山谷邊界層之觀測與模擬

Abstract

山谷邊界層氣象與一般平地不同，除了普遍的山谷風現象之外，複雜的地形也會造成風場變形，對邊界層的發展有相當大的影響。過去研究文獻指出，山谷地區除了熱力效應產生的山谷風環流之外，環境的盛行風也會影響山谷內的流場，但影響的程度卻遠不及山谷風。本研究以實際觀測和時空高解析度數值模式的風場模擬，來呈現臺灣中部山區臺灣大學實驗林溪頭地區(海拔1000m)之山谷邊界層氣象和流場特徵。 觀測結果顯示，不論白天或夜間，溪頭地區的垂直大氣剖面都與一般平地相似，但是混合層厚度較薄。當地農業氣象站和通量塔以及無線電探空儀皆顯示溪頭地區相當明顯的山谷風現象，並且發現山谷風侷限於山脊線(海拔2000公尺)以下高度，山谷高度外的風場則漸漸趨向盛行風。40m高度通量塔的溫度和風場垂直觀測，則顯示樹冠層高度是近地層的明顯邊界。 風場的電腦模擬部分，本研究以日本Cradle公司的STREAM Computatonal Fluid Dynamics model，模擬風場經過溪頭山谷實際地形的結果，並以穩定的北風(谷風)和兩次實際無線電探空的風場做為模式的風場邊界條件，以及植入40 m解析度數位地形模型資料進行模擬；結果顯示，開口朝向北方的溪頭山谷對於西風或西南風的盛行風來說，由於其位於綜觀環境風場的背風側位置，山谷內的風速偏低；換言之，盛行風對於山谷內的風場影響很小，並且我們不論從觀測和電腦模擬都發現山脊高度區隔風場的現象，因此山谷周遭的山脈高度也就是溪頭山谷邊界層的高度。除此之外，高解析度地形和時間之電腦模擬結果，合理呈現山谷複雜地形內三維小尺度區域的風場特徵。

The characteristics of atmospheric boundary layer in valleys differs from the same issue over plane terrain. Not only mountain-valley wind but also the deformation of wind field play a role in the development of boundary layer circulation. Some literatures showed that, in addition to mountain-valley wind circulation which is caused by the thermal effect, the prevailing winds have little impact on the wind flow in the valley. Through in-situ observation and high-resolution spatial-time numerical simulation, this research presents the phenomenon of valley boundary layer and the characteristics of wind flow in Xitou valley, which locates in the middle Taiwan with near 1000-m altitude and is part of the Experimental Forest, College of Bio-Resources and Agriculture, National Taiwan University. The in-situ observation showed that, except for the thinner mixed layer, the vertical atmospheric profile in Xitou is similar to that in a general plane. Obvious mountain-valley wind can be observed from local agricultural weather station、40-m height flux tower and the balloon radiosondes. It was also found the mountain-valley wind can only blow under the height of mountain ridges (~2000m MSL), and the wind would approach to prevailing winds above the mountain ridges instead. The temperature and wind field observation of 40m-height flux tower indicate the top of canopy is the critical level near surface. In computational simulations, this study uses STREAM Computational Fluid Dynamics model provide by Cradle Corp. of Japan, to simulate the wind field passing over the real topography in Xitou. In model settings, one stable northerly wind and two real vertical profiles of wind flow from radiosondes’ measurement were set as the boundary condition of wind. The 40m-resolution topography was also imported in these simulations. The results indicated that the northerly V-shape Xitou valley has small wind speed all the time which is caused by the leeside of the mountains for synoptic winds (westerly or west-southerly wind) at Taiwan. In other words, the prevailing winds have little impact on the wind field of Xitou valley. We also found that the wind field was separated by the height of mountain ridges in observation and simulations similarly. Obviously, the height of boundary layer of Xitou valley is decided by the topographic factor. In addition, the numerical simulations with the high-resolution topography gave reasonable results on the characteristics of 3D wind flow in this small-scale valley.