摘要
2012年7月21日北京发生了历史罕见的特大暴雨,最大累计降水量达到541 mm,最大雨强100.3 mm/h,强降水持续10多个小时,造成了巨大的损失。使用常规观测资料和NCEP再分析资料在讨论环境大气湿度条件与特大暴雨关系的基础上,分析了实现充沛的水汽远距离输送到华北并在北京上空积累的天气尺度动力过程及其形成的原因。结果表明,"7.21"特大暴雨产生在大气异常潮湿的环境中。在暴雨发生时,比湿最大值达到19 g/kg,而且,对流层中下层的比湿比北京区域性暴雨历史个例高40%。产生长时间强降水的重要原因是边界层以上高湿的特征在暴雨产生的过程中一直维持。充沛的水汽被一支从低纬度一直贯通到40°N附近的低空偏南气流从孟加拉湾和中国南海向北输送。偏南风持续增大形成低空急流,增大了水汽的输送。而且,随着急流核向东北方向移动逐渐靠近北京,在北京上空对流层低层产生了-17.7 g/(hPa·m^2·s)异常强烈的水汽通量辐合。同时,高空强烈辐散与低空辐合的耦合不断加强,不仅增加了低层水汽在暴雨区汇集,而且也通过增强垂直速度将更多的潮湿空气向上输送,使高层大气湿度增大。通过上述两个天气尺度动力过程,实现了水汽的远距离输送,并在暴雨区上空强烈辐合,导致北京地区水汽异常充沛。输送水汽的偏南风持续增大的原因主要有两个:一是台风外围环流的影响;二是在海上副热带高压稳定维持的情况下,大陆上低压加强、东移,造成东西向气压梯度增大,在地转偏向力的作用下,南风增强。最后,得到了实现水汽远距离输送的天气尺度动力过程机理模型。
An infrequent excessive storm rainfall hits Beijing on 21 July 2012. The observed maximum rainfall reaches to 541 mm and the top precipitation intensity is 100.3 mm/h. The severe rain storm lasts for more than 10 h, causing a serious loss. The observation and the NECP reanalysis data are used in the analysis in order to know why the rainfall could be so much. Firstly, the relation between the moisture condition and the rainfall is reviewed. And then, the reason that the abundant vapor could be transported to and accumulates in Beiiing is studied. The results show that this rainfall develops in a very wet environ- ment. The top specific humidity reaches to 19 g/kg. And the value of this variable in the lower and mid troposphere is 40% higher than that of regional severe rainfall cases in Beijing. The extreme humid situation maintains even though the severe rain- fall appears. It is favorable for the long-lasting severe rain storm. The moisture air is transported from the South China Sea and the Bay of Bengal to the Huabei areas by a southern flow which strengthens continuously to form a low level jet （LLJ）. The core of LLJ moves northeastwards to Beijing gradually and leads then to an abnormal moisture flux convergence （ - 17.7 g/ （hPa . m2 .s））. At the same time, the strong couple between upper-level divergence and low-level convergence not only in- creases the convergence of vapor but also strengthens the ascending velocity inducing more wet air to higher altitude and then thickening the moisture layer further. These two synoptic dynamic processes result in the vapor being transported far away and accumulating in Beijing. There are two reasons which lead to the acceleration in the south flow. One is the current from a ty- phoon joining into the monsoon. The other one is the geopotential height gradient increasing in the mid latitudes, which is caused by a eddy in mainland going eastwards while the subtropical high in the Pacific moving little. Finally, a schematic chart about the main idea of the dynamic process is given.
出处
《气象学报》
CAS
CSCD
北大核心
2013年第6期997-1011,共15页
Acta Meteorologica Sinica
基金
预报员专项(2013001)
公益性行业专项(GYHY201306008)
国家级预报员创新团队北方暴雨团队
北京市气象局精细化预报创新团队
关键词
特大暴雨
天气尺度动力过程
水汽远距离输送
超强水汽辐合
低空急流持续增强
Excessive storm rainfall, Synoptic-scale dynamic process, Long distance vapor transportation, Ultrastrong mois-ture flux convergence, LLJ sustaining acceleration
