摘要:0908号台风"莫拉克"（Morakot）在台湾岛引发了24 h超过1000 mm的强烈降水，与其缓慢西行过岛密切相关。本文利用NCEP-GFS实时分析场资料（0.5°×0.5°）、中央气象局台风资料库关于我国台湾地面气压场资料和中尺度数值模式WRF模拟结果，分析了台风Morakot在台湾岛附近缓慢西行成因。结果表明：（1）西太平洋副热带高压减弱东退、与台风"天鹅"（Goni，0907）的"藤原效应"是Morakot在台湾岛附近移速减慢的环境因素，而台风结构变化也是其缓慢过岛西行的一个主要原因；（2）台风过岛过程中，由于地形作用，Morakot环流内诱生低压活跃，先后或同时出现在岛屿东西两侧，使台风环流出现非对称松散结构；（3）在台湾岛西侧诱生低压中心取代东侧原台风中心形成不连续路径过程中，台风经历了低层环流从分裂到重组，正垂直涡度柱从垂直到倾斜再恢复垂直的变化过程，这是Morakot过台湾岛缓慢西行的一个重要原因；（4）Morakot西行登岛过程中环境引导气流主要是偏南气流，而包含诱生低压的扰动引导气流则为偏东气流。扰动引导气流虽是小量，但其纬向分量占环境引导气流纬向分量的比率从7%增至26%，较好地指示了Morakot的西行趋势，也说明地形诱生低压导致的台风结构变化是其过岛西行和缓慢移动的一个重要原因。
关键词:台风"莫拉克"/
缓慢西行/
诱生低压
Abstract:The heavy rainfall caused by typhoon Morakot (0908) with 24-h accumulative precipitation larger than 1000 mm in Taiwan of China is closely related to the slow moving speed of Morakot. Using real-time NCEP-GFS global analysis data (0.5°×0.5°), synoptic charts of surface observations in Taiwan area from Central Weather Bureau (CWB) Typhoon Data Base and numerical simulations of the Weather Research and Forecasting Model (WRF), the mechanism for the slow westward movement of typhoon Morakot around Taiwan Island is investigated. Results show that the weakening and eastward retreat of the western Pacific subtropical high and the Fujiwara effect of typhoon Goni (0907) are the main environmental factors that affected Morakot's slow movement. The structure change of Morakot is also an important reason for its slow westward movement. It is found that the terrain over the Islands of Taiwan also induced several lows within the typhoon circulation system and increased its asymmetry. The terrain-induced low that developed on the western side of the Central Mountain Range replaced the original Morakot center on the eastern side, resulting in a westward discrete track. During this process, the Morakot structure experienced complex changes. Its positive vertical vorticity column changed from vertical to tilting position and then returned to vertical, and its lower layer circulation experienced splitting and re-organization processes. The above changes were largely responsible for the slow moving of Morakot. The impact of these mesoscale vortexes on the environmental steering flow is further investigated using the Barnes filter method. It is found that the mesoscale disturbance, although a small amount, is favorable for the increase in the westward steering flow. However, the percentage of the total environmental steering flow accounted for by the mesoscale disturbance increased from 7% to 26% when crossing Taiwan Island, indicating the important role of structure change related to the terrain in Morakot's slow westward movement.
Key words:Typhoon Morakot/
Slow westward movement/
Induced low-pressure system
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