James E. Peak

Beta Test of the Systematic Approach Expert System Prototype as a Tropical Cyclone Track Forecasting Aid

The authors have developed error mechanism conceptual models with characteristic track departures and anomalous wind or sea level pressure patterns for dynamical tropical cyclone track predictions primarily occurring in tropical regions or those associated with midlatitude circulation patterns. These conceptual models were based on a retrospective study in which it was known that the 72-h track error exceeded 300 n mi (555 km). A knowledge-based expert system module named the Systematic Approach Forecast Aid (SAFA) has been developed to assist the forecaster in the information management, visualization, and proactive investigation of the frequently occurring error mechanisms. A beta test of the SAFA module was carried out for all available track forecasts for the western North Pacific cyclones 19W‐30W during 1999. The objective was to determine if the SAFA module could guide the team to apply the conceptual models in a real-time scenario to detect dynamical model tracks likely to have 72-h errors greater than 300 n mi. The metric was a selective consensus from the remaining model tracks that had smaller errors than the nonselective consensus track of all dynamical model tracks. This beta test demonstrated that the prototype SAFA module with the large-error mechanism conceptual models could be effectively applied in real-time conditions. The beta-test team recognized 14 cases in which elimination of one or more dynamical model forecasts before calculating the consensus track resulted in a 10% improvement over the nonselective consensus track. A number of lessons learned from the beta test are described, including that rejecting a specific model track is not normally successful if the tracks are tightly clustered, that the availability of the model-predicted fields is critical to the error detection, and that at least three of the five model tracks need to be available.

Diagnostic study of explosive cyclogenesis during FGGE

Abstract Two rapidly developing extratropical maritime cyclones are studied using the European Centre for Medium-range Weather Forecasting (ECMWF) level III-b analyses from the First GARP Global Experiment (FGGE). The first cyclone forms over the western North Pacific Ocean along an intense frontal zone south of Japan, while the second develops in a polar air mass over the North Atlantic Ocean. Quasi-Lagrangian diagnostic techniques in isobaric coordinates are used in a synoptic investigation and mass and vorticity budget diagnostic evaluation of storm development. Reliable diagnostics are obtained from the ECMMWF analyses over these ocean areas using 0000 and 1200 UTC data. Although the cyclones develop under almost unperturbed upper level flow resembling Petterssen type A development, rapid deepening in both cases occurs when an approaching upper tropospheric jet with appreciable shear vorticity advection becomes favorably superposed over the surface low. Stability decreases in the low troposphere durin...

Prediction of Tropical Cyclone Turning and Acceleration Using Empirical Orthogonal Function Representations

Abstract Prediction of tropical cyclone motion in terms of cross-track (CT) and along-track (AT) components is proposed as an alternative to geographic (zonal and meridional) components. Since the CT and AT components are defined relative to an extrapolated track based on the current and − 12 h wanting positions, the CT and AT components are representative of the important turning motion and apparent speed changes along the track. A discriminant analysis approach is used to determine which of the persistence-type and predictors and empirical orthogonal functions of the geopotential fields are most relevant. Classification functions are derived to predict the future CT and AT tercile group. The scheme correctly selects 45% of the CT and 50% of the AT classifications versus 33% due to random chance. Based on the results of the discriminant analysis, the sample of cases is stratified into five subgroups in terms of the past 12 h storm heading and speed. Separate regression equations are derived for the subgr...

Forecasting Tropical Cyclone Motion Using Empirical Orthogonal Function Representations of the Environmental Wind Fields

Abstract Tropical wind fields from the U.S. Navy Global Band Analyses (GBA) are studied to depict the synoptic flow surrounding tropical cyclones. The composite fields of the zonal and meridional wind components on a grid centered on the tropical cyclone indicate physically realistic flow patterns. Scalar empirical orthogonal function (EOF) analysis is used to represent the zonal and meridional GBA wind component fields. The representation of these components in terms of the first 35 out of a total of 527 EOF coefficients accounts for at least 80% of the total variance and eliminates much of the noise from the fields. This truncation requires only 7% of the storage needed for the original gridpoints. The eigenvectors can be interpreted as representing different synoptic flow patterns. Statistical regression equations are derived to predict the future zonal and meridional translation of the tropical cyclone. The EOF coefficients are used as predictors to represent the synoptic information for the scheme. T...

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

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 u...

Selection of Optimal Tropical Cyclone Motion Guidance Using an Objective Classification Tree Methodology

Abstract A number of tropical cyclone track forecast aids are available to the forecasters at the Joint Typhoon Warning Center (JTWC) at Guam. These aids typically provide conflicting guidance and no single aid provides consistently superior guidance in every situation. The basic assumption in this study is that synoptic factors and storm-related parameters can be used to predict the performance of each objective aid under various situations. The algorithm of Breiman et al. is used to derive objectively a classification tree to select which of eight objective aids has the lowest 72-h forecast error. The path by which each case traverses the classification tree consists of a series of branches or “decisions.” These branches, which ultimately result in the selection of an objective aid to be utilized in each case, may be physically interpreted in most cases. The branches of the classification tree in this study are highly dependent upon empirical orthogonal function coefficient values of the environmental w...

Dynamical–Statistical Model Forecasts of Southern Hemisphere Tropical Cyclones

Abstract The Navy Nested Tropical Cyclone Model (NTCM) is evaluated for performance on Southern Hemisphere storms near Australia. East of 135°E the model exhibits mean forecast errors of 246, 467, and 694 km at 24, 48, and 72 h, respectively. West of 135°E the mean forecast errors are 214, 511, and 745 km at 24, 48, and 72 h. The NTCM tends to have a poleward directional bias in the predicted tracks. This bias may be attributed to the lack of current data, which causes the analysis scheme to revert to climatological values. Storm tracks near the Australian coast also were not forecast well by the NTCM, especially in the western cases, presumably due to lack of consideration of land/sea effects. In a homogeneous sample comparison with an operational analog prediction technique (TYAN78), and with persistence of the past 12 hours motion, the NTCM performed worse in terms of forecast error at early forecast times and better at late forecast times east of 135°E. To the west of 135°E, the model performance was ...