中文摘要：

对2005年6月一次引发珠江（西江）流域致洪暴雨的环流特征和影响系统，尤其是β中尺度系统的环境场进行了研究。这次暴雨具有明显的多尺度特征，涉及到季风的活动、边界层的影响、地形的动力作用等。得到如下结果：（1）南海季风爆发推迟而后持续影响华南，西风带急流以及副热带高压位置均明显偏南，且副高脊线稳定维持在18°N左右，为6月中下旬暴雨在华南地区持续发生提供了稳定的大尺度环流条件。（2）这是一类华南前汛期锋前暖区暴雨。在该时段，华南地区始终维持一条近于东西向的准静止锋，这在卫星云图上清晰可见。锋生函数的计算结果表明，弱冷空气的作用明显。经向剖面分析揭示出：锋前有明显的上升运动区存在，非常有利于中尺度对流的发生发展。北方的弱冷空气与副高西北的暖湿气流之间存在明显的中低纬度系统的相互作用。（3）低空偏南气流的风速辐合场，在925hPa层上最明显。表明在这次华南暴雨过程中行星边界层有很重要的作用。不但从中南半岛和南中国海输送了大量水汽至暴雨区，而且其前方的风速强辐合区，对于对流的启动很有帮助。就启动机制而言，该风速辐合区可能与1998年华南暴雨的风向辐合区有一些差异。（4）南岭山脉的喇叭口地形，对偏南暖湿气流有明显的辐合抬升作用，使垂直运动进一步增强，有利于暴雨的发生。华南（尤其是西江流域）的特殊地形是持续性暴雨产生的重要条件之一。（5）中尺度对流雨团是这次致洪暴雨的直接影响系统。至少有4个口中尺度雨团（A、B、C、D）在西江流域发生发展。上述雨团A和雨团D分别出现于桂林和梧卅I等测站附近。该研究集中对中尺度雨团A、D的环境特征作了分析。低空风速辐合中心反复向暴雨区“冲击”，暴雨区对流有效位能一次次积

英文摘要：

The circulation characteristics and the influencing systems of the heavy rainfall causing flood in the Pearl River （the Xijiang River） valley in June 2005 are investigated. The heavy rainfall had the outstanding characteristics of multi scale systems and it occured in warm sector ahead of the front, relating to monsoon activity, the influence of the planetary boundary layer and the dynamic lifting of the topography. The conclusions are as follows： （1） The late onset of the South China Sea monsoon and its continuous impact on the South China, as well as the further south position of the uppe〉level jet in the westerlies and the subtropical high, which maintained in 18°N, provided the very favorable large-scale circulations for sustaining the heavy rainfalls in South China. （2） There were the heavy rainfalls happening in warm sector ahead of the front in South China during the pre-rainy season. There always existed an approximately east to-west oriented quasbstationary front in South China. The calculated results of the frontogenesis function indicate that the weaker cold air played an important role in occurrence of heavy rainfall. The analysis of the vertical cross section of horizontal wind and vertical velocity indicates that there was ascending motion obviously ahead of the front, which was very favorable for the formation and development of the mescrscale convection. The weaker cold air from the north interacted with the warm and wet air coming along the southwest edge of the subtropical high, there is a kind of interaction between middle- and low latitude systems. （3） The wind speed convergence of south-west wind at low level was the most obvious at 925 hPa. It indicates that the planetary boundary level played an very important role in the formation of the heavy rainfall, the current not only transported sufficient water vapor to the heavy rainfall area from the Sino-Indian Peninsula and the South China Sea, but also contributed to the initiation of the convection. （4） The trumpet sh