Impact of vertical wind shear on the Predictability of Tropical Cyclones

Recent studies of Zhang and Sippel (2009), Sippel and Zhang (2008, 2010) and Nguyen et al. (2008) suggest that the predictability of tropical cyclones may be intrinsically limited due to the presence of moist convection, which has a chaotic nature. Through cloud-permitting simulations with the Weather Research and Forecasting (WRF) model, the current study examines the impact of vertical wind shear and system-scale flow asymmetry on the predictability of tropical cyclone intensity during different stages of their life cycles (i.e., initial formation, rapid intensification, and quasi-steady mature phase). For varying magnitudes of vertical wind shear, a series of control simulations are performed, each initialized with an idealized weak tropical cyclone-like vortex. An ensemble of sensitivity experiments are then performed, whereby small random boundary-layer moisture perturbations are added to the control simulations, in order to examine the predictability of tropical cyclone intensity. These perturbations are added at various stages of storm development. It is found that the ensemble spread (i.e., the forecast uncertainty) of tropical cyclone intensity is considerably larger in the experiments with vertical wind shear than in the no-shear experiments. It is also found that the tropical cyclones are least predictable during the rapid intensification stage, with or without vertical wind shear. Finally, the timing of rapid intensification can be noticeably altered in the presence of vertical wind shear.