Intensification of an Asymmetric, Sheared Tropical Cyclone in a WRF Simulation

Although vertical wind shear of the environmental flow is well-known to be a negative influence on tropical cyclone (TC) intensity, cases of highly sheared, asymmetric tropical cyclones undergoing significant to rapid intensification have been documented and discussed in the literature. The physical understanding of these asymmetric intensification events remains incomplete. In this study, the asymmetric rapid intensification of Tropical Storm Gabrielle (2001) was simulated at 1-km horizontal resolution using the Weather Research and Forecasting (WRF) model. As the simulated tropical cyclone intensified, intense convective cells with associated cyclonic vorticity anomalies developed preferentially downshear. One particularly strong mesovortex developed initially downshear-right, revolved cyclonically around the TC center for several hours, and subsequently merged with the broader TC vortex. The contribution of this mesovortex to the intensification of the TC vortex will be quantified using tangential wind and potential temperature budgets. The formation and evolution of this mesovortex will be described by performing a vorticity budget analysis. The role of subsidence in the formation of the mesovortex will also be assessed. Throughout the simulation, the TC vortex was tilted in the downshear-left direction, with the tilt turning clockwise with height. The influence of the TC vortex tilt on the formation, axisymmetrization, and merging of localized cyclonic vorticity anomalies will be discussed.