摘要或動機

颱風是台灣地區常見的天然災害之一，而台灣地區複雜的地形變化，也為颱風路徑的變化投下一個變數。根據以往的資料顯示，颱風路徑受到地形影響往往是在登陸前六小時內，而颱風對台灣地區災害最嚴重的時候往往是登陸前後三小時，也就是說要是颱風路徑受到地形影響發生偏轉，則我們只有數小時的時間來進行防災的準備。本研究主要收集1987~2001年由台灣東部登陸的台風最佳路徑圖，分別統計其颱風路徑或颱風環流過山時的變化，並分類統計各種不同颱風要素（包括颱風強度、行進速度、移入角、登陸地段等）對颱風路徑變化的影響，以做為未來預報之參考。結果顯示，台灣地形的確對颱風路徑有所影響，並使颱風在接近時偏轉，偏轉角度大多在30度以內，且偏北較偏南略多。而當颱風已偏東北方向接近時（移入角小於60度）向南偏轉較多；以偏東南方向接近時（移入角大於161度）方向偏較多，顯然地形對颱風行進有阻礙作用；而正交颱風偏轉角度也比斜交颱風小。在颱風強度部分，越強的颱風路徑偏轉角也越小，颱風過山後路徑受到導引氣流影響回到原路徑的機率較低。而影響颱風移動速度最主要的因素是導引氣流的強弱，若導引氣流受到山脈阻擋時，會使颱風移動速度減慢。而台灣各地山脈高低不同，也會使某些地區出現較特殊的台風現象。例如花蓮南部及成功地區容易誘發副低壓造成分裂路徑；台東地區因颱風容易繞山而過因此偏南機率比其他地區高等。Typhoon is a natural hazard that often occurs in Taiwan. The complex terrain of Taiwan also adds to the uncertainty to typhoon tracks. Based on past data, typhoons often show changes in their tracks within six hours before making landfall, while they produce most of their damage within three hours of the landfall. In other words, if the track of a particular typhoon was to be affected by the terrain, people have only a few hours to prepare. The study collects best tracks of a total of 145 typhoons that made landfall from the east of Taiwan from 1987 to 2001. Their tracks and changes in circulation while they moved across the Central Mountain Range are recorded, and classified according to several different elements such as strength, traveling speed, incipient angle, and landfall area. The possible influence of these elements on typhoon tracks can be applied to forecasts in the future. Results indicate that the terrain of Taiwan does have an impact on typhoon tracks, and causes typhoons to turn when they approach from the east. Turning angle in most cases is within 30°, and cases turning northward are slightly more than those turning southward. When typhoons approach Taiwan from the northeast (incipient angle < 60°) they favor turning south, and when they approach from the southeast (incipient angle > 160°) they favor turning north. Also, the turning angle tends to be smaller for typhoons that approach the terrain at a right angle, and the opposite is favored when the incipient angle is more acute. This suggests that the terrain of Taiwan had a blocking and deflecting effect to typhoon tracks. For results related to typhoon strength, stronger typhoons tend to exhibit smaller turning in their tracks, but they are also less likely to return to their original direction of motion under the influence of steering flow after they leave the terrain. The most relevant factor to moving speed of typhoon is the strength of steering flow, which is slowed down if blocked by the terrain. The difference in terrain elevation can also influence the motion of typhoon and cause some rather special phenomena. For instance, a secondary low is more likely to form and result in a discontinuous track if a typhoon made landfall over southern Hwa-Lien or Cheng-kung, while a typhoon has a higher possibility to be deflected southward if it made landfall over Tai-tung, the southernmost area.