| 摘要: | 臺灣四面環海，地形崎嶇，局地環流(例如:海陸風、山谷風)對於區域天氣扮演著重要角色，在弱綜觀的天氣條件時，海陸風特徵明顯，同時臺灣中南部亦經常觀測到較高的懸浮微粒事件。因此，討論海陸風環流與其伴隨的污染物傳輸將是一重要的研究課題。若能瞭解局地環流對於區域空氣品質的影響，將對於未來天氣守視的重點及氣象預報能夠有所幫助。
本研究使用王聖翔老師實驗室自行開發的無人機大氣觀測系統，藉由提供較高時空解析度及較靈活的觀測調適能力等優勢，於2021年4月23至25日進行連續兩天多點同步無人機密集觀測實驗，共取得87筆定點垂直剖面資料，探討臺南地區在弱綜觀條件下，海陸風環流三維細微結構特徵與其伴隨污染物傳輸的演化，並藉由WRF模式模擬，比較模式是否能夠反映真實的局部環流結構。
研究結果顯示，在弱綜觀天氣型態下，臺南沿海至內陸40 km處都能觀察到海風環流的現象。海風環流於沿海地區在上午9-10時開始發展，海風鋒面於近地面平均風速約3.5-4 ms-1，海風厚度約600公尺，10-11時經過中部平原地區，海風環流厚度約800公尺，此時於海風鋒面之上有海風頭的結構，約在下午13時海風推進到山麓地區，但受到地形效應的影響，海風環流的結構已較不完整，海風厚度約1600公尺，海風鋒面風速僅3 ms-1，海風環流於14-16時風速達到最強，且於沿海和平原地區有較不穩定的KH波特徵。夜間垂直大氣結構可以分成三層，600公尺以下於日落過後轉換為陸風環流，600-1300公尺處為陸風與環境風場轉換層，且底部和頂部都有逆溫層的現象，是屬於夜間殘餘層的結構，有乾空氣沉降的特徵，1300公尺以上為受到環境風場的影響，以綜觀風場所主導。海風與陸風環流皆由沿海先開始發展。本研究期間，PM2.5的來源以本地污染為主，隨日出過後混合層逐漸發展，污染物垂直擴散條件改善，PM2.5濃度有下降的趨勢。近地面在海風鋒面前緣有污染物堆積的現象，當海風鋒面與內陸離岸風發生輻合，會將污染物沿著鋒面帶往高層。海風環流在中午發展完整，高層海風頭與KH波將海風環流帶入的污染物帶往離岸的方向，部分污染物會向下混合，再次經由海風環流帶入內陸地區。日落過後，在300-600公尺間經常有逆溫的構造發生，污染物主要堆積於此逆溫層頂之下，且風速微弱，導致相較於海風環流有較高的PM2.5濃度。在高層1300公尺之上受到綜觀環境風場的影響，會將上游的夜間殘餘層帶入臺南地區，造成高空有一高污染空氣團。
雖然模式對於2公里內的海陸風環流位置與發展的時序掌握的很好，但是於山麓地區的日夜風場和無人機現地觀測差異較大，凸顯未來的海陸風研究應該要著重瞭解綜觀環境風場和地形效應的影響，才能提升預報的準確性。;Taiwan is surrounded by the sea and characterized by rugged terrain, where local circulations, such as land-sea breeze (LSB) and mountain-valley wind, play a significant role in regional weather. Under weak synoptic conditions, the distinct features of LSB become evident. Moreover, central and southern Taiwan frequently experience extreme suspended particulate matter events. Therefore, investigating the interactions between local circulations and the transport of pollutants is of paramount importance. Understanding the influence of local circulations on regional air quality could prove vital for future weather monitoring and meteorological forecasting.
In my research, we employed a self-developed unmanned aerial vehicle (UAV) atmospheric observation system. Taking advantage of its higher spatiotemporal resolution and more flexible observational adaptability, three sites of synchronous day-night UAV observation was carried over Tainan during intensive observing periods from April 23rd to 25th, 2021. 87 UAV vertical profiles have been obtained at experiment to explore the three-dimensional structure characteristics of LSB and the evolution of accompanying pollutant transport in the Tainan region under weak synoptic conditions. Additionally, the WRF model was used to simulate and compare whether the model can reflect the real local circulations.
The results show that under weak synoptic weather, the LSB was observed up to 40 km from the coast in Tainan. The LSB developed in the coastal area around 9-10 AM, with an average near-surface wind of about 3.5-4 m/s and a thickness of approximately 600 m. Between 10-11 AM, it passed through the central plain region, with a thickness of about 800 m and sea breeze advanced to the foothill area, but due to the influence of terrain effect, sea breeze became uncomplete with a thickness of 1600m, and wind speed at sea breeze front reduced to 3 ms-1. The sea breeze reached its peak wind speed between 2-4 PM, with signs of unstable Kelvin-Helmholtz waves observed along the coastal and plain areas. During the night, the vertical atmospheric structure can be divided into three layers. Below 600 m, after sunset, it transitions into a land breeze. At 600-1300 m, it forms a transition layer between the land breeze and the ambient wind, characterized by the presence of inversion layers at both the bottom and top, indicating the structure of the nocturnal residual layer with dry subsidence features. Above 1300 m, it is influenced by the ambient wind field, dominated by synoptic winds.Both sea and land breeze circulations originate from the coastal region. During the study period, PM2.5 was primarily sourced from local pollution. With the development of the mixing layer after sunrise, the vertical diffusion conditions improved, leading to a decreasing trend in PM2.5 concentration. Near the surface, there was a phenomenon of pollutant accumulation at the front edge of the sea breeze. When the sea breeze front converges with the inland offshore wind, pollutants are transported along the front to higher altitudes. The sea breeze, well-developed at noon, carries pollutants brought in from offshore in the direction away from the coast through high-level sea breeze head and Kelvin-Helmholtz wave. Some pollutants mix downward and are brought back inland again through the sea breeze. After sunset, inversions occurred between 300-600 m, where pollutants mainly accumulated below the inversion layer , and weak winds resulted in higher PM2.5 concentrations compared to the sea-land breeze period. Above 1300 m, influenced by the ambient synoptic wind field, the nocturnal residual layer from upstream is transported to Tainan, leading to the presence of a polluted air mass at high altitudes.
While the model performs well in capturing the location and temporal development of LSB within 2 km, there are significant differences between the model and UAV observations in the foothill area′s diurnal wind fields. This highlights the need for future LSB research to focus on understanding the impact of synoptic environmental wind fields and terrain effects to enhance forecast accuracy. |
