梅雨雨带北跳过程研究

A Study on Northward Jump of the Meiyu Rainbelt

利用19792007年逐日再分析资料和高分辨率逐日降水资料，通过定义确定了每年梅雨雨带北跳的日期，对梅雨雨带的北跳过程及其可能的物理机制进行了研究。分析结果表明：梅雨雨带北跳日期存在明显的年际变化，本文合成得到的雨带北跳过程与前人的工作相一致。水汽输送的变化和对流层中层的垂直运动是影响梅雨雨带位置分布的关键因素。Omega方程诊断结果表明，在梅雨雨带北跳前期，对流层高层的环流异常导致江淮流域出现异常下沉运动，不利于梅雨雨带的北跳；而涡度方程的诊断结果表明，江淮流域的异常下沉运动导致的非绝热冷却在中国东部的对流层低层引起异常反气旋涡度倾向，有利于副热带高压西伸，从而有利于梅雨雨带的北跳。因此，当对流层高层环流发生变化（主要受纬向涡度平流影响），使得江淮流域的异常下沉运动转变为异常上升运动时，高低层相互配合，造成了梅雨雨带的突然北跳。

This study has exploited daily reanalysis data and high-resolution precipitation data for the period of 1979 to 2007 to investigate the northward jump of the Meiyu rainbelt and its possible physical mechanisms. The northward jump dates of the Meiyu rainbelt are determined. Results show that the northward jump dates of the Meiyu rainbelt vary greatly from year to year. Composite results of the Meiyu northward jump process revealed in this study are consistent with results of previous studies. Water vapor transport and vertical motions in the middle troposphere play a crucial role in determining the spatial distribution of Meiyu rainbelt. Diagnostic results of the Omega equation indicate that anomalous circulations in the upper troposphere induce descending motion anomalies over the Yangtze-Huaihe River basin, which inhibit the northward jump of the Meiyu rainbelt. Meanwhile, diagnostic results of a vorticity equation suggest that the anomalous diabatic cooling associated with the anomalous descending motions over the Yangtze-Huaihe River basin produces anomalous anticyclonic vorticity tendency over East China in the lower troposphere, which is favorable for the westward shift of the subtropical high and the northward jump of the Meiyu rainbelt. The above results show clearly that the sudden northward jump of the Meiyu rainbelt is attributed to anomalous ascending motions over the Yangtze-Huaihe River basin, which are associated with circulation anomalies in the upper troposphere （primarily driven by zonal vorticity advection）.