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.