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高空急流在北京“7．21”暴雨中的动力作用
利用常规观测、加密自动气象站降水资料以及NCEP/NCAR再分析资料等，使用天气动力学诊断方法，重点研究了高空急流对北京日（“7．21”）暴雨中降水突然增强时刻14时（北京时）和降水最强时刻19时的动力作用。结果表明，“7．21”暴雨的发生和西来的高空急流东移至北京上空有关，高空急流及其散度场和与高空急流相伴随的次级环流对“7．21”暴雨的发生起重要的动力作用。7月21日14时，高空急流轴的经向度开始增大，高空急流入口区右侧的散度场南北范围明显扩大，北京上空为深厚的对流运动，受来自低层的东南气流带来的暖湿空气的影响，北京地区降水量突然增大；20时，对流层高空急流轴的经向度进一步增大，此时由于北上西北低涡导致的低层辐合，使得北京上空对流层高层出现强辐散区，北京上空出现强烈的上升运动，加之来自东南的暖湿气流的影响，使得北京地区降水量在19时达到最大值。“7．21”暴雨中降水突然增强时刻和降水最大时刻，上升支均出现在高空急流入口区右侧，但是，次级环流的下沉支均发生在北京的东南部，这是影响“7．21”暴雨次级环流的一个重要特征。
Abstract：
Based on the data from the conventional meteorological observation,including the surface hourly precipitation and the NCEP reanalysis data,using the weather diagnosis and analysis methods,the dynamic processes of the upper-level jet stream for the torrential rainfalls in Beijing,both increased suddenly at the time of 14:00 BT and of the biggest amount at 19:00 BT 21 July 2012 (named simply as 7.21 hereafter)are studied.The results show that the upper-level jet stream moved from the west to over Beijing is related to the Beijing 7.21 rainstorm,and the upper-level jet stream and its divergence field,and the seconda-ry circulation associated with the upper-level jet play an important role in the Beijing 7.21 rainstorm.At 14:00 BT 21,along with the upper-level jet stream northward spreading,the range of the upper-level divergence,which is situated at the right side of the entrance region of the upper level jet stream,expanded significantly.The Beijing region is under the influence of the deep convective motion and affected by the favorable moisture conditions,which was associated by the southeast flow,and thus caused the precipitation of the Beijing area to increase abruptly.At 20:00 BT 21,the upper-level jet stream further northward spreads,and the low-level convergence generated by the northwest vortex,resulted in the strong upper-level divergence over Beijing.The vertical motion enhanced by the strong upper-level divergence over Beijing,which combined with the impact of warm air from the south-east,leading the precipitation of Beijing region to reaching the maximum at 19:00 BT 21.The ascend-ing motions appeared in the right side of entrance region of the upper-level jet,while the descending motions occurred in the southeastern part of Beijing,at either the precipitation-increased suddenly time or the precipitation maximum time of Beijing 7.21 rainstorm.This is an important feature of the secondary circulation of the 7.21 Beijing rainstorm.
QUAN Meilan
LIU Haiwen
ZHU Yuxiang
CHENG Long
作者单位：
成都信息工程学院 大气科学学院，高原大气与环境四川省重点实验室，成都，610225; 辽宁省抚顺市气象台，抚顺，113006
成都信息工程学院 大气科学学院，高原大气与环境四川省重点实验室，成都，610225; 重庆气象科学研究所，重庆，401147
中国气象局气象干部培训学院，北京，100081
成都信息工程学院 大气科学学院，高原大气与环境四川省重点实验室，成都，610225
年，卷(期)：
P458.1+21.1
Keywords：
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基金项目：
国家公益性行业(气象)科研专项，重庆市气象局开放式研究基金项目，重庆市博士后科研项目。
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