Simulation of the Downshear Reformation of a Tropical Cyclone

Previous studies have explored how tropical cyclones (TC) are able to vertically align in a sheared environment via precession or vortex Rossby wave damping. This study investigates an additional possible way that sheared TCs are able to reduce their tilt: downshear reformation. The downshear reformation of Tropical Storm Gabrielle (2001) was simulated at 1-km horizontal resolution using the Weather Research and Forecasting (WRF) model. Vortex-scale and convective-scale details of the simulated downshear reformation will be presented.

The shear tilted the initial parent vortex downshear-left and forced an azimuthal wavenumber-1 convective asymmetry. Within the downshear convection, vortex stretching played a key role in the spinup of an intense mesovortex. The circulation associated with this mesovortex and adjacent cyclonic vorticity anomalies comprised a developing upright “inner vortex” on the downshear-left (downtilt) periphery of the tilted parent cyclonic circulation. The reduction of TC vortex tilt from 65 km to 20 km within 3 hours reflected the emerging dominance of this upright inner vortex. We hypothesize that downshear reformation, resulting from diabatic heating associated with asymmetric convection, can aid the TC's resistance to shear by reducing vortex tilt and by enabling more diabatic heating to occur near the center, a region known to favor TC intensification.