形成2015年浙江省梅汛期暴雨的控制环流及梅雨锋结构

本文利用NCEP/NCAR全球再分析逐日资料、地面观测资料和自动站降水资料，在分析了2015年浙江省梅汛期强降水特征、水汽输送和局地环流的基础上，从西南季风进退、副热带高压、南亚高压及西风带波动等方面对2015年形成梅汛期暴雨的控制环流进行了分析。结果表明：2015年整个浙江省梅汛期降水量较常年显著偏多，浙江中部地区降水量比历史同期偏多接近一倍。丰沛的水汽从孟加拉湾经中南半岛向东输送，与西太平洋副热带高压西侧的西南气流相合并，在梅雨锋南侧形成异常辐合，为强降水提供了水汽条件。这次持续强降水由三次强降水过程构成并由西风辐合型锋生引起。第二次强降水过程中大气强对流性不稳定利于梅雨锋上中尺度对流系统发展，导致强降水呈现明显的局地性。而第一次和第三次过程中梅雨带附近大气基本处于对流稳定或中性，以斜压性降水为主。在对流层低层，副高较常年偏东偏南，其西北侧西南暖湿气流与北侧冷空气交汇于浙江省，利于梅汛期强降水集中期的出现。在对流层上层的南亚高压较常年位置偏东，其北侧的西风急流强度偏强，东亚急流核入口区右侧的强辐散利于造成强烈的上升运动。在对流层中层，贝加尔湖阻高的东侧有明显的波动能量向东向南传播并在长江中下游积聚，利于浙江地区扰动的维持，形成持续稳定的梅雨锋和中低空切变线，造成梅雨强降水过程的持续。2015年春夏季热带中东太平洋海温正异常分布有利于梅汛期降水偏多的异常环流的形成。

Circulation and Meiyu front in responsible for persistent heavy rainfall over Zhejiang during summer of 2015

By using the NCEP/NCAR daily reanalysis data, surface observation objective data and automatic precipitation data, based on the analysis of characteristics of heavy precipitation, water vapor transportation and local circulation over Zhejiang Province in 2015, the dominant circulation leading to heavy rainfall was analyzed, involving Southwest monsoon, subtropical high, South Asia high and wave activity of westerly belt. Results show that, in 2015, the precipitation in Meiyu period in Zhejiang Province was significantly higher than the rainfall mean climatology and over doubled in central Zhejiang. The water vapor transported mainly via Southwest airflow from the eastern part of the Bay of Bengal. The continuous heavy rainfall consisted of three heavy rainfall processes and was caused by west wind convergence type frontogenesis. The stronger atmospheric convective instability was beneficial for the development of meso-scale convective system on Meiyu front, making the uneven distribution of precipitation in the second rainstorm processes. In the first and third processes, the atmosphere is convective stable or neutral, and the precipitation was mainly due to baroclinic precipitation. In the lower troposphere, anomalous circulation was featured by more eastern and southern subtropical high, which facilitated the converging cold air and warm moist Southwest airflow on the northwestern side of subtropical high, resulting in continuous heavy precipitation over Zhejiang area. In the upper troposphere, strong convergence at the right side of entrance of the East Asia strong subtropical westerly jet at the northeastern side of the South Asia high enhanced strong vertical ascending motion. In the middle troposphere, Rossby wave energy in the east of the Baikal Lake blocking high propagated the south-eastward and accumulated in the lower reaches of the Yangtze River. These all were response for forming the steady Meiyu front as well as lower level shear line and being conducive of the maintenance of heavy rainfall. In the spring and summer of 2015, the anomalous SST forcing in the central and East equatorial Pacific is favorable of the occurrence of the severe precipitation events in Zhejiang.