stolen identity and possible stratosphere-troposphere exchange (STE). Diabatic forcing in the mesoscale proved to be most important in the transition of Gaston, whereas synoptic scale interactions were crucial to the evolution of Ernesto, which also occluded very quickly. The various spatial scales and rapid transitions of both these storms provide insight into the forecasting challenges during these transition events.