爬流和绕流对山地突发性暴雨的影响

利用ERA5再分析资料和融合降水数据,针对2018年5月21-22日发生在中国四川盆地西南部的一次山地突发性暴雨,首先对其降水强度和天气概况进行相关分析,并且通过绕流和爬流方程,将流场分解为绕流和爬流分量,重点探讨地形对于过山气流的影响及其对降水的作用。研究表明:受欧亚中高纬低槽槽后西北气流引导冷空气南下和西南低涡东移的共同影响,在四川盆地西南部山区发生了一次强降水过程。此次降水范围较广、强度大并且降水时间集中,是一次典型的山地突发性暴雨事件。由于地形的阻挡作用,使得来自东北方向的气流发生旋转,产生绕流运动,在盆地内形成局地涡旋。同时盆地和盆周山地之间的地形高度差强迫过山气流产生爬流运动,导致垂直上升运动加强。在绕流与爬流的共同作用下,为此次突发性暴雨的发生发展提供了有利的流场条件。进一步分析得出地形区域内爬流分量略大于绕流分量,即气流对于山地屏障的地形适应以爬流运动为主,绕流运动次之,地形爬流产生的垂直上升运动与雨带的分布密切相关。

Impact of Flow around and Flow over in Sudden Rainstorm on Mountains

Using ERA5 reanalysis data and CMORPH data,for a sudden rainstorm that occurred in the southwestern Sichuan Basin in China from May 21 to 22,2018,first the correlation analysis of its rainfall intensity and weather profile was carried out,and then the flow field was decomposed into flow around and flow over components through the flows around and over equations,further investigating the influence of flow around and flow over caused by terrain on rainstorm. The results indicate that due to the combined effects of the northwest airflow guiding the cold air southward and southwest vortex eastward,a heavy rainfall process occurred in the mountainous area in the southwest of the Sichuan Basin. This precipitation had a wide range,high intensity and concentrated precipitation time,which was a typical sudden mountain rainstorm event. Due to the blocking effect of the terrain,the airflow from the northeast direction rotates,generating the flow around and forming a local vortex in the basin. At the same time,the terrain height difference between the basin and the mountain forced the over-the-air flow to generate the flow over,resulting in increased vertical ascent. Under the combined effects of flow around and flow over,it provided favorable flow field conditions for the occurrence and development of this sudden rainstorm. Further analysis showed that the flow over component in the terrain area was slightly larger than the flow around component,that is,the response of the airflow to terrain at such altitudes as mountainous terrain was dominated by flow over and secondary flow around,and the vertical ascending motion caused by flow over was closely related to the distribution of rain belts.