Evolving Synoptic-Scale Precipitation Patterns in U.S. Landfalling Tropical Cyclones

Enhanced mid-level tropospheric moisture is known to be among the necessary ingredients for tropical cyclone (TC) genesis. However, when studying TC-environment interaction in mature TCs, inertial stability impedes the intrusion of mid-level dry air and thus, an existing TC may be somewhat protected from dry air intrusion (in low vertical wind shear environments). Yet, the TC is still vulnerable to thermodynamic changes in the boundary layer, where a warm underlying sea surface, air-sea fluxes, and moist inflow are theorized to be crucial for sustaining the symmetric vortex. In fact, there is mounting evidence that large-scale low-level moisture significantly impacts TC size, structure, and (perhaps) intensity. Recent research on this topic has focused on the influence of environmental humidity on the structure of TCs over the open ocean or in idealized modeling environments. Relatively few studies have investigated the influence of large-scale moisture on the evolution of precipitation in historical TCs during landfall. In this study, we utilize a shape metric methodology to evaluate North American Regional Reanalysis synoptic-scale precipitation patterns in 2004-2012 U.S. landfalling TCs. For each of the n=36 landfalling storms, we construct a 3-hourly time series of compactness measures that were developed to encompass characteristic geometries of TCs moving into the mid-latitudes: asymmetry (A), fragmentation (F), and dispersiveness (D). We then apply a moving Mann-Whitney U test to determine significant (p < 0.05) precipitation restructuring time windows. The results indicate preferred geographic regions for evolving precipitation patterns, with increasing (decreasing) compactness in the southern and eastern (western) Gulf of Mexico. Thus, northward moving TCs that make landfall in the panhandle and western coast of Florida are likely to increase in symmetry and cohesiveness. In contrast, westward moving TCs are likely to display more asymmetric, fragmented and dispersed precipitation patterns during the approach to landfall. Finally, we calculate and contrast TC composite structure in two subsets: storms in which all three shape metrics indicate 1) increasing and 2) decreasing compactness. In the increasing compactness subset, we frequently observe a prominent principal rainband that is co-located with a equatorward corridor of enhanced moisture flux convergence. Additionally, a strong moisture gradient exists to the northwest of the TC center, often to the front of storm motion. In the decreasing compactness subset, spiral banding is reduced, the circulation is cut-off from large-scale moisture sources, and moisture flux convergence is confined to the inner core region. Based on these results, TCs can evolve rapidly in the approach to landfall. To aid with forecasting the evolving structure during this crucial time period, we develop a conceptual model of TC synoptic-scale precipitation evolution based on large-scale moisture availability. Furthermore, we recommend continued research into the relationship between low-level moisture, spiral banding, and symmetrization of the TC vortex.