The Influence of Moisture, Vertical Wind Shear and Storm Motion on the Rainfall Distribution Pattern of Tropical Cyclones in the Southern Gulf Coastal States

Tropical cyclones (TCs) cause a lot of damage to society when they make landfall due to heavy rainfall and flash flooding. Many scientists have studied TC rainfall patterns to improve the forecasting of TC rainfall in order to reduce the impacts of flooding. However, there is still a need to improve the prediction of the spatial distribution of rainfall so that forecasts can be more accurate. A large supply of moisture is required to sustain TC rainfall production. When the relatively dry air that exists over the land surface intrudes into a TC, less rainfall should be produced on that side of the storm due to increased stability. Previous research has also found that of the vectors of vertical wind shear and storm motion work together to determine TC rainfall patterns.

In this research, total precipitable water (TPW) surrounding TCs is analyzed along with vertical wind shear and storm motion to investigate their influence on the symmetry of TC rainfall at the time closest to landfall. The 43 storms that made landfall over the Gulf states in the U.S. from 1998 to 2015 are analyzed. A Geographic Information System is used to examine the TC rainfall patterns to determine if they are symmetrically distributed on each side of the storm. The amounts of TPW on the left side and the right side of the storm are compared for 24 hours prior to landfall to determine if they are significantly different according to the results of Mann-Whitney U Tests. In addition, the shortest distance between a contour line representing 40 mm of TPW and the storm center was calculated to determine the proximity of the non-tropical air mass. Finally, the vector of the vertical wind shear was analyzed in conjunction with the storm motion vector.

The results show that TC rainfall patterns vary regionally, and these variations are related to moisture conditions, vertical wind shear and storm motion. TCs that made landfall over the western Gulf states usually produced more rainfall on the right side of the storm, and the rainfall patterns are influenced by not only shear/motion but also the dry air mass from the continent. For TCs that made landfall over the central Gulf states, storms usually produced more rainfall on the right side of the track, and the vertical wind shear is the main factor influencing the rainfall distribution. Many storms among those that made landfall over the Florida peninsula produced more rainfall on the left side of storm due to enhanced convergence along a moisture boundary. In addition, the distance of < 350-360 km between a storm center and TPW values of 40 mm was a threshold for interaction between the moist air mass from the TCs and a frontal boundary behind which a more dry and cool air mass was found.