The Dependence of Tropical Cyclone Response to Moderate Vertical Wind Shear on the Initial Storm Intensity

Tropical cyclones (TCs) interacting with vertical wind shear (VWS) usually have small chances of intensification. VWS tilts the vortex and displaces convection away from the surface center of circulation—both of these effects diminish the conversion from available to kinetic energy necessary to spin up a TC circulation. Yet, many TCs are able to intensify and even rapidly intensify in spite of moving through moderately sheared environments. Hurricanes Florence and Michael are two examples of TCs that underwent rapid intensification while interacting with moderate-to-strong (> 5 m s^-1) VWS. Both TCs were weak tropical depressions at the onset of their rapid intensification, which motivates the question: does the outcome of TC-VWS interactions (i.e., weakening, intensification, or neither) depend on the initial intensity when a TC encounters a sheared environment?

We investigate this issue through idealized numerical experiments with time-varying VWS. The simulated TC is first spun up in an environment with 5 m s^-1 westerly shear. Using two different modeling frameworks that allow for time-varying vertical wind shear, the background shear is then increased from 5 to 12 m s^-1 during different stages: tropical storm stage, rapid intensification stage, and mature, nearly steady-state stage. The effects of VWS upon the TC vortex are compared between simulations to understand if and why weak TCs can overcome the effects of VWS while strong TCs cannot. Additional simulations are also performed to test the sensitivity of intensity and structural changes to the shape of the environmental wind profile.