一个表征江南春雨的环流指数及其天气学特征

利用东亚和西太平洋对流层低层850 hPa纬向风的经向差异，定义了一个可以表征江南春雨变化特征的环流指数，通过诊断分析探讨该指数与降水和大气环流的关系。结果表明:该指数可以较好地反映江南春雨的年际和逐日变化特征。在年际时间尺度上，高指数年江南春雨偏多，而低指数年江南春雨偏少。该指数的逐日变化与江南地区同期逐日降水变化呈显著正相关。在高指数日，我国江南地区的低压系统和西北太平洋地区的副热带高压偏强，江南地区对流层高层辐散增强，低层辐合和高层辐散的增强为春雨发生提供动力抬升条件，有利于春季江南地区降水的产生；低指数日，西北太平洋副热带高压位置偏南，江南至华南地区对流层低层存在弱辐散，这种形势配合下不利于江南春雨的产生。

A Circulation Index of the Spring Persistent Rainfall in the South of the Yangtze and Its Synoptic Characteristics

Using daily precipitation dataset of 2466 stations over China, daily and monthly reanalysis dataset from 1961 to 2016 by National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR), a circulation index of spring persistent rainfall (I_SPR) is defined based on latitudinal differences of zonal winds in lower troposphere over the region from East Asia to the western Pacific. Relationships of I_SPR with spring persistent rainfall and general circulation is investigated. Results show that the westerly wind from the South of the Yangtze to South China and the easterly wind located in the region from Huanghuai to Jianghuai area in spring is beneficial to the spring persistent rainfall in the South of the Yangtze. Using this characteristic, the spring rainfall circulation index in the South of the Yangtze is defined. In high-index cases, rainfall increases in the South of the Yangtze; and in low-index cases, rainfall decreases. Meanwhile, the index defined not only reflect the annual variation of the spring persistent rainfall, but also can reflect the daily variation of the spring persistent rainfall. The index has good synoptic significance, and is positively correlated with the daily precipitation in the South of the Yangtze. A verification using data from 1961 to 2016 in the South of the Yangtze indicates that this definition of index can reflect the precipitation in most years in the South of the Yangtze in spring. Taking the year of 2016 as an example, results show that the index defined has a clear physical meaning. In high-index cases, the plateau trough and the southern branch of westerly trough are more active. The western Pacific high moves northward anomalously. Confluence of the southwesterly wind from the low trough, the western Pacific high and the cold air from the higher latitude occur from the South of the Yangtze to South China. Low level convergence and high level divergence provide dynamic uplifting conditions for spring persistent rainfall in the South of the Yangtze. In low-index cases, the western Pacific high moves southward anomalously, and anticyclone anomalies cover the mainland of China. The existence of weak divergence in the lower troposphere in the South of the Yangtze to South China goes against persistent rainfall.