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中国西南部一次东移型暴雨中涡旋发展的多尺度地形影响研究

Influence of Multi-scale Topographic Factors on Vortex Development during an Eastward-Propagating Rainstorm Event in Southwest China

  • 摘要: 由观测和数值模拟结果分析发现,2019年8月5~6日中国西南部的东移型致灾暴雨事件中存在三涡(南北双高原涡、西南涡)相继发展并导致暴雨加强和移动的现象。借助数值试验,研究了多尺度地形因子(青藏高原、横断山脉和四川盆地三大地形)各自对涡旋演变的作用。结果表明,横断山脉对西南涡的形成起关键作用,四川盆地影响着西南涡的位置和强度。对于高原涡(南侧高原涡)的移动,四川盆地地形只影响涡旋强度演变,但不会改变高原涡的移动路径。一旦横断山脉被移除,高原涡的东移现象随之消失。进一步分析青藏高原和四川盆地交界处的陡峭地形坡度改变对涡旋发展的影响发现,发现坡度越陡,高原涡移动速度越快,且盆地内二涡合并后的西南涡强度越强。最后借助于倾斜涡度发展理论,解释了不同坡度对涡旋强度演变的影响:随着坡度变陡,倾斜涡度发展系数沿涡旋下滑路径快速减小,对垂直涡度局地倾向的强迫作用,加剧了涡旋的快速加强。

     

    Abstract: Using observation and numerical simulation results, we reveal that three vortexes, namely the northern plateau vortexes (TPV1), southern plateau vortex (TPV2), and Southwest vortex (SWV), developed successively during a disaster-causing rainstorm event in Southwest China from August 5 to 6, 2019, which led to the intensification and eastward propagation of the rainstorm. Through numerical experiments, we study the effects of multi-scale topographic factors (Tibetan Plateau TP, Hengduan Cordillera HC, and Sichuan Basin SB) on vortex evolution. The results show that HC plays a key role in SWV formation, while SB influences the SWV location and intensity. The topography of the SB only affects the intensity of TPV2 but does not change the propagation path. In the absence of HC, the plateau vortex does not propagate. The influence of slope change of the steep terrain at the boundary between TP and SB on vortex development was further analyzed. The steeper the slope, the faster the propagation speed of the plateau vortex, and the stronger the SWV after the merging of TPV2 and SWV. Finally, the impact of the terrain slope on the evolution of vortex intensity was analyzed according to the theory of slantwise vorticity development. As the slope becomes steeper, the development coefficient of inclined vorticity decreases rapidly along the vortex slide path, and the forcing effect on the local tendency of vertical vorticity intensifies the rapid strengthening of vorticity.

     

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