The Tropical Cyclone Response to Smoothly Changing Wind Shear Using the Method of Time-Varying Point-Downscaling

Until now, idealized numerical simulations of the tropical cyclone (TC) response to time-varying wind shear have required instantaneous changes in the TC environment. This was typically accomplished by pausing the simulation, constructing a new environmental vertical wind profile, rebalancing the mass field, and then restarting the simulation. 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 developed for the Weather Research and Forecast (WRF) Model by Nolan (2011). It uses analysis nudging to smoothly transition between 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. Finally, TVPDS simulations of TCs transitioning from environments characterized by large shear into low-shear environments are performed to determine if a similar but opposite response occurs. The results presented in this study demonstrate the utility of the TVPDS method in terms of investigating time-varying TC environments.