Effects of Vertical Wind Shear on the Predictability of Tropical Cyclones

Dandan Tao and Fuqing Zhang

Pennsylvania State University

Through cloud-resolving simulations with the Weather Research and Forecasting (WRF) model, this study examines the effect of vertical wind shear and system-scale flow asymmetry as well as the randomness of moist convection on the predictability of tropical cyclone (TC) intensity during different stages of the TC life cycle under different environmental conditions. A series of ensemble experiments are performed with varying magnitudes of vertical wind shear and environmental moisture under two different sea-surface temperature conditions. Each ensemble simulation is initialized with the same idealized weak TC-like vortex but with different realizations of small-scale, small-amplitude random perturbations added to the initial conditions. It is found that the environmental shear can significantly affect the predictability of tropical cyclone intensity; the larger the vertical wind shear, the larger the forecast uncertainty (and thus the more inherently limited the predictability), especially during the formation and rapid intensification stages.

In the presence of environmental shear, initial random noise may result in changes in the timing of rapid intensification by as much as 1-2 days, through the randomness (and chaotic nature) of moist convection. Upscale error growth from differences in moist convection first alters the tilt amplitude and angle of the incipient tropical storms, which leads to significant differences in the timing of precession and vortex alignment. During the precession process, both the vertical tilt of the storm and the effective (local) vertical wind shear are considerably decreased after the tilt angle reaches 90 degree to the left of the environmental shear. The tropical cyclone intensifies immediately after the tilt and the effective local shear reach their minima. The variation in the timing of rapid intensification among ensemble members increases with environmental vertical wind shear. In many simulations the tropical cyclone may never reach rapid intensification at all if the shear exceed 7.5 m/s unless the constant SST is increased substantially (from 27 to 29 C tested). The higher SST condition also shortens the RI onset time and reduces the spread of TC genesis given the same vertical wind shear. It is also found that the environmental moisture availability may also change the intrinsic predictability of the tropical cyclones but the largest sensitivity comes from the simulations with moderate relative humidity (in comparison to very dry or very moist conditions). In essence, the predictability of tropical cyclone intensity is intrinsically more limited in the presence of stronger vertical wind shear which can be further altered by other environmental conditions such as moisture content and sea-surface temperature.