Environmental Helicity and Its Effects on Development and Intensification of Tropical Cyclones

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. 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. 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. 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 the Interim ECMWF Re-Analysis (1979-2011) and Global Forecast System analyses (2004-11) 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 but statistically significant correlation between environmental helicity and TC intensity change with positive helicity being more favorable for intensification.