Tropical Cyclone-Relative Environmental Helicity and the Pathways to Intensification in Shear

Tropical cyclone-relative environmental helicity (TCREH) is a measure of how the wind vector changes direction with height, and it has been shown to modulate the rate at which tropical cyclones (TCs) develop both in idealized simulations and in reanalysis data. The channels through which this modulation occurs remain less clear. This study aims to identify the mechanisms that lead to the observed variations in intensification rate. Results from idealized numerical simulations suggest that the difference in intensification rate between TCs embedded in positive versus negative TCREH primarily results from the position of convection and associated latent heat fluxes relative to the wind shear vector. When TCREH is positive, convection is more readily advected upshear and air parcels that experience larger fluxes are more frequently ingested into the TC core. Simulations with the addition of spatially constant background flow show that certain storm motions relative to the wind shear vector are more favorable for intensification. As is the case when TCREH is positive, certain background flows promote a more rapid advancement of convection into the upshear quadrants and a better overlap of surface latent heat flux and low-level convergence. Trajectories computed from high resolution simulations demonstrate the recovery of equivalent potential temperature downwind of convection, latent heat flux near the TC core, and parcel routes through updrafts in convection. Differences in trajectory characteristics between TCs embedded in positive versus negative TCREH are presented. Contoured frequency-by-altitude diagrams (CFADs) show that convection is distributed differently around TCs embedded in environments characterized by positive versus negative TCREH. They also show that the nature of the most intense convection differs only slightly between cases of positive and negative TCREH. The results of this study emphasize the fact that significant variability in TC intensification rate result from vertical variations in the environmental wind direction, even when the 850 – 200 hPa wind shear vector remains unchanged.