Tropical Cyclone Tilt and Precession in Moderate Shear: Precession Hiatus in a Critical Shear Regime

Abstract One source of uncertainty associated with vertical wind shear (VWS) on tropical cyclone (TC) intensity evolution arises when the VWS becomes sufficiently strong such that the TC vortex is unable to overcome the inhibiting effects of VWS (the critical shear regime), resulting in a transition from vortex realignment and eventual reintensification to persistent vortex misalignment and failure of reintensification. To uncover the initiation mechanism of the behavioral transition, this study examines the dynamical evolution of the vortex tilt and precession through a set of CM1 ensemble simulations in moderate shear (7.5 m s−1) that includes the behavioral transition by systematically enhancing the TC vorticity amplitude aloft (vortex resiliency) at a restart point. In this critical shear regime, all experiments exhibit a common precession hiatus behavior, during which the tilt magnitude increases and later leads to divergent outcomes in intensity and tilt evolutions. Volume-averaged horizontal vorticity budget reveals an anomalous differential vorticity flux that emerges in the downtilt-left quadrant during the hiatus period. This differential vorticity flux generates horizontal vorticity that points toward the downtilt-right direction, simultaneously increasing the vortex tilt and slowing down the precession rate. This downtilt-left differential vorticity flux is due to midlevel vortex stretching at the rainband terminus region, where there is a transition from convective to stratiform precipitation. Meanwhile, the downdraft associated with stratiform precipitation also causes vorticity compression at the low levels. These results indicate that the stratiform rainband region is important for increasing the vortex tilt and pausing the precession.