The Impact of Vertical Wind Shear on the Intensity and Organizational Mode of Mesoscale Convective Systems Using Numerical Experiments
摘要: 采用我国实际观测的探空作为中尺度模式Weather Research and Forecasting(WRF)的理想试验的背景场,分别改变整层、低层和中层的垂直风切变,研究其对中尺度对流系统强度和组织结构的影响。结果表明,改变整层垂直风切变对对流系统的强度和组织结构影响最显著,增加整层垂直风切变,对流强度增强且易组织成线状,减小整层垂直风切变,对流强度弱且呈分散状态。从垂直速度、水平风场、散度场和冷池的三维结构特征分析了其影响的机制:(1)风切变增加,上升气流与下沉气流的相互干扰减弱,有利于垂直速度的维持和增强;(2)垂直风切变增加造成水平涡度增加,扭转项的作用分别使上升和下沉运动得到加强;(3)垂直风切变增加,冷池强度和高度增加且集中在系统后部,使系统线状组织性增强。研究还发现,增加垂直风切变造成近地面大风和降水增强,且强降水出现在大风之后,这主要是因为在对流发展阶段上升运动与下沉运动互不干扰情况下,强下沉运动造成的近地面大风,而成熟阶段上升运动不断增强或维持造成雨水比湿不断增加形成强降水。Abstract: Weather Research and Forecasting(WRF) model numerical simulations were used to study the impact of vertical wind shear-including the entire, low-level, and mid-level shear-on mesoscale convective systems.The initial background field of the WRF idealized simulation was the observed radiosonde data in China that produced severe convective systems.The results demonstrated that the simulations of changing vertical wind shear in the entire layer had the greatest impact on the intensity and organizational mode of mesoscale convective systems.For increasing(decreasing) vertical wind shear, the intensity of convection increased(decreased) and the convective systems organized into convective line(decentralized).The mechanisms involved were explored by analyzing the three-dimensional characteristics of vertical velocity, horizontal wind, divergence, and the cold pool, and can be summarized in the following three aspects:(1) The interference between ascending and descending motion weakened as stronger vertical wind shear increased, which led to a longer maintenance and enhancement of vertical velocity.(2) The horizontal vorticity increased with stronger vertical wind shear, and then vertical vorticity increased(decreased) at the ascending(descending) motion under the tilting term in the vorticity equation, which led to intensification of the ascending and descending motion, respectively.(3) The intensity and height of the cold pool increased with stronger vertical wind shear, which led to linear organization of mesoscale convective systems.Other results included the fact that the near-surface wind speed and precipitation amount increased with increased vertical wind shear, and heavy precipitation appeared after the strongest wind.Strong descending motion led to strong surface wind because of the noninterference between ascending and descending motion in the development stage of the mesoscale convective system.During the mature stage, rainwater accumulated because of the constant reinforcement or maintenance of ascending motion leading to heavy rainfall.
- Vertical wind shear /
- Idealized experiment /
- Three-dimensional wind field /
- Cold pool
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