The structure and climatology of boundary layer winds in the Southeast United States and its relationship to nocturnal tornado episodes

This talk will report on the results of a comprehensive study of cool season tornadoes in the southeast United States. Previous research has shown that there is a relatively high frequency of tornadoes in the overnight to early-morning hours during the cool season in the Southeast, particularly in areas close to the Gulf of Mexico. In fact, most strong and violent tornadoes (F2 or greater) in Florida occur during the cool season, associated with extratropical cyclones. The cause of this nighttime maximum during the winter months is not well understood, and one focus of this research will be on gaining a better understanding of the causes of this phenomenon.

Previous research in the development of severe weather scenarios along the Gulf coast has also documented the importance of return flow warm, moist tropical air inland from the Gulf, which may commence several days after the passage of a cold front into the Gulf. The warm Loop Current in the Gulf also can increase fluxes of heat and moisture into this return flow air, which can lead to rapid air mass destabilization. It has also been shown, however, that forecasting the trajectories of return flow air is difficult, and that operational numerical prediction models are not able to accurately forecast the modification of the boundary layer (partially due to lack of data over the Gulf), which can be important in determining the severe weather potential over the Southeast.

Composite results of this study suggest that nocturnal cool-season tornado episodes in the Southeast are associated with a stronger mid-level trough and upper-level jet than their daytime counterparts, as well as stronger flow and more moisture in the low-levels (i.e., 850 hPa and below). This talk will report on the results of current research, in which we use boundary layer wind/temperature data (including surface, radiosonde, and pilot ballon) to examine the climatological structure of the boundary layer in the Southeast, as well as the boundary layer structure prior to and during tornado episodes. The diurnal variation of the low-level winds near the Gulf coast will be examined to determine if there is evidence of more backed surface winds than further inland (particularly during the overnight hours), which would increase the low-level directional shear and contribute to a more favorable environment for supercell development near the immediate Gulf Coast. Additionally, use of pilot balloon and radiosonde data will be made to examine the low-level (0-3 km) wind shear profiles in the vicinity of nocturnal tornado episodes.