Moisture Budgets in Major US Landfalling Hurricanes & Implications for Rainfall

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Stephanie Zick (University of Florida)
Corene Matyas (University of Florida)

There is a strong need to characterize the origins and pathways of moisture in tropical cyclones (TCs) to improve rainfall forecasts. Previous research has shown that moisture convergence dominates evaporation in the TC moisture budget. Large-scale moisture has also been identified as a controlling factor in TC size and outer spiral rainband activity. Thus, it is hypothesized that TC rainfall will be modulated by drier large-scale moisture regimes, particularly when these systems interact with continental air masses. The present study investigates the spatial distribution of TC moisture budget fields in the period around landfall. Using the North American Regional Reanalysis (NARR), moisture budgets are calculated for U.S. landfalling TCs during 1998-2012. All calculations are performed every 3 hours over a 96-hour period centered on landfall. To investigate the spatial distribution of each moisture budget term, spatial metrics are formulated to quantify symmetry, spatial cohesivness, and dispersiveness. In major hurricanes, the TC moisture budget begins to redistribute 12-24 hours prior to landfall. The moisture becomes more dispersed with respect to the TC center but often maintains a cohesive, although asymmetric, structure as the TC moves inland. These results suggest that midlatitude air masses may influence TC structure prior to landfall, when the inner core of the TC is still over water. It is hypothesized that interaction with drier continental air and higher vertical wind shear are the main contributors to this reorganization of convection. In terms of precipitation forecasting, regions of heavy rainfall are likely to be maintained by persistence of moisture convergence over large areas, and the regions of heavy rainfall are likely to occur farther removed from the TC center as the TC moves inland.

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