Much attention has been given to the impact of environmental wind shear in the 850 200 hPa layer on tropical cyclones (TCs). However, even with the same magnitude of shear, helicity in this layer can vary significantly. TC vertical tilt is often attributed to wind shear. Different values of helicity modulate this tilt and certain tilt configurations are more favorable for development or intensification than others, suggesting that mean positive environmental helicity is more favorable for development and intensification than mean negative helicity. A new parameter is presented, the tropical cyclone-relative environmental helicity (TCREH). Positive TCREH leads to a tilted storm that enhances local storm scale helicity in regions of convection within the TC. This enhanced local scale helicity potentially allows for more robust and longer lasting convection which is more effective at generating latent heat and subsequent TC intensification. Idealized modeling simulations demonstrate the impact of environmental helicity on TC development and intensification. Results show that wind profiles with the same 850-200 hPa wind shear but different values of helicity lead to different rates of development. TCREH also is computed from Era-Interim reanalysis (1979 2011) and GFS analyses (2004 2011) to determine if a significant signal exists between TCREH and TC intensification. Mean annular helicity is averaged over various time periods and correlated with the TC intensity change during those periods. Results suggest a weak (R ~ 0.35) but statistically significant correlation between environmental helicity and TC intensity change with positive helicity being more favorable for intensification. The correlation between TCREH and intensity change is similar in magnitude to the correlation between 850 200 hPa wind shear and intensity change, particularly at time ranges longer than 72 h. This suggests that TCREH explains a similar amount of TC intensity variance when compared to shear.
Environmental Helicity and its Effects on Development and Intensification of Tropical Cyclones