一次青藏高原东北侧边坡强对流暴雨的中尺度对流系统演变特征

The Variation Characteristics of Mesoscale Convection System in a Severe Convective Torrential Rain over the Northeast Slope of the Qinghai-Xizang Plateau

2018年7月22日20:00(北京时,下同)至23日08:00在青藏高原东北侧边坡的临夏北部、兰州西部等地区出现了罕见的大暴雨,最大降水量达176.8 mm,小时最大降水量61.6 mm。利用FY-4A卫星、多普勒雷达、区域自动站及再分析资料,重点分析引发强对流暴雨中尺度对流系统(MCS)的发生发展特征及其触发机制。结果表明:(1)强对流暴雨是在西太平洋副热带高压边缘暖湿气流配合纬向多波动气流带动冷空气东移的有利配置下发生的,高温高湿的环境为强对流暴雨的发生创造了有利条件。(2)先后形成并发展的2个MCS造成临夏北部、兰州西部出现强对流暴雨,孤立的对流云团加强合并发展为第一阶段的MCS,向西南方向传播的MCS与从甘南高原向东北方向移动的云团在大暴雨区逐渐合并为第二阶段的MCS。大于10 mm·(10min)^(-1)的强降水出现在上风向云团边缘,强降水与对流云团的发展程度有密切关系。(3)强对流暴雨期间,第一阶段的强回波有后向传播特征,强对流单体对大暴雨区的影响频次高,显著的“列车效应”自南向北移动造成持续6 h以上的间断性强降水。(4)边界层的中尺度风速脉动、地面辐合线及特殊地形等是触发MCS发生、发展的主要因子,地面中尺度辐合线的形成、维持及移动,与强降水的位置有很好的对应关系。

A rare torrential rain occurred in the northern Linxia and western Lanzhou on the northeast side of the Qinghai-Xizang Plateau from 20:00(Beijing Time,the same as after)on 22 to 08:00 on 23 July,2018,with the maximum total rainfall of 176.8 mm and the maximum hourly rainfall of 61.6 mm.Based on the data of satellite,CINRAD,automatic weather stations and reanalysis data,the occurrence and development characteristics of the mesoscale convective system(MCS)causing severe convective rainstorm and its trigger mechanisms were analyzed.The results show that:(1)Severe convective rainstorms occurred under the favorable configuration of a warm and humid air flow of the periphery of Western Pacific subtropical high combined with multiple zonal fluctuations driving cold air eastward.The environment of high temperature and humidity created favorable conditions for the occurrence of severe convective rainstorm.(2)Two MCS developed successively caused severe convective rainstorms in the northern Linxia and western Lanzhou.The isolated convective cloud clusters strengthened and merged into the first stage of MCS.The MCS spreading to the southwest and the clouds moving from the Gannan plateau to the northeast gradually merged into the MCS of the second stage in the heavy rain area.Heavy rainfall over 10 mm·(10min)^(-1) occurred at the edge of cloud cluster in the upwind direction,and the heavy rainfall is closely related to the development of convective cloud clusters.(3)During the severe convective rainstorm,the strong echo in the first stage had obvious backward propagation characteristics.The strong convective cell had a high frequency of impact on the heavy rainstorm area,and the significant"Train Effect"moving from south to north caused intermittent heavy rainfall lasting more than 6 hours.(4)Mesoscale wind speed pulsations in the boundary layer,surface convergence line and special terrain are the main factors that triggered the occurrence and development of mesoscale convective systems.The formation,maintenance and movement of the surface mesoscale convergence line had a good correspondence with the location of heavy rainfall.