Forecasting Tropical Cyclone Recurvature Using an Empirical Othogonal Function Representation of Vorticity Fields
NAVAL POSTGRADUATE SCHOOL MONTEREY CA
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An empirical orthogonal function EOF representation of relative vorticity is used to forecast recurvature change in storm heading from west to east of 000 deg N of western North Pacific tropical cyclones. The time-dependent coefficients of the first and second EOF eigenvectors vary in a systematic manner as the tropical cyclone recurves around the subtropical ridge and tend to cluster in a different region in EOF space. Exploiting this Euclidean distance approach, additional EOF coefficients are identified that best represent the vorticity fields of recurving and straight-moving storms. Classification of an individual case is then into the closest time-to-recurvature in 12-h intervals or straight-moving storm category as measured in multidimensional EOF space. Although rather subjective, the Euclidean method demonstrates skill relative to climatological forecasts. A more objective discriminant analysis technique is also tested. A final version that involves the first six EOF coefficients of the 250 mb vorticity field is useful 72 correct in identifying recurvers or straight-movers during the 72-h forecast period. Skill in classifying situations within 12-h time-to-recurvature groups is low, but might be improved using other analysis techniques or in combination with other predictors.