Abstract
The aim of this dissertation is to attain a better understanding of how tropical cyclones (TCs) respond to variations in the three-dimensional environmental wind field. Much attention has been given to the impact of environmental wind shear in the 850 – 200 hPa layer on tropical cyclones. 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. Initially we proposed that this enhanced local scale helicity may allow for more robust and longer lasting convection which is more effective at generating latent heat and subsequent TC intensification. Further investigation shows that this is a secondary influence on TC intensity and that variations in the azimuthal and radial position of convection in the TC play a stronger role. Vertical tilt of the vortex 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 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 but statistically significant correlation between environmental helicity and TC intensity change with positive helicity being more favorable for intensification. Another goal of this dissertation is to identify the mechanisms that lead to the observed variations in intensification rate. Results 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. 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. Trajectory characteristics show that low-level unstable air is lofted into deep convection near the radius of maximum winds more frequently when TCREH is positive. 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. Finally, the implications of time-varying environments around TCs are examined. Until now, idealized numerical simulations of the tropical cyclone (TC) response to time-varying wind shear have applied instantaneous changes in the TC environment. A new modeling framework allows for smoothly transitioning environmental wind states: time-varying point-downscaling (TVPDS). TVPDS is an enhancement of the point-downscaling technique (Nolan 2011) developed for the Weather Research and Forecast (WRF) model. It uses analysis nudging to smoothly transition between different environmental vertical wind (and/or temperature and moisture) profiles while coordinating the point-downscaling method such that the environment remains in balance. Using this new framework, results from previous studies are reexamined to test whether the instantaneous ‘shock’ to the environment has implications for TC intensity evolution. Results suggest that instantaneous changes to the TC environment indeed do lead to an unrealistic response to an increase in shear. TVPDS simulations of quasi-steady state, moderately intense (~50 ms-1) TCs show that the response to increasing wind shear is a steady reduction in intensity without a recovery to the pre-shear intensity. TVPDS simulations also show that the rate at which the TC weakens depends on how rapidly the environment transitions from low to high shear. Analyses of surface fluxes and regions of convection are presented to determine how the time-varying shear affects the TC.