Environmental Helicity and its Impact on Development and Intensification in 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. TC vertical tilt is often attributed to wind shear. How do different values of helicity modulate this tilt? Are certain tilt configurations more favorable for development or intensification than others? We hypothesize that mean positive environmental helicity is more favorable for development and intensification than mean negative helicity. Positive environmental helicity leads to a tilted storm that enhances local storm scale helicity in regions of convection within the TC. This enhanced storm scale helicity allows for more robust and longer lasting convection which may be more effective at generating vorticity. This vorticity can then be absorbed by the parent TC leading to intensification. Two modeling techniques are combined to analyze the impact of environmental helicity on TC development and intensification in the Weather Research and Forecasting (WRF) model. These techniques, Point Downscaling (PDS; Nolan, 2011) and analysis nudging (FDDA; Stauffer and Seaman 1990,1991), can be used to maintain vertical profiles of wind, temperature, and humidity in the environment surrounding the TCs that are nearly constant in time. Results show that wind profiles with the same 850-200 hPa wind shear but different helicities lead to different rates of development.

In addition to the WRF simulations, reanalysis data are analyzed to see if a significant signal exists between environmental helicity and TC intensification. Era-Interim reanalysis data from 1979 – 2011 and GFS analyses from 2004 – 2011 are used to calculate environmental helicity in annuli around TCs. 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 exists between environmental helicity and TC intensity change with positive helicity being more favorable for intensification. These results from reanalysis data are in agreement with the WRF simulations.