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

Tropical cyclone-relative environmental helicity (TCREH) has been shown to modulate the rate at which tropical cyclones (TCs) develop in idealized simulations and in reanalysis data. The channels through which this modulation occurs remain less clear. This study aims to identify these channels and the mechanisms at work that lead to the observed variation in intensification rate. Results suggest that the difference in intensification rate between TCs embedded in positive versus negative TCREH primarily results from the positioning of convection and associated heat and moisture fluxes relative to the wind shear vector. When positive TCREH is present, convection is more readily advected upshear and air characterized by larger fluxes is more easily ingested into the TC core. Idealized simulations demonstrate that a sufficient magnitude of TCREH over a deep layer of the atmosphere is required for TC intensification rate to vary significantly when the 850 – 200 hPa shear vector remains constant. 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 slightly between cases of positive and negative TCREH. Trajectories are computed from high resolution simulations to show the paths that air parcels travel in TCs. These trajectories demonstrate properties such as equivalent potential temperature recovery rate downwind of convection, latent heat flux near the TC core, and parcel routes through updrafts and downdrafts in convection. Differences in trajectory properties between TCs embedded in positive versus negative TCREH are presented.