Forecast of Typhoon Motion in the Vicinity of Taiwan during 1989–90 Using a Dynamical Model

Abstract

A limited-area numerical model designed specifically for forecasting typhoon tracks has been operational at the Central Weather Bureau (CWB) in Taipei, Taiwan, since January 1990. It is a primitive equation model with nine σ levels and a grid size of 70 km. The model domain of 8500 km ? 6000 km is centered near Taiwan, and covers the western part of the Pacific Ocean and southeast China. A model-balanced vortex is bogussed into the analysis to initialize the forecast. To ensure the maintenance of the vortex circulation throughout the forecast period, artificial heating options are incorporated to supplement the Kuo-type cumulus parameterization in the model. The statistics of track errors for all forecast cases conducted during the development and operational checkout period (before December 1989) and during 1990, the first year of real-time operation, are reported. During the operational checkout period, 12 typhoons were forecasted, with an average 48-h track error of 415 km (62 forecast cases). For the 1990 season, there were 11 typhoons, with an average 48-h error of 392 km (63 forecast cases). The errors are compared with the One-Way Interactive Tropical Cyclone Model (OTCM), which is considered as the best long-term operational numerical track model for the western Pacific, using a homogeneous sample. The results indicate that the two models have similar average errors. The model had larger errors than the climatology and persistence (CLIPER) method. However, for all three typhoons with erratic movements, the model outperformed the CLIPER. The model was modified in several ways prior to the beginning of the 1990 season. The most beneficial modification appears to have been the enlargement of the forecast domain. However, the domain was still not large enough to cover important synoptic fields for Typhoon Marian, which was the westernmost typhoon forecasted by the model. Postoperational experiments were conducted and the forecast track of Typhoon Marian improved when the model domain was expanded to the west. Examination of the synoptic patterns indicates that the track forecast depends closely on the forecast of the subtropical high circulation.