||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.
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