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2020年“6·26”四川冕宁暴雨降水物理过程模拟诊断研究

Simulation and Analysis of the Physical Processes Underlying the Heavy Rainfall in Mianning on June 26, 2020

  • 摘要: 利用WRF模式,借助三维降水诊断方程和降水效率定义,针对2020年6月26日四川冕宁一次突发性暴雨过程开展高分辨率数值模拟诊断研究,从不同强度降水的垂直动力结构、水凝物分布及各种宏微观过程对降水的贡献等角度进行对比分析,进一步揭示复杂地形区强降水过程中与水物质(水汽和各种水凝物)相关的宏微观特征。结果表明,此次冕宁暴雨过程可分为两个阶段:第一阶段(6月26日18:00~22:00;北京时,下同),强降水区上升运动强度最大,在水汽辐合达最强的同时,促使较弱降水区的液相水凝物向强降水区辐合,强降水区收集的丰富液相水凝物一部分转化为冰相水凝物供应云系增长(此时>35 dBZ的对流性回波伸展高度最高),另一部分转化为强降水降至地面,造成第一阶段的强降水发生;第二阶段(26日23:00至27日01:00),强降水区的局地上升运动有所减弱,导致强降水区液相水凝物辐合减弱,但仍保持强烈的水汽辐合,上升运动极值中心有所降低,有助于云、雨滴碰并和水汽凝结等过程发生,上升运动呈现双极值分布,回波强度在垂直方向表现为强—弱—强的结构,高层云中的冰相粒子对低层云系具有一定播撒效应,有助于液相水凝物高效转化为强降水。不同降水强度间的降水效率差异显著,变化范围为5%~70%,越大的降水效率对应越强的地面降水。

     

    Abstract: Herein, using a high-resolution simulation produced by the weather research and forecasting model, along with a three-dimensional precipitation equation and the concept of precipitation efficiency, a heavy rainfall event that occurred in Mianning on June 26, 2020, is studied. The vertical dynamic structure, hydrometeor distribution, and various physical processes involved in precipitation are compared and analyzed. The characteristics related to water substances (i.e., water vapor, rain, snow, hail, graupel, ice, and liquid cloud) in complex terrain areas are further revealed. Results indicate that the rainstorm process can be divided into two stages. In the first stage, spanning from 1800 BJT to 2200 BJT (Beijing time) on June 26, 2020, the intense uplift within the strong precipitation area promotes the convergence of water vapor and liquid-phase hydrometeors from the weak precipitation area. The abundant liquid-phase hydrometeors collected in the strong precipitation area convert into heavy rainfall and ice-phase hydrometeors to induce cloud growth. At this time, a strong convective echo exhibiting a reflectivity of >35 dBZ reaches the maximum height. In the second stage, spanning from 2300 BJT 26 to 0100 BJT 27 June 2020, the uplift in the strong precipitation area weakens, leading to a reduction in the convergence of liquid-phase hydrometeors, but. However, the convergence of water vapor remains strong, and the location of maximum ascending motion descended, which facilitates the condensation of water vapor as well as the collision between clouds and raindrops. Two maximum centers of ascending motion are observed in the vertical direction. The echo intensity displays a strong–weak–strong pattern. Ice particles in the upper atmosphere likely serve as seeds for lower clouds, aiding the conversion of liquid-phase hydrometeors into heavy rainfall. Furthermore, the precipitation efficiency varies considerably across different rainfall intensity ranges, from 5% to 70%; the greater the precipitation efficiency, the stronger the surface precipitation.

     

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