地形產生之低層水氣輻合效應與逆溫強度對上坡霧的控制:溪頭個案研究

Abstract

山區雲霧森林為經常雲霧繚繞的山地森林，在此環境下雲水及霧水可對生態系統提供額外的水文收支，此為山區雲霧森林一個重要且獨特的特性。本研究中透過雲冪儀的觀測以及理想數值模擬，針對溪頭谷地中地形產生之低層水氣輻合及成霧過程進行分析，此為首次以高解析度雲解析模式對溪頭地區進行模擬，嘗試了解起霧過程有關的局地環流之研究。觀測分析顯示雲冪儀觀測資料不但適合作為地面起霧與否之判定，亦可獲得更多低雲雲底高度變化的訊息。在2016年1月7日的起霧個案中，雲冪儀觀測顯示溪頭山谷的起霧過程乃是先有低層雲形成後，雲底逐漸下降至地面而形成霧的現象，而此一雲底降低的過程亦與溪頭谷地谷風的發展有所關連。為了進一步了解與起霧相關之水氣傳送過程，本研究使用具有高解析度(500 m)臺灣真實地表狀況與地形高度資料之渦度向量方程雲解析模式(TaiwanVVM)進行理想數值模擬，以探討溪頭山谷局地環流對於起霧過程之影響。數值模擬結果顯示，谷風沿著谷底上升之上坡風與其前緣所引發渦流均是溼化山谷邊界層進而導致山谷起霧的主要局地過程。而透過敏感度測試則進一步發現，溪頭山谷局地起霧持續時間長短受到了綜觀尺度環境的逆溫強度所控制。研究結果顯示，對於溪頭山谷的水氣供應而言，最主要是透過地形引起的低層水氣輻合效應來提供，而山谷邊界層上方的逆溫層覆蓋，則是限制了山谷內對流發展的高度，造成將水氣留存在山谷邊界層中的效果，故當山谷上方覆蓋的逆溫強度較強時，溪頭山谷內起霧的持續時間也會較長。

Montane cloud forests (MCF) are characterized by forests that are frequently immersed in clouds or fog so that the interception of cloud/fog water provides extra hydrological input to this ecosystem. In this study, we examine the effects of orographically induced moisture convergence and the fog formation at Xitou valley of Taiwan by ceilometer observation and idealized cloud-resolving simulations. This work is the first attempt to understand the local circulation associated with fog at Xitou using a high-resolution cloud-resolving model. Observation analysis shows that the ceilometer is not only reliable to detect fog occurrence but also provides more information about low-level cloud base evolutions. In a fog case on Jan. 7th, 2016, the low-level cloud base lowering is observed before fog formation on the valley surface, which is also associated with the valley winds at Xitou valley. To understand the processes of the moisture transport associated with the fog formation, we perform idealized simulations using high-resolution vector vorticity equation cloud-resolving model (TaiwanVVM) with realistic land surface processes to evaluate the local circulation associated with the fog development. The simulations indicate that both the upslope winds and the turbulent eddies initiated by the upslope winds are primary local processes to moisten the boundary layer in the valley which leads to fog formation at Xitou. Sensitivity experiments show that local fog duration is controlled by synoptic temperature inversion strength. The results suggest that the effect of orographically induced low-level moisture convergence is the essential process to supply moisture in the Xitou valley, and the capping inversion helps the fog formation by limiting the development of convection and preserves moisture in the valley. The sensitivity experiments also suggest that the fog duration is longer with a stronger temperature inversion.