青藏高原大地形对华南持续性暴雨影响的数值试验

利用新一代中尺度数值预报模式WRF3.2及NCEP/NCAR 逐日4次1°×1°的FNL再分析资料,通过有、无青藏高原以及将高原高度降低到临界高度的数值试验,研究了青藏高原大地形对我国华南地区2010 年5 月一次持续性暴雨过程的影响。试验结果表明:高原大地形对降水的影响显著,随着高原高度的升高,降水增多,高原以东地区的雨带也由北向南移动;高原地形的机械阻挡作用使迎风坡一侧的近地面层附近为强上升运动,背风坡为下沉运动,并分别对应降水的峰值和谷值区;高原对西风气流的爬流、绕流作用明显,高原升高后爬坡作用减弱,以绕流作用为主;高原的加热作用使气流过高原时南支减弱,北支加强,并加强了高原及其东部地区低层的正涡度和高层的负涡度,使高原上空为强烈的上升运动;高原的热力作用使西太平洋副热带高压位置偏南、偏西并稳定维持;高原大地形对形成稳定的高原季风环流圈有重要作用;高原地形高度的作用有利于定常波的形成,波动中心对应强上升运动,形成降水的大值区,稳定维持的定常波使得降水持续集中在同一地区,造成持续性暴雨。

Numerical Experiments on Influence of Tibetan Plateau on Persistent Heavy Rain in South China

Using the next generation of the mesoscale numerical prediction model, WRF3.2, and the NCEP/NCAR four-times-daily FNL reanalysis data (1°×1°), experiments are conducted with or without the Tibetan Plateau and with the plateau's height reduced to a critical height in order to study the influence of the plateau on a persistent heavy rain process that happened in May 2010 in South China. The results show that the plateau significantly affected the precipitation; with the uplift provided by the plateau, precipitation in the eastern area of the plateau increased and the rain belt moved from north to south. The mechanical blocking effect of the plateau produces a strong upward motion in the side of the windward slope's near-surface layer, whereas the leeward slope shows a descending motion; these motions correspond with the peak and valley areas of precipitation, respectively. The effect of the plateau's climb flow and flow around on the westerly flow is obvious; the uplift of the plateau weakens the climbing effect, and makes the airflow mainly with the effect of flow around. The heating effect of the plateau weakens the airflow's southern breach while strengthening the northern breach when it flows over the plateau; it also strengthens the low-level positive vorticity and high-level negative vorticity over the plateau and its eastern region, bringing strong upward motion over the plateau. The plateau's thermal effect on the West Pacific subtropical high is to make its position southerly, westerly, and stable. Further, the plateau plays an important role in forming a stable plateau monsoon circulation cell; the height of the plateau facilitates the formation of stationary waves, the centers of which correspond to strong upward motion where considerable precipitation forms. Because of the stable and long-lasting steady waves, the precipitation continues to be concentrated in the same place, resulting in persistent heavy rain.