Assimilation of synthetic tropical cyclone observations into the Navy Operational Global Atmospheric Prediction System

In June 1990, the assimilation of synthetic tropical cyclone observations into the Navy Operational Global Atmospheric Prediction System (NOGAPS) was initiated at Fleet Numerical Oceanography Center (FNOC). These observations are derived directly from the information contained in the tropical cyclone warnings issued by the Joint Typhoon Warning Center (JTWC) and the National Hurricane Center. This paper describes these synthetic observations, the evolution of their use at FNOC, and the details of their assimilation into NOGAPS. The results of a comprehensive evaluation of the 1991 NOGAPS tropical cyclone forecast performance in the western North Pacific are presented. NOGAPS analysis and forecast position errors were determined for all tropical circulations of tropical storm strength or greater. It was found that, after the assimilation of synthetic observations, the NOGAPS spectral forecast model consistently maintained the tropical circulations as evidenced by detection percentages of 96%, 90%, and 87% for 24-, 48-, and 72-h forecasts, respectively. The average forecast position errors were 188, 299, and 434 km for the respective forecasts. The respective errors for the One-Way Influence Tropical Cyclone Model (OTCM), one of JTWC's primary track-forecasting aids, were 215, 364, and 529 km. In homogeneous comparisons the percent improvement of the NOGAPS 48- and 72-h forecasts was 14% and 12% over the OTCM and 31% and 33% over JTWC's operational Climatology-Persistence Model.