中文摘要：

通过WRFV2．1．2模式数值模拟试验并结合长期观测数据，研究了中国西北半干旱区长期存在和维持的森林山区（兴隆山区，103．84°E、35．86°N）的降水特征及其与周边地区的降水差异，并探讨了造成这种差异的主要原因。结果表明，兴隆山区与周边地区的降水差异主要表现在夏、秋季。在夏、秋季兴隆山区受东南湿润气流的影响，获得较多的水汽输入和较稳定的水汽来源，而山地地形则有利于截留东南气流携带的水汽并形成降水；兴隆山区及其周边地区局地的蒸散差异对二者之间降水差异的贡献不大。另外，兴隆山区土壤堆积覆盖的石质山构造和森林下垫面也有利于降水的截留和贮存以及植被的生长。因此，有利于水汽输入的大尺度环流形势、地形对空中水汽的截留以及特殊的地质因素是兴隆山山区孤立森林岛在半干旱区长期存在和维持的原因。

英文摘要：

Using a numerical model and the long Lc between an isolated mountain forest （Xinglong, 103.84°E, 35.86°N） and its surrounding area over the ar- id semi-arid region of Northwest China is studied in order to understand the mechanisms for the persist- ence of the isolated forest. The results indicate that the precipitation differences between Xinglong Moun- tain and its surrounding area are the largest in summer and autumn. The Xinglong Mountain area is domi- nated by the southeastern wet {low in summer and autumn and consequently a stable water vapor source. In addition, the meso-scale high mountainous terrain blocks the water vapor from the southeastern flow and enhances prempitation. The contribution of local evapotranspiration to the precipitation is less than that o{ advection effects of water vapor. On the other hand, the rock tectonics of Xinglong Mountain cov- ered by soil and forest also help to hold and store water for the growth of plants. Therefore, the persist- ence of the forest island of Xinglong Mountain in semi-arid regions is the consequence of the large-scale cir- culation, which transports more water vapor, the orography and the special geology which tend to help and store more water vapor for this area.