| 摘要: | 台灣在每年夏、秋兩季常受颱風影響，伴隨颱風與其外圍環流出現的強風及豪雨更常為台灣帶來嚴重之災害。環保署監測資料顯示，颱風中心距離台灣約1000 ~ 2000公里時，外圍環流因中央山脈影響，可能造成台灣西半部地區空氣品質惡劣。本研究一方面分析1994 ~ 2003年6 ~ 11月中部空品區之臭氧濃度變化與颱風或熱帶低壓活動之相關，另一方面，選取明顯的空氣污染案例，利用MM5 (Fifth-generation Penn. State/NCAR Mesoscale Model) 模式進行颱風對中部空品區空氣品質影響之分析與模擬。 統計結果顯示，當發生高臭氧污染時(O3 > 120 ppb)，颱風或熱帶低壓的位置大致集中在南海及台灣-菲律賓之間，路徑以第5、6及第9類居多，即颱風西行通過台灣-菲律賓之間、颱風北行，以及南海生成之颱風。而在1994 ~ 2003年所有第5、6類及第9類颱風路徑中，以第5、6類路徑造成高臭氧污染之持續時間較長，第9類路徑造成中部空品區高臭氧污染之發生率最高(66 %)。 另外，本研究針對2005年9月16 ~ 18日之高汙染事件進行模式模擬，分析綜觀天氣與氣象場之特徵。模擬結果顯示，台灣地區受到高壓脊與韋森特颱風(為第9類颱風路徑)影響，為穩定、下沉的天氣，綜觀風場為東南風。底層氣流受地形阻擋及尾流效應影響，中部地區之風速明顯減弱；高層氣流越過山脈後下沉，導致混合層高度發展較低。在垂直速度、探空斜溫圖及逆軌跡之模擬結果中可發現，底層至700 hPa皆為沈降運動，約在900 hPa高度附近有一逆溫層存在，大里及南投測站之逆軌跡線皆由西北方向傳送過來，表示沿岸地區的污染物(台中火力發電廠)受海風影響而被傳送至較內陸地區，加上逆溫效應及沉降運動，將污染物侷限在約900 hPa高度之下進而導致9月17日中部空品區發生高臭氧污染。 On average during summer and fall season, there are three to four typhoons affecting Taiwan per year. With their violent wind and heavy rainfall, these typhoons often cause severe disaster. The data analysis from the monitor stations of EPA (Environmental Protectional Administration) shows, high ozone episodes may occur in the western central Taiwan when the center of typhoon locates 1000-2000 km away from Taiwan. This study analyzed the relationship between typhoons or tropical depressions and the variation of ozone concentration in the central region during 1994-2003 (June to November). And further more, we used the mesoscale model (Fifth-generation Penn. State/NCAR Mesoscale Model, MM5) to do a case simulation to investigate the influence of typhoon activities on the air quality of the central Taiwan region. The statistical result shows that when high ozone concentration episodes occur (O3 > 120 ppb); the center of typhoons or tropical depressions is mainly located in the South China Sea and the region between Taiwan and the Philippines. Most tracks of such typhoons are classified to the track of 5, 6 and 9 follow the Central Weather Bureau’s classification, and the episodes affected by typhoons with tracks 5 and 6 seem to last longer. Besides, we also find a high correlation (66%) between typhoons with track 9 and the occurrence of high ozone episodes, which is the highest correlation in all tracks. In addition, this study also used MM5 model to simulate the high ozone episode during 16-18 September 2005. The simulation results exhibit that the atmospheric condition in Taiwan is stable and the synoptic wind is southeastly, as a result of the anti-cyclonic flow of the subtropical high pressure and Typhoon Vicente. The low-level prevailing flow is subject to splitting due to the blocking effect of the Central Mountain Range (CMR), a weak wind speed zone is notable in the western central region, which is located in the leeward side of the CMR. The simulation results also show that the middle prevailing flow pass over the CMR and then move downward to the lower levels, causing the height of mixing layer lower. According to the vertical wind velocity, skew-T diagram and backward trajectory analysis, there is sinking motion from surface to 700 hPa, the top of inversion layer is about 900 hPa and the airflows are from the northwest, which means that the sea-breeze may transport the pollutants from coastal area to inland. The inversion layer and sinking motion trapped the pollutants under the height of about 900 hPa, which lead to the occurrence to high ozone episode in the western central region on 17th of September. |
