Abstract: A mesoscale convective system (MCS) which happened around Shangdong Province in the middle spring 2003 induced heavy rainfall. In the unstable large scale atmosphere, trigger conditions determine when and where the convections come into being. However, these conditions lie in boundary level above ground with small scales, and the general surface and rawinsonde data can not reveal them. A method of numerical simulation and sensitivity simulation together with objective analysis are conducted, and its trigger mechanism can be diagnosed. The results of diagnosis indicate that two developments of MCS bring up this heavy rain. The first can be called a typical Huabei MCS, whose initial convections are arisen by terrain forced climb; the second initial convections are arisen by boundary convergence lines in the afternoon, meantime the lower level cold air from Huabei Plain also contributes to their formation. So the maximum convective available potential energy (MCAPE) is able to give right information on where the initial convections will occur.

Key words: heavy rainfall, numerical simulation, diagnostic analysis, convective available potential energy, boundary convergence line, MCC

摘要： 2003 年 4 月中旬发生在山东一带的中尺度对流系统引发了一次罕见的春季大暴雨。在不稳定的大尺度环境中，触发条件决定了对流系统在何时、何地发生。触发条件通常位于近地面层中，水平尺度也很小，常规资料无法揭示出来。为此，采用数值模拟和数值模拟敏感性试验并结合客观分析对它的发生机理进行了诊断。结果表明，此次暴雨是由 MCS（中尺度对流系统）的两次发展造成的。第 1 次是比较典型的华北 MCC（中尺度对流复合体）过程，它是由地形强迫抬升触发了初始对流。第 2 次初始对流是由午后的边界层辐合线触发的，从华北平原南下的浅薄冷空气也起了重要作用。另外，最大对流有效位能（MCAPE）对初始对流的发生位置有很好的指示意义。

关键词: 暴雨, 数值模拟, 诊断分析, 对流有效位能, 边界层幅合线, MCC