Abstract: The horizontal filtering technique is employed to separate meso-scale field from large scale field for a case of heavy precipation over Yangzi river occurred on 1-5 July 1991. The northern large-scale jet stream has a maximum velocity center exceeding 50 m/s on 200 hPa. Meanwhile, the southern jet stream corresponding to the large-scale precipitation area, has a maximum wind velocity on 500 hPa exceeding 20 m/s. It is also found that when the meso-scale center of high field and wind vector fieldare nearly the same, a heavy rain center is always located. Finally, the contribution term to the development of meso-scale low on 700 hPa at different levels are evaluated. It is found that 700 hPa term is the biggest, while 500 hPa and 200 hPa terms have the same magnitude. They have same variation step.

Key words: smoothing filter, large-scale background field, meso-scale system

摘要： 利用水平平滑滤波方法对不同等压面上大中尺度场的分析结果表明：在1991年7月1—5日的暴雨过程中，从高层到低层青藏高原北部有一急流中心，急流中心位于40°N附近。和这次暴雨过程直接对应的急流中心位于29°N附近。1—5日这两个急流中心均稳定少动且中心风速比常年大。大范围降水区中的强暴雨中心和中低层的流场、风场辐合区以及扰动低压中心吻合较好。低层等压面上大中尺度系统相互作用项的区域平均值对700hPa中尺度系统发展的贡献最大，高层200hPa和中层500hPa贡献数值接近，高中低层大中尺度系统相互作用项对700hPa中尺度系统发展的贡献具有同步性。

关键词: 平滑滤波, 大尺度背景场, 中尺度α系统