梅雨雨带北跳过程研究

2020年梅雨持续时间长，雨量偏多；暴雨范围广、过程雨量大；雨带南北摆动大，但强降雨落区重叠度高。利用NCEP/NCAR再分析资料，针对行星尺度环流系统演变进行分析发现，梅雨期内，亚欧中高纬维持稳定的阻塞形势，环流经向度大，冷空气活动频繁并向江淮流域渗透。同时西太平洋副高（简称副高）显著偏强偏西，副高引导的水汽向江淮流域输送明显偏强；季风爆发后，来自孟加拉湾和南海的西南气流携带充沛的水汽往江淮流域的输送也较强。此外，副高北跳偏晚，冷暖空气在江淮流域长时间交汇，致使梅雨锋偏强，降雨明显偏多。6—7月，南亚高压和副高存在频繁的南北摆动，造成雨带南北摆动大，但主流业务模式在中期时效内对副高的预报偏差仍然较大。目前集合预报模式对500 hPa大尺度环流系统的可用预报时效大约5 d；而200 hPa环流系统的可用预报时效能达到9 d左右。因此在中长期预报中，应更关注高层系统的演变和调整，及时抓住预报信号。

Interannual variability of the Bonin High associated with the propagation of Rossby waves along the Asian Jet

In 2020, the duration of Meiyu is very long and the amount of precipitation is more than normal. The rainstorm covers a wide range. The rain belt swings greatly from north to south, although the strong rainfall areas overlap. Using NCEP/NCAR reanalysis data and analyzing the evolution of the planetary-scale circulation systems, it is found that during the Meiyu season, strong blocking high systems maintain in the mid-high latitudes of Asia and Europe, and frequent cold air outbreaks penetrate the Jianghuai basin. At the same time, the Western Pacific subtropical high is significantly strong and westward, and the water vapor transported by the subtropical high to the Jianghuai Basin is significantly strong. After the monsoon erupts, the southwest airflow from the Bay of Bengal and the South China Sea carries abundant vapor to the Jianghuai Basin. The cold and warm air converge in the Jianghuai Basin for a long time, resulting in a stronger Meiyu front and significantly more rainfall. From June to July, the South Asian High and the subtropical high have frequent north-south swings, causing large north-south swings of rain belt during the Meiyu period. However, the medium-range forecast deviation of the subtropical high is still large in the mainstream operation models. At present, the ensemble forecast models have a usable forecast time of about 5 days for the 500 hPa large-scale circulation systems, while that for the 200 hPa circulation systems may reach about 9 days. Therefore, more attention should be paid to the evolution and adjustment of high-level systems to grasp the forecast signals in medium and long-range forecasts.