Further examination of the thermodynamic modification of the inflow layer of tropical cyclones by vertical wind shear

Abstract

Recent work has developed a new framework for the impact of vertical wind shear on the intensity evolution of tropical cyclones. 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, &theta;_e) air into the storm's inflow layer. These previous results have been based on idealised numerical experiments for which we have deliberately chosen a simple set of physical parameterisations. Before efforts are undertaken to test the proposed framework with real atmospheric data, we assess here the robustness of our previous results in a more realistic and representative experimental setup by surveying and diagnosing five additional numerical experiments. The modifications of the experimental setup 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. <br><br> In all experiments, the depression of the inflow layer &theta;_e values is significant and all tropical cyclones exhibit the same general structural changes when interacting with the imposed vertical wind shear. Tropical cyclones in which strong downdrafts occur more frequently exhibit a more pronounced depression of inflow layer &theta;_e <i>outside</i> of the eyewall in our experiments. The magnitude of the &theta;_e depression <i>underneath</i> the eyewall early after shear is imposed in our experiments 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 description of intensity modification in our suite of experiments.