| 摘要: | 本研究使用Weather Research and Forecasting(WRF)模式，以2008年西南氣流實驗[Southwest Monsoon Experiment(SoWMEX)]探空資料作為模式初始場，模擬颮線通過理想台灣地形時，其結構變化與降雨分佈情形，以及山脈背風處颮線重新生成，並增強的物理過程。吾人並比較依照RKW理論(Rotunno–Klemp–Weisman theory)估算之颮線移動速度與陣風鋒面移速之關係，探討兩種方法估算結果的差異。理想地形實驗包括對稱橢圓山脈、非對稱橢圓山脈以及雙峰山脈三種，探討颮線系統以不同方向通過理想山脈時，理想地形對於颮線結構的改變情形。在本研究中，主要探討的颮線系統移動方向分別為西-東走向和梅雨季常見之西北-東南走向。
在對流系統以西向東走向的實驗中，通過對稱橢圓山脈地形時，颮線兩端氣流及冷池以繞山的形式通過山脈。而在地形最高的中間部分，颮線因受到山脈阻塞作用最大而消散，造成線狀對流斷成南北兩段；當颮線過山水躍發生後，原本消散的中段對流將再次生成。若將理想地形改為非對稱橢圓山脈後，其氣流繞山與系統再次增強的過程與對稱山脈時類似，主要差別在於較平緩的迎風面山坡讓阻塞效果減弱，而背風面下沉抑制增加，造成迎風面及背風面降水皆減少。雙峰地形實驗中，山谷地形造成明顯的通道效應，導致山谷背風面冷池較弱且降雨量降少。若將系統改以西北-東南走向並通過地形時，所有理想地形模擬結果將呈現颮線系統的水平結構及降水分佈上出現南北不對稱分佈；且系統通過地形時的角度改變，造成通道效應的現象變得不明顯。針對使用RKW理論所計算之理論密度流速而言，當颮線過山時其結果會低於陣風鋒面移速，而將系統改為西北-東南走向後，由於地形阻塞作用較少，冷池過山時其理論密度流速隨時間的變化量將減少。; Numerical simulations of a squall line traversing an idealized mountain terrain are performed using the high-resolution Weather Research and Forecasting (WRF) model and the homogeneous base-state environment is taken from a sounding data of Southwest Monsoon Experiment (SoWMEX) in June 2008 to investigate the convection structure, cold pool, redevelopment process, and precipitation fields. The theoretical density current speed is calculated following Rotunno, Klemp, and Weisman (1988). The differences between the density-current speed and gust-front speed are discussed. The idealized simulation includes three kinds of mountain terrain, the symmetric, asymmetric and multiple-ridge terrain, respectively. In this study, the direction of squall line includes eastward-moving and southeastward-moving, which is common in the Mei-Yu season.
The simulation of eastward-moving squall line traverses the symmetric terrain represents that air parcel and cold pool can flow around the mountain. The maximum blocking-effect occurred at mountain ridges, which causes the middle segment of convection dissipated. Thus, squall line is separated into two parts. After squall line traverse mountain ridges, the middle segment of squall lines restrengthen via hydraulic jump process and generate lee vortexes. After the mountain terrain changes from symmetric to asymmetric terrain, a smoother windward slope can leads to more lifting and a steeper slope at lee side causes more adiabatic warming, both lead to weaker cold pool and less precipitation. The simulation of an eastward-moving squall line traversing multiple-ridge yields obvious channel effect, which leads to a weaker cold-pool strength and less precipitation. After the squall line direction change from eastward-moving to southeastward-moving, the system of horizontal structure and precipitation fields represent the asymmetric distribution in the north-south direction. When the eastward-moving system encounters mountain ridges, the comparison of the calculated density current speed shows that it underestimates the gust-front speed. The simulations of the southeastward-moving system represents that the squall line aligns at an angle to the mountain ridge, which leads to a system traverse a smoother slope and then reduce the blocking effect and the variation of cold-pool speed. |
