臺灣中部山區埔里盆地之局部環流與邊界層結構特性;Characterization of the Boundary Layer Structure and Local Airflows over the Complex Terrain in Puli Basin, Taiwan

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題名:	臺灣中部山區埔里盆地之局部環流與邊界層結構特性;Characterization of the Boundary Layer Structure and Local Airflows over the Complex Terrain in Puli Basin, Taiwan
作者:	王鈺慈;Wang, Yu-Tzu
貢獻者:	大氣科學學系
關鍵詞:	大氣邊界層結構;熱力環流;複雜地形;Boundary Layer Structure;Thermal Circulation;Complex Terrain
日期:	2019-08-20
上傳時間:	2019-09-03 12:20:50 (UTC+8)
出版者:	國立中央大學
摘要:	臺灣中部山區埔里盆地，周圍高山環繞地形複雜，在弱綜觀天氣形態下，大氣污染物經由海風從西半部傳送至山區，造成埔里盆地空氣品質不佳，常有高污染事件發生。此時，山區的熱力局部環流，如上下坡風、山谷風環流，以及邊界層的發展將主導污染物擴散與累積。　　於個案期間透過探空觀測發現埔里盆地之大氣結構特性，白天，底層大氣加熱形成均勻混合的大氣邊界層，同時伴隨由熱力環流產生之西風分量，並於午後達到風速最高值，引起污染物由西部沿岸的傳送並累積；夜間，地表冷卻形成穩定邊界層，限制底層風速的發展，並在距地表約400m處有強東風帶產生。為瞭解受複雜地形作用引起的大氣結構變化，以及其所影響的污染物傳遞過程等，在山區觀測資訊缺乏的情況下，此研究藉由高解析氣象模式，以及少數地面測站、探空觀測等資料，探討埔里山區日夜局部環流結構與邊界層發展特性。　　研究方法使用Weather Research Forecast (WRF)氣象模式進行模擬，分別討論不同水平網格解析度與地形資料使用下之氣象場結構，模擬結果顯示：(1)提高模式網格解析度，於晝夜皆呈現模擬環境風場較粗網格解析度弱的情形，改善模擬風速的高估，並且近地表溫度的模擬，與觀測溫度之關係性可達到「高度相關」。(2)使用National Aeronautics and Space Administration所發布的Shuttle Radar Topography Mission -90m解析度地形資料，可對於臺灣山區地形有更細微且準確的描述，並增進模式對山區局部熱力環流結構特性的呈現。(3)利用90m高解析地形資料進行WRF-600m氣象模擬，發現白天熱力環流受山脈地形影響，海風過山後加速，並與上坡風及谷風結合，增強盆地內的西風分量；夜間地表冷卻產生穩定邊界層結構，同時山風挾帶冷空氣塊下降至埔里盆地，並在重力作用下，於穩定邊界層上方發生東風加速的特性，形成夜間東風噴流。　　考慮到當模式網格解析度選擇在0.1至1km之間時，將面臨邊界層參數化方案的Gray-Zone問題，因此進一步使用Shin-Hong scale aware scheme，根據模式網格大小，計算對流邊界層內次網格相對於所有紊流通量之占比，瞭解所需參數化的紊流比例，並改進對流邊界層內紊流熱通量傳遞的計算方式。與Yonsei University scheme模擬結果比較後顯示，尤其能增進模式於白天對流邊界層內的描述，垂直混合作用的減弱使得整體大氣溫度降低，改善模擬的暖偏差，同時水平風場因更多紊流能量直接被模式解析而不受邊界層參數化的估計所限制，於底層大氣能量增加，導致底層風速增強，進而於盆地上方更突顯探空觀測的底層西風特徵。 ;Puli basin is located in the central mountainous area in Taiwan, the surrounding terrain height is about 800 to 2000 m. Under a weak synoptic weather system, the thermally driven circulations such as the upslope/downslope wind and mountain/valley wind generally develop and dominate the local airflow in Puli basin. 　　The observed sounding data indicated some characteristics of atmospheric structure. During the daytime, there is a strong westerly wind preduced by the thermal heating in the near surface layer and upper level over Puli basin. During the nighttime, the radiation cooling causes the nighttime inversion layer in the lower level and forms the nighttime stable layer (SBL). Strong easterly wind is also observed above SBL. 　　However, it is difficult to accurately simulate the development of atmospheric motions over the mountainous area. In order to understand the characteristics of the planetary boundary layer (PBL) structure and the local circulations over the complex terrain in Taiwan, the Weather Research and Forecasting (WRF) Model at a fine resolution (600-m) was applied with the high resolution terrain data. Simulation results were evaluated with the observed sounding data and surface stations to characterize the PBL structure and local airflows over the central mountainous area of Taiwan. 　　The model performance is better represented at fine scale resolution than the coarse resolution. The ambient wind speed decreases significantly, and it reduces the overestimation of the surface wind speed in the whole Taiwan area. The performance is not only good at the wind field but also good with a correlation coefficient of temperature at 0.9 (highly correlated). The use of the high resolution topographical data from Shuttle Radar Topography Mission which was published by National Aeronautics and Space Administration enhances the WRF simulation performance, particularly over the complex terrain. And it also helps characterize the PBL structures and local air flow in Puli basin. In the morning, the nocturnal stable boundary layer (SBL) disappears and the wind speed remains low due to the divergence induced by developing upslope wind. In the afternoon, atmosphere become well-mixed and the westerly flow which is composed of the sea breeze and up-valley wind prevailing over the basin. During the nighttime, there is strong easterly wind above the nocturnal SBL by the downslope wind. 　　In addition, two different PBL scheme (Yonsei University, YSU and Shin-Hong scale-aware, SH) were applied to study the influence of the PBL physical processes on the simulated vertical structures over the complex terrain in Taiwan. The main difference between the two PBL schemes is the algorithm for nonlocal PBL parameterization and the scale dependency of the subgrid-scale transport. The SH PBL scheme considers a more accurate nonlocal heat flux profile and multiplies the grid-size dependency function with the vertical transport term which is found in large-eddy simulation. By using SH PBL scheme, the resolved motion can be improved and the simulated convection structure can be maintained at the gray zone resolution. The results from SH PBL scheme show the better simulation of the theta and wind speed profile compare to the YSU scheme, and highlight the characteristics of observation such as a strong westerly wind in the near surface in the afternoon.
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