Assessment of Tropical Cyclone Moisture Budgets and Thermodynamics in the North American Regional Reanalysis

Recent research emphasizes an important but fine distinction between tropical cyclone (TC) energetics and moisture potential, which is of particular importance when forecasting TC rainfall impacts. Many studies are utilizing reanalsysis datasets to examine the role of TCs in the climate system. However, because of the relatively low horizontal resolution in these datasets, it is unclear whether reanalysis models are able to capture the sub-gridscale convective processes that are crucial to TC energetics. The present study investigates moisture budgets and moist thermodynamics in U.S. landfalling TCs in recent (2005 and 2008) North Atlantic hurricane seasons using the North American Regional Reanalysis (NARR). Detailed moisture and thermodynamic budgets are calculated using output from the 32-km NARR dataset and are compared with results from previous studies of TCs in high-resolution numerical models and satellite data. To assess the importance of each moisture budget term, volume-averages are derived for radii of 100 km, 500 km, and 1000 km from the TC center every six hours around the period of landfall, when forecasting precipitation processes is most crucial to human populations In addition, to assess the performance of NARR parameterized physical schemes within a TC regime, the thermodynamic budget is investigated during and prior to TC landfall. The volume-averaged moisture budget quantities in the NARR dataset agree well with previous studies, with moisture convergence accounting for the bulk of precipitating TC moisture. Moisture convergence is also spatially and temporally correlated with NARR-derived precipitation as well as with satellite- and radar-derived precipitation datasets. Although evaporation contributes a small proportion of the total mass of TC rainfall, a statistically significant linear correlation does indicate that evaporation rates are important in predicting TC rainfall potential. Our spatial analysis suggests that advection of moisture from the large-scale toward the mesoscale vortex is an important process and we investigate whether this moisture is sourced from the large-scale environment through total precipitable water or is extracted from the ocean surface in dry-air subsiding regions outside the TC circulation. Overall, the NARR dataset is shown to perform well in its representation of TC convection and rainfall, as verified against NEXRAD level III reflectivity data, satellite-derived rainfall, and previous research. We find that the NARR dataset may be used to study the relationship between the thermodynamic environment and convective patterns in landfalling TCs.