| 摘要: | 由於地理位置上的特殊性，台灣位於冬季極地大陸冷氣團自源地爆發到出海的必經之路，且本身地形高聳陡峭，所以台灣成為北半球同一緯度地區，在冬季經常會有降雪事件發生的地方。降雪現象的發生，除了必須溫度到達冰點外，還有兩個必要的條件：一是充足的水汽，另一個條件是空氣裡必須有凝結核(condensation nuclei)。本文的研究目的為以氣候觀點，深入瞭解影響台灣的降雪機制，以及東亞沙塵在台灣降雪事件過程中所扮演的角色，同時說明在氣候變化的影響下，台灣降雪現象的改變。
本文採用1995～2011年的中央氣象局玉山站、環保署空氣品質監測站及SYNOP電碼資料，背景大氣採用NCEP/DOE的大氣資料。為了能以量化來描述降雪活動及沙塵活動，文中定義SAI指數用以表示降雪活躍度，以及採用Yu et al. (2010)定義春季DAI的方法，定義冬季的DAI指數，用以表示冬季沙塵活躍度 。
台灣降雪與東亞冬季沙塵活動在1995～2011年間，存在著年際間的變化。SAI的時間序列的分析顯示，降雪活躍度在活躍年（如：2008、2010、2011）與不活躍年（如：1996、1999、2003）有數百倍的差異，而由DAI時間序列也看出，沙塵活動活躍度在不活躍年（如：1997，2003）與活躍年（如：2001，2002）也有數十倍的差別。
在降雪事件與沙塵活動的活躍年與不活躍年比較分析中，由大氣環流距平分析結果顯示，在降雪活躍年裡，東亞大槽的位置偏東，北方低層大氣裡的冷空氣由華北一路到台灣附近，在南方從印度洋夾帶豐沛水氣的西南氣流跨過中南半島後，若遇到西太平洋面上相對性高壓的出現，其所引發的反氣旋式環流，則這兩股氣流在南中國海合併後往東北方輸送，沿著大陸東南沿岸到台灣，使得整著華南地區大氣環境比正常年濕潤，有利於降雪的發生，此時若有沙塵隨著強烈西北氣流而來提升了PM10濃度，也就是增加了冰晶凝結核，則會提高降雪活躍度（如：2010年）。降雪不活躍年，則與降雪活躍年相反，東亞大槽位置偏西，加強了低層的西北氣流，且來自印度洋的暖濕氣流強度減弱，加上沒有西太平洋上的相對性高壓出現，所以暖濕的水氣也不會被往北抬，所以在華南地區及台灣附近的大氣環境比正常年乾燥，不利降雪發生。
SVD分析綜觀大氣環流結果指出，東亞大槽的位置與異常氣壓偶極子的關係是相當緊密的，其表現在1998年、1999年、2001年、2002年、2003年、2010年及2011年等年之間，明顯影響沙塵活躍度及降雪活躍度的強弱。
模式模擬結果顯示，台灣冬季降雪、降水的水氣主要來源，均來自印度洋的暖濕氣流。個案20100326發生當年空氣品質較為混濁，但該個案的降雪強度大於個案20090310，其該年的空氣品質較為良好。
Taiwan has relatively frequent snowfall in mountain during winter among regions of the same latitude. The phenomenon is contributed by Taiwan’s unique topography – high and steep mountains, and geographical location - sitting on the route the continental polar air mass travels from its birthplace to the ocean, contribute to this phenomenon. Snow occurence, in addition to the freezing-point temperature, when two requirements are met: sufficient vapor and the condensation nuclei in the air.
This study pursues the causes of the snowfall activity in Taiwan, the relations between the East Asian dust aerosol and the snowfall activity in Taiwan, and the impacts the climate changes have on the snowfall activity in Taiwan. In this study, Yushan snowfall activity from 1995~2011 and related atmosphere circulations were examined using SYNOP data, NCEP/DOE reanalysis atmospheric data, the observations of the Central Weather Bureau’s Yushan Weather Station and the Taiwan Air Quality Monitoring Network of the Environment Protect Administration, Executive Yuan.
To provide a quantitative measure of snowfall events and dust activity, a snowfall activity index (SAI) and the DAI Index by Yu et al. (2010) were defined. The time series of yearly SAI and DAI show that East Asian dust storm activity and Taiwan snowfall marked interannual variations during 1995 ~ 2011. For active years such as 2008, 2010, and 2011, SAI was hundreds of times larger than that for inactive years such as 1996, 1999 and 2003; and DAI in active years such as 2001 and 2002 was several tens of times larger than that in inactive years such as 1997 and 2003.
In active years when the EAT (East Asian Trough) was shifted eastward, the strength of WPH (West Pacific High) increased in the south and an anticyclone thus occurred. This anticyclone introduced anomalous southwesterly flows along the southeastern coast of mainland China and over Taiwan, resulting in a wetter-than-normal atmosphere in support of snowfall. Oppositely, for inactive years, drier-than-normal atmosphere appeared and consequently sluggish snowfall seasons followed.
A SVD (singular value decomposition) analysis of the Asian synoptic circulation indicated that the connection between the pressure dipoles and the position of EAT is strong in 1998, 1999, 2001, 2002, 2003, 2010, and 2011. It significantly affects both of the snowfall and dust activities.
In summary, snowfall active years usually occurred when the East Asian dust storm was inactive. Nevertheless, the snowfall activity increased in Taiwan if there was dust event and the dust aerosol successfully transported to Taiwan. This finding is also demonstrated in the model simulation of this study. |
