A Study On The Characteristic Of Unusual Tropical Cyclone Tracks Near Taiwan Island And The Uncertainty In Track Forecasting

Despite steady improvement has been made in tropical cyclone(TC)track forecasts recently,the unusual TC tracks are still difficult to predict.Taiwan Island is one of areas with frequent unusual TC tracks due to its complex topography.In this study,the statistical characteristics of unusual TC tracks in the area of 116°E-126°E,19°N-29°N around Taiwan Island during the 66-yr period from 1949 to 2014 are examined.Then typhoon Morakot(2009)is chosen to investigate the reason for its unusual track and the uncertainty in track forecasting based on diagnostic and ensemble synoptic analysis.Finally a series of numerical experiments is conducted to explore the influence of Taiwan terrain on TC unusual track.Statistical analysis shows about 10% more TCs were deflected to the right than to the left when moving across Taiwan's Central Mountain Range(CMR).Anomalies change in TC moving direction deflection and speed often occur in Taiwan's eastern coast,northwestern and southern strait.About 33% TC approaching Taiwan Island were accompanied with terrain-induced SC(secondary low pressure center),and the SCs prefer to appear in the northwestern quadrant of the island.Generally,the centers of TC and SC distribute in the east and west side of CMR separately.And 36% of TC centers making landfalling in Taiwan are replaced by SCs.Numerical simulation data from the Weather Research and Forecasting model(WRF)show that the weakening and eastward withdrawing of the western Pacific subtropical high and the Fujiwara effect interacting with typhoon Goni(0907)are the main environmental systems influencing Morakot' slow movement.Moreover,the development of SC within TC circulation increases its asymmetry and influences typhoon movement.When Morakot crossing Taiwan Island,its vortex becomes tilt vertically with the low-level circulation splitting.The terrain-induced asymmetric structure is benefit for Morakot long-time stagnation over Taiwan Island.Ensemble synoptic analysis of Morakot using ECMWF ensemble forecasting data indicated that the forecasting uncertainty of TC track presents high in both of early and late periods.The differences of forecasting mainly results from the differences of simulating the Pacific subtropical high,westward zonal wind and the binary typhoon.The good group move westward crossing Taiwan while poor group turn sharply northward in advance.In the later period,SCs are more intense in good group which lead typhoon crossing Taiwan Island with a discontinuous track.In contrast,the SCs are weaker in poor group which result in typhoon staying in the eastern coast of the island and then moving northward.Finally,a set of numerical sensitivity experiments with various Taiwan terrain heights are designed to explore the impacts of terrain on typhoon tracks.Results show that the blocking of terrain can slow down TC moving speed,resulting in a southward track deflection and then replaced by SC to present an uncontinuous tracks.Sensitivity experiments with Taiwan terrain elevation decreasing to 50% or 0% show that TC will cross Taiwan Island with faster speeds without the generation of SC.In addition,Taiwan terrain increases the mosescale convection and which is beneficial for the development of SC and TC sustenance.