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

Abstract

The 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 21 ) show that when the standard deviation of the random shear perturbations among different ensemble members is as small as 0.5 m s 21 , 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 21 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).

On the Predictability and Error Sources of Tropical Cyclone Intensity Forecasts

“…(We did not examine the effects of initial structural error, as the specification of the radius of maximum wind is effectively the only free structural parameter in CHIPS, but this source of error may be important in real storms.) The high sensitivity to initial intensity error when the initial intensity is small may be owing to the striking sensitivity of the timing of rapid intensification of weak storms to shear documented by Zhang and Tao (2013) and Tao and Zhang (2015).…”

“…We interpret this as stemming from the greater potential for rapid intensification of initially weak storms compared to storms that are already not far from their potential intensities and to the great sensitivity of the timing of rapid intensification to shear when the initial intensity is small (Zhang and Tao 2013;Tao and Zhang 2015). This suggests that future work might focus on predictability of rapidly intensifying events and/or on events that achieve major hurricane intensity.…”

“…(In the highshear cases, storms can be reliably forecast to dissipate.) Zhang and Tao (2013) and Tao and Zhang (2015) showed that tropical cyclones are particularly sensitive to shear in their formative stages, with small differences in shear or initial intensity strongly affecting the timing of subsequent intensification.…”

Gravity Wave Activities Associated With Convective Phenomena at a Tropical Location Near Land‐Sea Boundary

“…Earlier studies have shown an interrelation between the variation of GW momentum fluxes in the LS and the intensity of zonal wind (Sato & Dunkerton, ; Vincent & Joan Alexander, ). GW energy is reduced in the presence of strong vertical wind shear (Tao & Zhang, ; Tsuda, ; Zhang & Tao, ). In the present study, the seasonal pattern indicates that the lowest zonal and meridional wind shears correspond to the maximum GW energy in the LS in monsoon months.…”

A Conceptual Framework for the Scale‐Specific Stochastic Modeling of Transitions in Tropical Cyclone Intensities

“…To account for the differences in the behavior of multiscale processes, researchers have previously used ensemble simulations of perturbed initial conditions to achieve an improved forecast as compared to individual deterministic forecasts (Berner et al, 2011;Finocchio & Majumdar, 2017;Tao & Zhang, 2015;Zhang & Tao, 2013). did a Fourier decomposition of the flow field within the vortex and investigated the error growth of individual wave numbers (WNs) within TCs while imposing stochastic perturbations at various scales internal as well as external to the TC vortex.…”