FORECASTING TROPICAL CYCLONE RECURVATURE .2. AN OBJECTIVE TECHNIQUE USING AN EMPIRICAL ORTHOGONAL FUNCTION REPRESENTATION OF VORTICITY FIELDS

An empirical orthogonal function (EOF) representation of relative vort icity is used to forecast recurvature (change in storm heading from we st through north to east of 360-degrees) of western North Pacific trop ical cyclones. A pattern recognition approach is adapted in which the synoptic conditions at recurvature time and each 12-h interval up to 9 6 h prior to recurvature are to be distinguished from the synoptic pat tern for straight-mover storms. Synoptic descriptors are defined in te rms of the time-dependent principal components of the vorticity fields for the individual maps. A standard discriminant analysis approach us ing 250-mb vorticity fields correctly identifies recurvers and straigh t movers in 80% and 66%, respectively, of the 782 cases. For a specifi c discriminant analysis that is derived to separate recurvers (74% cor rect) from straight movers (81% correct), the accuracy is higher than for the operational track prediction techniques and the official forec asts considered in Part I of this study. Although the accuracy of the discriminant analysis in identifying the time to recurvature in 12-h i ntervals is less than desired for operational use, this new technique has higher accuracy than the techniques evaluated in Part 1. Better ac curacy 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).