Vertical Wind Shear as a Predictor of Tropical Cyclone Motion.

The effects of vertical wind shear on tropical cyclone motion are investigated for 1357 western North Pacific Ocean cases. An empirical orthogonal function EOF analysis is used to represent the vertical shear of the zonal and meridional components between the 700, 400 and 250 mb levels. Composite wind-shear fields are developed for five past-motion storm categories and analyzed to determine their statistical differences. The significant differences between these categories represent differing synoptic forcing by the wind shear that affects storm motion. Within the category of storms moving northwest, significant differences occur between right- and left-turning cyclones. This demonstrates that the vertical shears derived from operationally analyzed wind fields contain synoptic forcing information that is revelant to tropical cyclone motion. A regression analysis is used to identify potential predictors from among the wind-shear EOF coefficients. These predictors are included in regression equations to post-process the 24-, 48- and 72-h OTCM One-way Tropical Cyclone Model forecasts for 1982-1983. The modified OTCM tracks using this scheme have an average error of 419 km at 72 h, which is an improvement of 174 and 212 km over the unmodified OTCM and official Joint Typhoon Warning Center 72 h forecast errors, respectively. Wind-shear EOF coefficients included in a decision tree to select the best objective forecast aid only nominally increase the accuracy of the optimum forecasts.