Effects of Vertical Wind Shear on the Predictability of Tropical Cyclones: Practical versus Intrinsic Limit

The effect of environmental shear on the predictability of tropical cyclones (TC) is first explored through a series of cloud-permitting ensemble sensitivity experiments with small, random initial condition perturbations. It is found that under the existence of vertical wind shear: the larger the shear magnitude, the less predictable the TCs especially the onset time of the rapid intensification (RI) until the shear magnitude is large enough to prevent the TC formation. Based on ensemble correlation and spectrum analysis, it is shown that 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 and eventually leads to deviations in the development of the vortices. The location and strength of moist convection at later times are greatly influential to the precession and alignment of the TC vortex as well as the final RI onset time. A series of "fake-dry" sensitivity experiments further confirm the critical role of moist convection and the attendant diabatic heating in the later stage of TC development and the predictability of RI onset. Two additional sets of ensemble sensitivity experiments with different magnitude random perturbations to the mean environmental shear are conducted to understand the transition from intrinsic to practical predictability. It is found that when the standard deviation of the random shear perturbations among different ensemble members is as small as 0.5m/s, the difference in shear magnitude overwhelms the randomness of moist convection in influencing the TC development and rapid intensification (a sign of limited practical predictability given the current accuracy in specifying environmental shear). However, for the ensemble with standard deviation of random shear perturbations of 0.1m/s, the uncertainty in TC onset timing is as big as the ensemble that is perturbed only by small random moisture conditions in the initial moisture field (indicative of the limit in intrinsic predictability).