1998及1999年乌山阻高突变对长江中下游大暴雨过程的影响

The Influence of Sudden Change of Ural Blocking High on HeavyRainstorm Processes in the Middle-Lower Reaches of YangtzeRiver in JuneJuly of 1998 and 1999

1998年和 1999年在长江中下游地区发生了特大暴雨 ,为了了解这两年长江中下游地区强暴雨系统的大尺度环流背景和寻找发生大暴雨的前兆因子 ,我们分析研究了乌拉尔山地区阻塞高压的逐日变化对长江中下游地区强暴雨过程的影响。本文利用 1998年 6月、 7月和 1999年 6月逐日12 :0 0 (世界时 ,下同 ) 5 0 0hPa高度场和乌拉尔山及附近地区 (4 5°6 5°N ,30°10 0°E)的阻塞高压强度指数 (UBHII)资料。重点分析研究了这两年长江中下游地区大暴雨发生前后的UBHII和环流的逐日变化特征 ,发现这两年持续性大暴雨过程均发生在乌山阻高从强高峰向低谷下降期间 ,即在长江中下游地区发生大暴雨过程之前乌山阻高就先有一个突变。若欧亚环流比较稳定 ,则长江中下游地区降水较少。这个研究结果说明乌山阻高从异常加强期向减弱期转折这一特征是 1998及 1999年长江中下游地区将要发生特大暴雨过程的重要前兆之一。文中还对 1998年 7月下旬和 1999年 6月下旬欧亚地区逐日 5 0 0hPa高度的环流演变过程作了分析探讨 ,特大暴雨的产生必须同时有强冷空气与强暖湿气流的相互作用 ,在乌山阻高开始建立和加强期 ,亚洲上空环流比较稳定 ,长江中下游上空缺乏冷暖气流的剧烈活动 ,所以没有明显的降水过程。而乌山强阻高开始减弱是强?

The heavy rainstorm processes occurred in the middle-lower reaches of Yangtze River in 1998 and 1999. To understand the large-scale circulation background of the heavy rainstorm system in the middle-lower reaches of Yangtze River in 1998 and 1999, and explore the precursor factors of producing heavy rainstorm, we analyzed and studied that the day-to-day changes of Ural Blocking High (UBH) and show they have a great influence on the heavy rainstorm processes in the middle-lower reaches of Yangtze River. The daily 12:00UTC UBH intensity indexes (UBHII) at 500 hPa over the area 45°~65°N, 30°~100°E in June^July 1998 and 1999 are calculated by using the observed data. The UBHII before and after the heavy rain in the middle-lower reaches of Yangtze River in recent two years and the daily circulation change feature are mainly analyzed. We find that the persistent heavy rainstorm processes in the middle-lower reacher of Yangtze River in 1998 and 1999 occurred usually in the descending periods of the daily UBHII. When the UBH sets up and intensifies, Asian-European circulation is stable, there is a little rainfall in the middle-lower reaches of Yangtze River. The result shows that the abrupt turnning of UBH changing from devoloping period into weaken period is one of the precursor factors producing heavy rain in the middle-lower reaches of Yangtze River. The day-to-day evolution processes of height field at 500 hPa and its mechanism are also discussed in this paper.