| The cases with larger errors in24h forecast of typhoon tracks in China during2005-2012and the associated large-scale circulations are examined in this study. It is found that the mean forecast errors decrease by8.5%after removing the largest forecast error cases which account for3.9%of the total. About60%of the cases are characteristic of small direction errors but lower translation speed than observation. Associated large-scale surrounding flows can be classified as cyclonic circulation, weak background and the west side of subtropical high, respectively. Three quarters of the total cases are related to the background of cyclonic circulation, in which half of them are affected by monsoon gyre (MG). Analysis on mean translation speed shows that the sudden increasing of typhoon mean speed around the first forecast time is the main reason for lower forecast speed. This is one of the major difficulties in the24h-forecast of typhoon tracks in China.Detailed analysis on each category of cases shows that most cases lie on typhoon tracks which have turned to northeast Except two cases associated with cyclonic vortex, mean translation speed of other two categories of selected cases among cyclonic circulation has the characteristics of sudden change. Though two cases of weak background are in the acceleration process, their translation speeds don’t have the characteristics of sudden change. The acceleration process of two cases lying on the west side of subtropical high is characterized of sudden change. Analysis on high-frequency and low-frequency circulation of four cases associated with MG shows that the northeastward turning of typhoon tracks and the sudden change of mean translation speed could be explained by interactions between low-frequency and high-frequency circulation as well as evolution of the asymmetric wind flow. |
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