Forecasting Tropical Cyclone Recurvature. Part II. An Objective Technique Using an Empirical Orthogonal Function Representation of Vorticity Fields

Abstract

An empirical orthogonal function (EOF) representation of relative vorticity is used to forecast recurvature (change in storm heading from west through north to cast of 360°) of western North Pacific tropical cyclones. A pattern recognition approach is adapted in which the synoptic conditions at recurvature time and each 12-h interval up to 96 h prior to recurvature are to be distinguished from the synoptic pattern for straight-mover storms. Synoptic descriptors are defined in terms of the time-dependent principal components of the vorticity fields for the individual maps. A standard discriminant analysis approach using 250-mb vorticity fields correctly identifies recurvers and straight movers in 80% and 66%, respectively, of the 782 cases. For a specific discriminant analysis that is derived to separate recurvers (74% correct) from straight movers (81% correct), the accuracy is higher than for the operational track prediction techniques and the official forecasts considered in Part I of this study. Although the accuracy of the discriminant analysis in identifying the lime to recurvature in 12-h intervals is less than desired for operational use, this new technique has higher accuracy than the techniques evaluated in Part I. Better accuracy can be achieved if the time resolution requirements are relaxed, for example, into three groups (0?24 h, 36?72 h, and greater than 72 h until recurvature).