Examining Planetary, Synoptic and Mesoscale Features that Enhance Precipitation Associated with Landfalling Tropical Cyclones in North Carolina.

by Croke, Meredith Suzanne

Landfalling tropical cyclones (TCs) over North Carolina often produce excessive rain well inland from the location of landfall. A three-part study consisting of a precipitation analysis, climatological analysis and numerical model simulations was performed to determine the dominant mechanisms that influence precipitation associated with TCs that move over North Carolina. The goal was to determine the intrinsic or environmental features that enhance precipitation associated with the TCs influencing North Carolina and create a conceptual model to enable forecasters to better assess the likelihood of enhanced precipitation during TCs. In the precipitation analysis of 28 TCs that made landfall or tracked along North Carolina?s immediate coastline from 1953-2003, the spread of precipitation and track of each storm across North Carolina was considered. The potential correlation between several intrinsic features (i.e. maximum storm intensity, landfall intensity and translation speed) and the 3-day storm average precipitation from 52 rain gauge stations across North Carolina were examined. The results indicated no statistically significant correlation between precipitation and any of the above features. Due to this lack of correlation, the preexisting synoptic/dynamic environment the TC was entering at landfall was examined to determine if the environmental features could be the dominant precipitation enhancing mechanism. In the climatological analysis the 28 TCs were divided into relatively heavy and relatively light rainfall groups so that composite analyses of several environmental features from 72 hours prior to landfall could be examined. The features included 250 hPa geopotential heights, 850-700 hPa and 500-250 hPa potential vorticity, 925-850 hPa moisture flux, 1000 hPa frontogenesis, temperatures, winds and mean sea level pressure. The results indicated that there are several significant planetary, synoptic and mesoscale climatological differences from 72 to 6 hours prior to landfall between the heavy and light rainfall groups. The numerical simulations served to test the agreement between the climatological analysis and two case studies. The two case studies used were Hurricane Floyd (1999) and Tropical Storm Arthur (1996) which represented heavy and light rain events, respectively. All numerical simulations were performed using the Non- Hydrostatic Mesoscale Atmospheric Simulation System (NHMASS) model. The model was run at four different scales to allow features from the synoptic to meso- ß scales to be examined. The results were consistent with the climatological study, showing that preexisting environmental features influence the mesoscale environment, leading to enhanced precipitation well inland from the location of landfall. A conceptual model from 72-hours prior to landfall is now available for forecasters to incorporate into the ir tropical forecasting routine.