夏季长江流域两类中尺度涡旋的统计与合成研究

A Statistical and Compositional Study on the Two Types of Mesoscale Vortices over the Yangtze River Basin

利用2000～2013年夏季6 h一次、水平分辨率为0.5°（纬度）×0.5°（经度）的CFSR（Climate Forecast System Reanalysis）再分析资料,对产生于四川盆地的西南涡和产生于大别山地区的大别山低涡进行了识别,统计出西南涡和大别山低涡的发生频数、初生时段、移动路径、三维结构等气候特征;在此基础上根据涡旋生成前的地面气压场和降水特征,对西南涡和大别山低涡分别进行了分类与合成研究,并细致对比了两类涡旋的异同点,主要结论如下：（1）西南涡在7月上旬最活跃,而大别山低涡则在6月上旬发生频数最高。凌晨时段是两类涡旋的高发期;西南涡日间的生成数目多于夜间,而大别山低涡则与之相反。（2）绝大多数西南涡和大别山低涡维持时间少于12 h;绝大多数西南涡维持准静止,而大别山低涡则主要向东北方向和偏东方向移动。（3）两类涡旋均为对流层中低层的低压系统,其中大别山低涡的垂直伸展层次较西南涡更低。相比于西南涡,由于水汽条件更优,大别山低涡所引发的降水更强,强降水的凝结潜热释放使得大别山低涡的平均生命史比西南涡更长。（4）产生前有降水的西南涡/大别山低涡相比于产生前无降水的西南涡/大别山低涡而言,对流层高层南亚高压的强度更强、辐散更显著;对流层中层与500 h Pa西风带短波槽的配置条件更好;对流层低层涡旋中心附近的辐合更显著、切变更强;并且对流层中低层的上升运动更强。这些都是有利于降水发生与维持的有利条件,而与降水凝结潜热密切相关的热力强迫使得产生前有降水的西南涡/大别山低涡相比于产生前无降水的西南涡/大别山低涡拥有更长的生命史长度,更大的水平半径和更大的涡旋生命史内降水量。

Using the 6-hourly, 0.5°×0.5° CFSR（Climate Forecast System Reanalysis） data for the summers（June– August） of 2000–2013, the climate characteristics of the southwest vortices（SWVs） generated over the Sichuan Basin and Dabie vortices（DBVs） around the Dabie Mountains（e.g. frequency, initial time, moving tracks, three-dimensional shape）, are detected and statistically analyzed. Based on the associated surface pressure characteristics and precipitation characteristics before their initiation, the SWVs and DBVs are classified, composited, and compared. The main results are as follows：（1） Both types of vortices occur frequently in summer. SWVs are most frequent in early July and DBVs are most frequent in early June. 0200 LST to 0800 LST is the most frequent time of occurrence for both types of vortices. SWVs are more frequent in the daytime, whereas DBVs are more frequent at night.（2） Most SWVs/DBVs persist for less than 12 hours. Under the influences of the Sichuan Basin, most SWVs are quasi-stationary, while in contrast, most DBVs move significantly along eastward and northeastward tracks.（3） Both types of vortices are mainly located in the middle and lower troposphere, but DBVs are generally located at lower levels than SWVs. Compared with SWVs, the precipitation associated with DBVs is heavier, mainly due to their more favorable moisture conditions. The larger latent heat release associated with these heavier rainfall events provide conditions that are more favorable for the persistence of DBVs.（4） Compared to those without precipitation before their formation, those SWVs/DBVs with obvious precipitation before their initiation are characterized by a stronger South Asia high and more intense divergence in the upper troposphere, a more favorable configuration relative to the 500 h Pa trough, stronger convergence and shear in the lower troposphere, and more intense ascending motion in the middle and lower troposphere. The thermodynamical conditions associated with the latent heat release before the formation of both types of vortices are conducive to a longer life span, larger horizontal radius, and heavier precipitation.