地形降水對於環境條件與地形特性之敏感度測試：3維理想地形模擬研究

The Sensitivity of Orographic Precipitation to Ambeint Conditions and Terrain High: An Idealized Three-Dimensional Modeling study

林宥丞(You-Cheng Lin)

指導教授 ： 楊明仁

國立中央大學/地球科學學院/水文與海洋科學研究所/碩士(2008年)

摘要

本研究利用WRF模式模擬氣流在不穩定的天候條件下，氣流跨越3維理想鐘形地形時，地形降水的變化情形。根據不同的Fr數可發現，當氣流遇到3維理想地形時，會出現4種降水移動型態：(1)降水初期於山脊區域產生對流性降水，隨時間其降水會往上游處擴展。(2)長延時的對流降水系統滯留於山脊區域。(3)山脊區域存在長延時之層狀降水與對流降水的混合性降水，模擬後期，山脊區域的降水會些微往上游與山脊的背風處擴展。(4)在山脊區域存在長延時的對流性降水或層狀性降水，亦或者混合性降水，而在下游處具有一獨立的對流性降水系統。
　　當Fr數較小時，地形阻塞效應明顯，在背風處均可發現背風面渦漩的產生，而背風面渦漩會壓抑住山脊區域降水往下游處的擴展。而隨著風速增加，Fr數越大時，氣流逐漸以越山的形式來取代氣流繞山的形式通過地形的阻擋。當改變氣流入射地形之入射角時，氣流遇到地形時產生不對稱的氣流繞山與越山的運動，導致背風面單一渦漩的生成，同時亦改變地形降水的分佈情況。
　　研究中以Fr數來討論地形降水之物理機制，難免有些缺失，未來研究可加入空氣塊軌跡線的變化來討論氣流的偏斜，以及加入地形半幅寬的變化，來討論氣流遇到不同坡度地形時地形降水的差異性，再進一步可加入真實地形與柯氏力的影響來做相同的模擬，以求對台灣地區地形降水的物理特性有更明確的認知。

關鍵字

地形降雨 ； 3維理想地形 ； 氣流過山

並列摘要

In this study, idealized simulations are performed to investigate the propagation and type of precipitation systems for a conditionally unstable flow over a three-dimensional mountain. Four moist regimes based on the unstaturated Froude number, can be identified from a series of idealized simulation. The characteristics of these four flow regimes are : (1)Regime I–flow with an upstream-propagating convective system and an early convective system over the mountain ridge; (2)Regime II–flow with a long-lasting orographic convective system over the mountain ridge, upslope, or lee-side slope; (3)Regime III–flow with a long-lasting mixed convective / stratiform precipitation system over the mountain, and a downstream-propagating convective system at later period; (4)Regime IV –flow with an orographic stratiform, convective or mixed stratiform and convective precipitation system over the mountain, and possibly a convective precipitation system downstream.
When the Froude number is smaller, we can find the lee-side vortex downstream, and this lee-side vortex will brake the precipitation systems propagation. When the Froude number is large, we can find a flow-over regime instead of a flow-around regime. When we change the angle of flow incidence, the airstream will encounter differential flow deflection, and a single lee-side vortex can be found downstream, at the low-Froude-number regime.
Although Froude number serves as a control parameter for the propagation of orographic precipitation, it may not completely represent all flow characteristics. Trajectory analyses can be used to further investigate the flow attributes associated with orographic precipitation.

並列關鍵字

propagation and type of precipitation systems ； flow regimes

參考文獻

Chen, S.-H., and Lin, Y.-L., 2005a: Effect of moist froude number and CAPE on a conditionally unstable flow over a mesoscale mountain ridge. J. Atmos. Sci., 62, 331-350.

Chen, S.-H., and Lin, Y.-L., 2005b: Orographic effects on a conditionally unstable flow over an idealized three-dimensional mesoscale mountain. Meteor. Atmos. Phys., 88, 1-21.

Colle, B. A. 2004: Sensitivity of orographic precipitation to changing ambient conditions and terrain geometries: An idealized modeling perspective. J.Atmos. Sci., 61, 588-606.

Grossman, P. A., and D. R. Durran, 1984: Interaction of low-flow with the Western Ghat Mountains and offshore convection in summer monsoon. Mon. Wea. Rev., 112, 652-672.

Hill, F. F., 1983: The use of average annual rainfall to derive estimates of orographic enhancement of frontal rain over England and Wales for different wind direction. J. Climate., 3, 113-129.