东亚夏季风时期冷空气活动的位涡分析

Potential Vorticity Analysis of Cold Air Activities during the East Asian Summer Monsoon

通过多时间尺度分析方法,比较了夏季风时期表征冷空气活动的多种指标,得到表征夏季风时期冷空气活动的最优指标——位涡。分析表明,位涡(potential vorticity,简称PV)的低频振荡(20～80天和40～80天振荡)在表征冷空气活动中效果最好。利用该指标对东亚夏季风中冷空气的来源和作用进行了研究,重点分析了梅雨时期中高纬和中高层系统与夏季风系统相互作用的过程和机制。得出几点重要的结论:中高纬平流层下部和对流层顶的高位涡库以及60°N附近的亚洲大陆东部低层的高位涡库是东亚夏季风系统中冷空气的主要来源。冷空气的侵入路径对不同的雨季表现出不同的特征:华南前汛期,有两次冷空气的侵入,第一次来自东北方向低层,第二次来自对流层高层的高位涡库;梅雨期,爆发时主要来自高层的高位涡库,中后期鄂霍次克海附近850hPa的高位涡库是冷空气加强和维持的主要来源;华北雨季,冷空气主要来自对流层高层。倾斜等熵面是垂直涡度最易发展的区域,它也是高层冷空气侵入的路径。特定的位涡分布对应特定的降水类型,位涡和降水的季节内振荡(尤其是低频部分)蕴含了中高纬系统与夏季风系统相互作用的关键信息。

Using multi-scale analysis,several indicators of cold air activities during the East Asian summer monsoon （EASM） are compared,and the results show that potential vorticity （PV） is the optimal indicator in tracing cold air activities during EASM. The further study shows that it is best that the low-frequency oscillations of PV （20-80 day and 40-80 day oscillations） represent cold air activities. Then,the source and role of cold air during EASM are studied by use of the indicator,and the process and mechanism of interactions between EASM system and extratropical and mid-upper level systems during the Meiyu period are analyzed in detail. The conclusions are as follows：the two high PV pools are the main sources. One exists in the extratropical low stratosphere and tropopause,and the other exists in the low troposphere over the east of the Asian continent. To the intrusion paths,different rainy seasons show different features：during the pre-flood season in South China,the cold air intrudes twice respectively from the northeast at mid-lower levels and the high PV pool of the tropopause, during the Meiyu period,the cold air origins mainly from the high PV pool in the extratropical low stratosphere and tropopause during the onset of Meiyu,and from the high PV pool in the low troposphere over the Sea of Okhotsk during the mid-late period of Meiyu, during the rainy season in North China the cold air origins chiefly from the upper troposphere. Inclined isentropic surfaces are the areas where vertical vorticity grows quickly most easily,and the paths along which the upper cold air intrudes. Each rainy season over eastern China in summer is likely the reflection of the intraseasonal oscillations of the interactions between the EASM system and extratropics,especially the low-frequency part playing a more important role. Finally,the sources and characteristics are sketched.