本研究使用中央氣象局1961~2011 年臺東和臺中測站資料，按氣溫、相對濕 度和平均風速定義焚風事件，並分成伴颱與非颱二類。觀測資料顯示，臺東伴颱、 非颱焚風皆以西南風最多，東北風次之；而臺中非颱焚風以西南風最多，伴颱焚 風則以偏北風最多。 利用1979~2011 年歐洲中期天氣預報中心之高解度重分析資料，將兩地之焚 風事件前後二日進行其850 百帕渦度場的旋轉性擴張經驗正交函數分析，並以顯 著正相位個案做其他變數的組合分析，觀察各模態的環流發展特徵。經診斷分析， 造成兩地焚風的主因為颱風和鋒面：當颱風在琉球一帶或路徑往北臺灣 (菲律賓 附近或路徑往南臺灣) 時，常導致臺東 (臺中) 焚風的發生，臺中 (臺東) 焚風 則為偶發；而鋒面經過致使西南氣流或東北季風吹拂時，亦會誘發焚風。 氣流遇到地形產生的越山、穿谷和分流輻散沉降，為本研究中焚風形成的主 要可能機制。當風向與山勢較垂直 (平行) 時，以越山 (穿谷) 沉降為主；當較 弱氣流經過臺灣島分流後，會引發迎風側高空輻散、誘發下沉，加深背風側的沉 降。以上三種機制常共同作用，造成不同時空的焚風事件。

This study adopted observed Central Weather Bureau (CWB) hourly data from 1961 to 2011 in Taitung and Taichung. Foehn events were defined in terms of three major factors, high temperature, low relative humidity and strong wind speed. All the foehn events in Taitung and Taichung were divided into two types: events associated with typhoons (TYs) near Taiwan and independent events. The results of local winds based on the observational data are as follows. In both areas, the prevailing winds were mainly from the southwesterly direction, and the secondary winds were from the northerly to northeasterly directions. However, in Taichung, the northerly local winds of the foehn events related to adjacent TYs had a higher percentage to occur. This work used the European reanalysis (ERA)-Interim data of European Centre for Medium-Range Weather Forecasts (ECMWF) with high spatial resolution from 1979 to 2011. First, the rotated-extend empirical orthogonal function (REEOF) was applied by employing 850 hPa vorticity fields for 48 hours in each foehn case.Second, a composite analysis was applied for other variables (e.g. vertical velocity, winds) respect to the significant positive phases of the four major REEOF principal components in each type. From the composited circulation patterns, it can illustrate the time evolution characteristics for the foehn events associated with the four major REEOF modes for both types. Analytical results indicate that TYs and fronts are the main causes of foehn. As TYs were located around the Ryukyu Islands, or moved toward Nothern Taiwan (located around the Philippines or moved toward the Southern Taiwan), foehn events would occur in Taitung (Taichung) usually, sometimes in Taichung (Taitung). Otherwise, foehn could be also induced by southwesterly flow or northeasterly flow because of fronts. This investigation identifies crossing mountains, passing through valleys and splitting as the three main possible mechanisms to induce foehn when airflow interacts with terrain. As the wind flow is perpendicular to (parallel with) the mountains, mountain-crossing (valley-passing) subsidence may be the main mechanism. Furthermore, weaker winds tend to be split when they blow over Taiwan island. This may cause divergence in the windward side, which extends to strengthen the subsidence over the leeward side owing to the downslope winds. The three mechanisms may work together when the airflow crosses/passes the mountains, causing foehn in different time and space over Taiwan.