Flow Boundaries and Ventilation of Tropical Cylones in Idealized Scenarios with Varying Wind Shear

It is a long-standing idea that vertical shear of the environmental winds promotes the exchange of air masses between a tropical cyclone’s (TC) inner core and the environment, so-called ventilation. Ventilation affects TC intensity by i) diluting the high moist entropy (θ_e) of the inner-core convection with lower-θ_e air from the environment or by ii) promoting detrainment of high-θ_e air from the inner-core convection. Different ventilation pathways are documented in the literature but their relative importance and their preferred conditions for occurrence, in terms of storm and environment characteristics, are not yet well understood. In particular, recent work has demonstrated the importance of the vertical profile of vertical shear.

This contribution seeks to make progress in our conceptual understanding of different ventilation pathways. To this end, we combine the examination of flow boundaries with a trajectory analysis of the actual origin and fate of air masses in idealized scenarios. Flow boundaries are identified based on finite-time Lyaponov exponents in synthetic data of TC-like vortices. Using synthetic data allows us to sweep the parameter space spanned by shear magnitude, shear vertical profile, vortex intensity, and vortex radial wind profile. The results are largely consistent with expectations based on simpler, two-dimensional models: Environmental air is pushed closest to the storm center at those levels where the ratio of the shear-induced storm-relative flow and the strength of the TC’s swirling winds is largest. Results of a series of full-physics simulations of idealized TC-shear scenarios with varying vertical shear profiles are interpreted in the light of the flow boundaries. Trajectory analysis of the thermodynamic properties of inner-core convection reveals that the actual ventilation is modified by convective-scale motion that is not well represented by the synthetic data. While flow boundaries provide one useful conceptual building block, further processes, on which we speculate in our conclusions, play a role in determining the prevalence of different ventilation pathways.