内蒙古干旱半干旱带2012年“720”极端暴雨事件的特征及成因

Characteristics and Causes of the Extreme Rainstorm July 20,2012 in the Arid and Semiarid Zone in Inner Mongolia

2012年7月下旬,处于内陆干旱-半干旱地带的内蒙古中-西部地区出现了有气象记录以来的极端降水事件,使该区气象与气候预测面临新问题.7月20日河套地区出现大范围暴雨天气,其中两个台站日降水量超历史极值,一个台站超历史阈值,属极端天气事件.利用常规及精细化监测资料和NCEP再分析资料对此次极端降水天气事件进行分析.结果表明:从天气背景看,贝加尔湖低槽内冷空气侵入副高西侧的暖空气中,在对流层低层激发出低涡系统,低涡前侧的南风急流使南来水汽到达41°N以北,并在河套地区聚集.对流层低层水汽通量维持在8～10 g·cm-1·hPa-1·s-1,大气比湿达12～17g·kg-1,为极端降水事件提供了丰沛的水汽.该事件是通过MCC强烈发展形成的,河套西北部不断有中尺度对流系统MCS发展东移,河套南部新生的β中尺度系统发展并入MCS中,MCS系统发展为中尺度对流复合体MCC,MCC中心的TBB值达-40～-83℃.近地面雷达监测显示,河套东北部、中部存在强雷达回波群,回波群内对流单体中心的反射率因子均达到50～55dbz,构成超级对流单体.地面上,不断新生的中尺度辐合线长时间存在于河套东北部并促发对流性暴雨.暴雨前期对流层低层增温作用显著,中高层"干侵入"使大气不稳定能量进一步增加.冷锋前暖空气强烈的上升运动促发了大气不稳定能量释放.该事件很可能与当前全球变暖密切相关,随着全球变暖,北极冰量减少,夏季风增强,雨带北移,使得中纬度内陆干旱-半干旱带发生前所未有的极端降水事件过程.

In late July 2012, an extreme heavy rainstorm swept the inland arid and semiarid zones in the central area of Inner Mongolia, China, which was a new problem for meteorology and climatology, especially for weather forecast. In Hetao region the heavy rainstorm event occurred on 20 July. The daily precipitation records in two stations exceeded historical extreme. In another station the record exceeded historical threshold. In this paper, the causes of the event are analyzed with NCEP reanalysis data, conventional observations and monitoring. It is found that： 1） the cold air from Lake Baikal invaded into the warm air of the subtropical high. Vortex system was inspired in the lower troposphere. The water vapor taking by south wind along the front of vortex system reached to north of 41N and gathered in Hetao. The water vapor flowed from lower troposphere to the mid-level with a flux of 8~10 gcm-1hpa-1s-1 and an atmospheric specific humidity up to 12~17 gkg-1, which provided abundant moisture to the extreme storm;2） the extreme rainstorm was due to the strong development of the MCC. A meso-scale convective system MCS developed in northwest Hetao. A β meso-scale convective system from south incorporated it. A MCC was developed with its TBB value of -40 ℃ to -83 ℃ at the centre. There were β meso-scale convective echo groups in northeast and central Hetao, within which the echo of each convective cell reached 50-55 dbz, forming super convective cells. The continuously growing ground convergence lines triggered convective rainstorms in northeast Hetao;3） before the rainstorm, high unstable energy accumulated owing to the lower troposphere warming. Dry intrusion increased atmospheric instability further. Then, Hetao extreme rainstorm occurred, when the atmospheric instability energy was released owing to strong upward movement of the front warm air;4） the event was closely related to global warming. Along with global warming, the ice volume in high latitude and Arctic is decreasing, causing the cold vortex activity decreasing, while summer monsoon is increasing, causing the rain belt moving northwards. Therefore,it is not occasional that extreme rainstorm occurs in the middle latitude semiarid inland.