Effects of vertical wind shear on the predictability of tropical cyclones: Practical versus intrinsic limit

Abstract

AbstractThe effects of environmental shear on the dynamics and predictability of tropical cyclones (TCs) are further explored through a series of cloud‐permitting ensemble sensitivity experiments with small, random initial condition perturbations on the low‐level moisture fields. As an expansion of earlier studies, it is found that larger the shear magnitude, less predictable the TCs, especially the onset time of the rapid intensification (RI), until the shear is too large for the TC formation. Systematic differences amongst the ensemble members begin to arise right after the initial burst of moist convection associated with the incipient vortex. This randomness inherent in moist convection first changes the TC vortex structure subtly, but the location and strength of subsequent moist convection are greatly influential to the precession and alignment of the TC vortex as well as the RI onset time. Additional ensemble sensitivity experiments with different magnitude random perturbations to the mean environmental shear (6 m s−1) show that when the standard deviation of the random shear perturbations among different ensemble members is as small as 0.5 m s−1, the difference in shear magnitude overwhelms the randomness of moist convection in influencing the TC development and rapid intensification (indicative of limited practical predictability). However, for the ensemble with standard deviation of 0.1 m s−1 in random shear perturbations, the uncertainty in TC onset timing is comparable to the ensemble that is perturbed only by small random moisture conditions in the initial moisture field (indicative of the limit in intrinsic predictability).