10B.5 VORTICITY-BASED DETECTION OF TROPICAL CYCLOGENESIS Michelle M. Hite 1 *, Mark A. Bourassa 1,2 , Philip Cunningham 2 , James J. O’Brien 1,2 , and Paul D. Reasor 2 1 Center for Ocean-Atmospheric Prediction Studies, Florida State University, Tallahassee, Florida 2 Department of Meteorology, Florida State University, Tallahassee, Florida 1. INTRODUCTION Tropical cyclogenesis (TCG), although an already well researched area, remains a highly debatable and unresolved topic. While considerable attention has been paid to tropical cyclone formation, little attention has focused on observational studies of the very early stages of TCG, otherwise referred to as the genesis stage. In the past, the early stages of TCG were unverifiable in surface observations, due to the paucity of meteorological data over the tropical oceans. The advent of wide swath scatterometers helped alleviate this issue by affording the scientific community with widespread observational surface data across the tropical basins. One such instrument is the SeaWinds scatterometer, aboard the QuikSCAT satellite, which infers surface wind speed and direction. Launched in 1999, this scatterometer has encouraged various studies regarding early identification of tropical disturbances (Liu et al. 2001; Katsaros et al. 2001; Sharp et al. 2002). These studies, though operational in intent, hypothesized the potential for SeaWinds data to be applied towards research applications (i.e., genesis stage research). The main goal of this study is to develop an objective technique that will detect the early stages of TCG in the Atlantic basin using SeaWinds data. Liu et al. (2001), Katsaros et al. (2001), and Sharp et al. (2002) demonstrated the ability to identify tropical disturbances, discrete weather systems of apparently organized convection that maintain their identity for 24 hours or more and are too weak to be classified as tropical cyclones (i.e., tropical depressions, tropical storms, or hurricanes), by the National Hurricane Center (NHC). Each technique utilized surface wind data obtained by the SeaWinds scatterometer. However, the criteria that defined their identification method differed. Sharp et al. (2002) employed vorticity in their detection condition, whereas Liu et al. (2001) and Katsaros et al. (2001) relied upon closed circulations apparent in the scatterometer data. Using a threshold of vorticity over a defined area, Sharp et al. (2002) identified numerous tropical disturbances and assessed whether or not they were likely to develop into tropical cyclones. Detection was based on surface structure, requiring sufficiently strong vorticity averaged over a large surface area. Unlike Sharp et al. (2002), Katsaros et al. (2001) and Liu et al. (2001) concentrated on disturbances that would develop into classified tropical cyclones. They examined surface wind patterns and looked for areas of closed circulation, successfully detecting tropical disturbances before designation as depressions. These studies illustrated the usefulness of SeaWinds data towards tropical disturbance detection, with the intent of improving operational activities. The early identification of surface circulations presented in these studies suggests an opportunity to detect the early stages of TCG, setting the basis for this paper. The detection technique described herein has the potential for applications in the scientific and operational communities. In operational applications, the forecasting community can implement the detection technique as an additional observational tool. In doing so, the technique can enhance the current observing system employed to identify and monitor tropical weather systems, thereby reducing the time forecasters spend examining the tropics for incipient systems. In research applications, identification of the early stages of TCG can enhance understanding in regions where little research has been conducted due to the lack of surface observations, prior inability to conclusively locate TC precursor disturbances, and consequently the lack of observation studies (Reasor et al. 2005) on the establishment of the initial surface vortex. This study focuses on the Atlantic basin, but the detection technique can be applied to other tropical regions, such as the Pacific basin, after adjusting the threshold values to account for regional differences in TCG mechanisms. The ability to detect the early stages of TCG provides an opportunity to classify tropical disturbances in the Atlantic basin based on the source of initial surface cyclonic vorticity. ____________________________________ Corresponding Author address: Mark A. Bourassa Center for Ocean-Atmospheric Prediction Studies, Florida State University, Tallahassee, FL 32306- 2840. Email: bourassa@coaps.fsu.edu Phone: (850) 644-6923