On the Contributions of Incipient Vortex Circulation and Environmental Moisture to Tropical Cyclone Expansion

Forecasting the lateral extent of a tropical cyclone (TC) wind field is a critical element in determining the potential scope of risks to be expected prior to landfall. As infrastructure and populations within coastal communities continue to grow, a corresponding increase in vulnerability motivates a thorough understanding of physical mechanisms underlying the lateral expansion of TC wind fields to minimize potential casualties and economic losses. This study investigates the contributions of incipient vortex circulation and mid-level moisture to TC expansion within an idealized numerical modeling framework. We demonstrate that the incipient vortex circulation strength places the primary constraint on TC expansion, and in part establishes the expansion rate. Increasing the mid-level moisture further promotes expansion, thereby modulating the expansion rate; however, in the absence of additional factors contributing to expansion other than incipient vortex circulation, varying the mid-level moisture mostly acts to delay or expedite the intensification process. One of the more common findings related to TC expansion in the literature illustrates a proclivity of relatively small TCs to stay small and relatively large TCs to stay large. Findings reported herein suggest that an initially large vortex can expand more quickly than its relatively smaller counterpart; therefore, all other factors contributing to expansion held constant, the contrast in size between the two vortices will increase with time. As the incipient vortex circulation strength is decreased, a notable reduction of potential vorticity (PV) generation is found in association with a relative lack of rainbands. On the contrary, as the incipient vortex circulation strength is increased, PV generation associated with rainbands abounds, and is further accompanied by a concurrently expanding PV core and radius of maximum winds when the vortex is embedded in mean flow. We propose that TCs may possess a sort of "memory" with respect to their incipient circulation, such that in the absence of processes inhibiting expansion, the cumulative generation of PV over time permits an initially large vortex to expand more quickly than its relatively smaller counterpart. Thus, TC expansion rate in light of the incipient vortex circulation may be a fundamental property of TCs, which further motivates research exploring factors external to the TC that may inhibit or facilitate expansion.