江淮流域夏季极端降水过程中天气尺度扰动结构特征及形成机制的研究

The prediction of summer extreme rainfall over the Yangtze-Huai River Basin is always the hot topic of weather forecasters and scientists. The case study reveals that the midlatitude synoptic-time scale (3-8-day) disturbances are the key systems responsible for the occurrences of those extreme rainfall events. However, these disturbances were mostly removed in previous extreme rainfall studies because they were regarded as high frequency noises. These situations inspire the ideas for this project. Based on 29-year rainfall and circulation data, this project will try to investigate the following issues from the aspect of climate statistics. First, the propagation tracks and three dimentional structural characteristics of the synoptic-time scale disturbances associated with the Yangtze-Huai River Basin summer extreme rainfall will be explored. Then, the influence of these disturbances on summer extreme rainfall events over the Yangtze-Huai River Basin will be studied in detail, including the interactions between disturbance and Meiyu front, as well as the moisture transportation contribution from disturbances to extreme rainfall. Furthermore, the roles of East Asian large-scale background circulation systems and the Tibetan Plateau in regulating the synoptic-time scale disturbances' origins and energy dispersions will also be investigated. The successful implementing of this project will enhance the understanding of midlatitude synoptic-scale disturbances and provide new references for the short-term summer rainfall forecast over the Yangtze-Huai River Basin.

江淮流域夏季的极端降水预报一直是国内外气象界关注的热点问题。个例分析表明，中纬度大气环流中时间周期为3-8天的天气尺度扰动是造成江淮流域夏季极端降水过程的关键系统。然而，它们在江淮流域强降水事件的已有研究中往往被作为高频噪声滤掉了。因此，本项目拟以已有个例研究为基础，利用29年的降水和环流资料，在国际上首次从气候统计的角度，研究江淮流域夏季极端降水过程中天气尺度扰动演变特征和三维空间结构，天气尺度扰动对江淮流域夏季极端降水事件形成和发展的作用，以及东亚大尺度环流系统和青藏高原对天气尺度扰动生成发展和传播过程的影响。项目的成功实施将增进我们对中纬度天气尺度扰动的认识，并为江淮流域夏季短期降水预报提供新的参考依据。

江淮流域夏季极端降水事件频发，是多种频次扰动作用的结果。本项目基于29年夏季平均的日降水率及降水标准偏差的空间分布，将江淮流域分为南、北两个子区域，揭示出高频天气尺度扰动在江淮流域北部极端降水事件形成过程中起绝对主导作用，而在江淮流域南部，高频、中等频次、低频扰动的作用相当。进一步以天气尺度扰动作为研究对象，根据对流层低层扰动发展传播特征将南、北子区域的扰动分为三类：1. 起源于青藏高原东侧、 伴有气旋-反气旋式波列的气旋式扰动；2. 起源于青藏高原东侧的气旋式扰动；3. 起源于中高纬度地区的气旋式扰动。通过对扰动的追踪分析、三维剖面分析，结合原始环流场等变量的合成分析，揭示出上述三种扰动生成发展和传播的主要决定因素：第一和第三类气旋式扰动的发展传播由对流层高层中纬度东传的罗斯贝波列决定；第二类扰动则由对流层中层东传发展的低槽、对流层高层东传发展的副热带西风急流决定。上述所有类型的扰动中，西太平洋副热带高压的位置决定了降水在两个子区域的落区，扰动初期降水的潜热释放导致对流层低层气旋式扰动增强，最终与对流层高层反气旋式扰动形成强大的斜压系统，使得降水达到最强。上述成果厘清了造成江淮流域极端降水事件天气尺度扰动的源区和发展机制，为江淮流域夏季短期降水预报提供新的参考依据。