Tropical cyclones in vertical shear: dynamic, kinematic, and thermodynamic aspects of intensity modification

Recent work has developed a new framework for the impact of vertical wind shear on the intensity evolution of a mature tropical cyclone. A focus of this framework is on the frustration of the tropical cyclone's power machine by shear-induced, persistent downdrafts that flush relatively cool and dry (lower equivalent potential temperature, θe) air into the storm's inflow layer. The kinematics of the combined vortex – vertical-shear flow favors the formation of strong, persistent downdrafts.

These previous results have been based on idealized numerical experiments for which we have deliberately chosen a simple set of physical parameterizations. After summarizing our previous results, we here survey and diagnose five additional numerical experiments with some modifications of the experimental setup to assess the robustness of our previous results. The modifications comprise the values of the exchange coefficients of surface heat and momentum fluxes, the inclusion of experiments with ice microphysics, and the consideration of weaker, but still mature tropical cyclones.

In all of the experiments, the depression of the inflow layer θe values is significant and all simulated tropical cyclones exhibit the same general structural changes when interacting with the imposed vertical wind shear. Tropical cyclones with a higher downdraft activity exhibit a more pronounced depression of inflow layer θe outside of the eyewall. The magnitude of the θe depression underneath the eyewall soon after shear is imposed correlates well with the magnitude of the ensuing weakening of the respective tropical cyclone. Based on the evidence presented, it is concluded that the newly proposed framework is a robust and therefore useful description of intensity modification in our suite of experiments. A potential sensitivity of the downdraft formation to the radial structure of the tropical cyclone vortex will be discussed briefly.