An Evaluation of the Real-Time Tropical Cyclone Forecast Skill of the Navy Operational Global Atmospheric Prediction System in the Western North Pacific

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Abstract
The meteorological quality and operational utility of the Navy Operational Global Atmospheric Prediction System (NOGAPS) in forecasting tropical cyclones is evaluated and it is shown that the model can provide useful predictions of motion and formation on a real-time basis in the western North Pacific. The evaluation was conducted during the 1990 operational testing of a procedure to improve the initial analysis or specification of tropical cyclones (TCs) in NOGAPS by the U.S. Navy Fleet Numerical Oceanography Center (FNOC). The NOGAPS TC analysis procedure generates synthetic TC observations based on operational vortex data (e.g., location and maximum surface wind speed) and then adds the observations to the observational data base with flags to force their assimilation. Results from the first year of testing were favorable, despite intermittent application of the procedure. The meteorological characteristics of the NOGAPS tropical cyclone predictions were evaluated by examining the formation of low-level cyclone circulation systems in the tropics and vortex structure in the NOGAPS analysis and verifying 72-h forecasts. Analyzed circulations were found in the vicinity of developing TCs for nearly all cyclones during the operational test period. This finding implies that the model is “primed” for assimilating the synthetic observations and may be accurately simulating the large-scale environments favorable to TC formation. The analyzed TC circulations had greater than observed horizontal extent due to coarse grid spacing (δx∼ 160 km) in the global model; however, the vortices, in general, were vertically stacked and maintained during the forecast by realistic amounts of thermodynamic forcing from the cumulus parameterization. Despite the large size of the NOGAPS TC vortices, the track forecasts were not overly biased with regard to track or speed. The operational utility of the NOGAPS track forecasts was analyzed through a comparison with the real-time runs of a baseline climatology persistence aid and with the best dynamical model used by the Joint Typhoon Warning Center, Guam. To ensure a realistic comparison of the forecasts and to improve the appearance of the global model tracks, a postprocessing adjustment procedure was employed that accounts for the observed initial motion and position. The adjusted NOGAPS track forecasts showed equitable skill to the baseline aid and the dynamical model. In fact, NOGAPS successfully predicted unusual equatorward turns for several straight-running cyclones. Overall, the adjusted NOGAPS track forecasts were judged to be competitive with other aids used by the operational forecasters at JTWC and it is suggested that global models may make important contributions to improving TC forecasting in the future.

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