Abstract/Summary

Recent studies have demonstrated the importance of moist dynamics on the intensification variability of tropical cyclones (TCs) in directional shear flows. Here, we propose that dry dynamics can account for many aspects of the structure change of TCs in moist simulations. The change of vortex tilt with height and time essentially determines the kinematic and thermodynamic structure of TCs experiencing directional shear flows, depending on how the environmental flow rotates with height; i.e., in a clockwise (CW) or counter-clockwise (CC) fashion. The vortex tilt precesses faster and is closer to the left-of-shear (with respect to the deep-layer shear), with smaller magnitude at equilibrium in CW hodographs than in CC hodographs. The low-level vortex tilt, and accordingly more low-level upward motions are ahead of the overall vortex tilt in CW hodographs, but are behind of the overall vortex tilt in CC hodographs. Such the configuration of vortex tilt in CW hodographs is potentially favorable for the continuous precession of convection into the up-shear region, but in CC hodographs is unfavorable. Most of the upward motions within a TC undergoing CW shear are concentrated in the down-shear-left region, whereas those in the CC shear are located in the down-shear-right region. Moreover, the upward (downward) motions are in-phase with positive (negative) local helicity in both CW and CC hodographs. Here we present an alternative mechanism that is associated with balanced dynamics in response to vortex tilt to explain the coincidence and also the distribution variability of vertical motions, as well as local helicity in directional shear flows. The balanced dynamics could explain the overlap of positive helicity and convection in both moist simulations and observations.