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An Evaluation of Tropical Cyclone Forecast Aids Based on Cross-Track and Along-Track Components

Russell L. ElsberryDepartment of Meteorology, Naval Postgraduate School, Monterey, CA 93943

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James E. PeakDepartment of Meteorology, Naval Postgraduate School, Monterey, CA 93943

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Abstract

Official and objective forecast aids for tropical cyclone tracks in the western North Pacific during 1979–83 are evaluated in terms of cross-track (CT) and along-track (AT) components relative to an extrapolated track based on warning positions (a persistence forecast). The focus of the study is the 72-h forecasts, which are essential for timely evasion planning. The CT and AT components are divided into three classes (terciles). A scoring system that assess penalty points for forecasts in the incorrect tercile is used to rank the official and objective aids. The One-way Tropical Cyclone Model appears to be most skillful at 72 h based on the tercile score. When a finer resolution (five-class) distribution is tested using a nonlinear penalty point assessment, the Nested Tropical Cyclone Model is shown to provide the most skillful path forecasts at 72 h. Some of the less skillful forecasts are shown to have systematic biases within the quintile distributions. The past 12-h speed and direction are used to define five categories that approximately represent different synoptic forcing. differences in forecast performance of the aids are indicated across the five categories. This study can provide guidance as to the relative merits of the different objective aids, and where improvements in the official forecast might be made.

Abstract

Official and objective forecast aids for tropical cyclone tracks in the western North Pacific during 1979–83 are evaluated in terms of cross-track (CT) and along-track (AT) components relative to an extrapolated track based on warning positions (a persistence forecast). The focus of the study is the 72-h forecasts, which are essential for timely evasion planning. The CT and AT components are divided into three classes (terciles). A scoring system that assess penalty points for forecasts in the incorrect tercile is used to rank the official and objective aids. The One-way Tropical Cyclone Model appears to be most skillful at 72 h based on the tercile score. When a finer resolution (five-class) distribution is tested using a nonlinear penalty point assessment, the Nested Tropical Cyclone Model is shown to provide the most skillful path forecasts at 72 h. Some of the less skillful forecasts are shown to have systematic biases within the quintile distributions. The past 12-h speed and direction are used to define five categories that approximately represent different synoptic forcing. differences in forecast performance of the aids are indicated across the five categories. This study can provide guidance as to the relative merits of the different objective aids, and where improvements in the official forecast might be made.

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