Date of Award

5-2012

Document Type

Honors Thesis

Degree Name

Bachelor of Science

Department

Atmospheric and Environmental Sciences

Abstract

Hurricane Gustav was one of two major hurricanes that occurred in the Atlantic basin during the 2008 hurricane season. The storm caused more than $6 billion U. S. dollars of damage in Haiti, Jamaica, Cuba, and along the coast of the Gulf of Mexico from 25 August 2008 to 5 September 2008. The greatest amount of damage and highest number of casualties produced by the storm can be attributed to the rapid intensification of the hurricane. Rapid intensification (RI) of a tropical cyclone is defined as an increase in maximum sustained winds by 15.4 m s⁻¹ (30 kts) in a continuous 24 h period. Between 1200 UTC 29 August and 2200 UTC 30 August, the minimum pressure of Gustav decreased by 48 hPa and the maximum sustained winds increased by 80 kts from a tropical storm to a category four hurricane on the Saffir-Simpson scale. This increase in intensity exceeds the definition of RI.

Traditionally, the parameters of ocean heat content, vertical wind shear, and atmospheric moisture content have been evaluated to assess changes in tropical cyclone intensity. Kaplan and DeMaria (2003) have shown that sea surface temperatures (SSTs) in excess of 28.4°C and low vertical wind shear values (approximately 5 m s⁻¹) are conducive for RI. A moist atmosphere (i.e., high values of relative humidity) also signals an atmospheric environment conducive to tropical cyclone intensification. The SST field from the National Oceanic and Atmospheric Administration Optimum Interpolation 1/4° Daily SST Analysis (NOAA-OISST) dataset was used to assess the ocean heat content in the vicinity of the storm during its lifespan. Data from the initialization of the Global Forecasting System (GFS) was used to evaluate 200-850 hPa vertical wind shear, 850-700 hPa layer averaged relative humidity, and 200 hPa divergence. Lightning flash data from the World Wide Lightning Location Network (WWLLN) is used to investigate the role of convection in the RI of the storm.

The SST and atmospheric relative humidity fields in the vicinity of Hurricane Gustav were conducive to the storm's intensification, but values of vertical wind shear in the vicinity of the storm were greater than 10 m s⁻¹, a magnitude typical of tropical cyclone weakening. A convective band formed 12 h prior to the onset of the storm's RI, which led to the broadening of a 200-hPa divergent area in the vicinity of the storm. It is hypothesized that the broadening of this area allowed for a positive feedback loop that allowed for the RI of the storm.

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