Semester of Graduation
Master of Science (MS)
Geography and Anthropology
Upper-level (horizontal) divergence (ULD) is an important variable in tropical weather systems. As part of the circulation within a tropical cyclone (TC), it carries air in the upper troposphere away from the center of circulation (COC). To date, most research assumes the 200 hPa pressure level (approximately 12 km, varying with latitude and time of year) as the height for maximum ULD in a TC, possibly because weather observation at the 200 hPa level by radiosonde have remained mandatory for aviation purposes. The more recent availability of gridded, high-spatial-resolution, global “reanalysis” data at multiple levels, along with improvements in spatial interpolation techniques, has allowed for more precise and accurate determination of the heights at which peak ULD actually occurs, how that level varies temporally prior to and after tropical cyclogenesis (TCG), and how the spatial and temporal attributes of the three-dimensional zone of the maximum ULD field vary by storm. This research addresses these questions.
Prediction of TCs is improving rapidly as scientific understanding of the atmospheric and oceanic conditions that characterize TCG, along with tools available for measuring the associated variables, are becoming more advanced and widespread. Results using the 2005 Atlantic tropical cyclone season suggest peak mean ULD during TCG occurs predominantly at 175 hPa and is typically located in the northeastern quadrant of the storm, hundreds of kilometers from the COC. The mean conditions showed a steady increase in magnitude of ULD from TCG ‒12 hours to TCG +6 hours and levels off or reduces at TCG +12 hours. Mean ULD is less organized on most levels before TCG and becomes more organized and concentrated between 200 hPa and 150 hPa from TCG to TCG +12 hours.
Peak ULD during individual cyclogenesis occurrences varies widely in magnitude, temporally, and vertically. Therefore, each cyclone should be evaluated individually. In general, this research supports the notion that confinement of ULD analysis to a single pressure level could diminish research or model outcome as ULD location is variable in four-dimensional space. These results may be useful in weather and climate modeling as evolution of TC outflow can be better understood.
Ates, Sara A., "Evolution of the Maximum Upper Level Divergence Field in Gulf-Atlantic Tropical Cyclogenesis" (2018). LSU Master's Theses. 4370.