2012年7月内蒙古极端降水事件及成因

Procession and Causes of the Extreme Rainstorm Event in Inner Mongolia in July 2012

2012年7月下旬内蒙古中部出现罕见的极端降水事件(简称"7·27"暴雨)。利用NCEP再分析资料、常规观测和精细化监测等资料分析了"7·27"暴雨成因。结果表明:(1)贝加尔湖低涡异于常年。巴尔喀什湖到鄂霍茨克海为阻塞高压,其底部横槽加深生成贝加尔湖低涡。低涡内有冷空气活动,在对流层低层及地面激发出低涡和气旋。(2)该过程的中尺度特征明显。地面中尺度切变线不断新生、稳定维持,形成多个中尺度雨带。高空β中尺度对流云团不断新生、合并加强,形成对流复合体M CS。河套西北部、河套南部、锡林郭勒盟中西部三个暴雨区均是中尺度对流复合体M CS发展的结果。(3)垂直方向上,暴雨发生前到暴雨期从地面至700 h Pa的大气比湿达10~22 g·kg-1,且850~700 h Pa水汽垂直输送达到最强;水平方向上,南风风力辐合,末端到达45°N以北,河套南部地区为"水汽汇"。(4)对流层高层大气涡旋运动是该过程的直接诱因。大气涡旋运动由200 h Pa向700 h Pa传播,正涡度平流在700 h Pa到200 h Pa随高度增加,高层水平辐散、低层补偿辐合,大气强迫上升产生暴雨。(5)对流层中层持续"干侵入",低层持续的暖湿输送,大气不稳定能量不断重建,这是极端暴雨过程的热力机制。(6)随着全球气候变暖,亚洲夏季风加强,来自孟加拉湾和南海的水汽向东亚副热带地区输送加强,水汽输送进一步向北扩展到我国华北内陆地区,是"7·27"暴雨过程的重要原因。

An extreme rainstorm named as ‘7·27 'occurred in the middle of Inner M ongolia in China during late July 2012. Using data from NCEP reanalysis,conventional observations,and monitoring,the causes of the‘7·27'extreme heavy rain were analyzed and discussed. The results showing:( 1) Bell-Lake vortex happened in ‘7 ·27 'rainstorm w as quite different from that in perennial years. The area from Balkhash Lake to the Ochotsk Sea w as controlled by blocking high,the bottom of the blocking high w as occupied by Bell-Lake vortex.( 2) ‘7·27 'rainstorm has strongly the characteristics of the meso-scale. M any meso-scale rain belts w ere generated by surface shear lines formed constantly and sw ung north and south. The β meso-scale convective clouds w ere produced constantly and enhanced by amalgamation and further formed the convective complexes( MCS or MCC). The rainstorms in the northwestern and southern Hetao area,and the western Xilin Gol League w ere all resulted from the meso-scale convective complex( M CC) or M CS and ground shear lines,( 3) The distribution characteristics of the w ater-vapor w ere: the atmospheric specific humidity in vertical w as up to 10 ~ 22 g·kg- 1from ground to 700 h Pa. The w ater vapor transported strongest in the scope from 850 h Pa to 700 h Pa.Horizontally,the south w inds reached 45°N,w hich made the 'moisture sink'of Southern Hetao.( 4) Atmospheric vortex in Troposphere w as the direct cause of the event. The atmosphere vortex spread from 200 h Pa dow n to 700 h Pa. Heavy rains w ere hence produced by the upper atmosphere divergence and the low er compensate convergence.( 5) The thermodynamical mechanics of ‘7·27'rainstorm is: The instable energy of atmosphere w as rebuilt constantly with the mid-level dry intrusion and the low-level humid transport.( 6) With global w arming,the Asian summer monsoon intensifying,from the bay of Bengal and the South China Sea to the East Asian subtropical region w ater vapor transport strengthening,w ater vapor transport further northw ard expansion into North China's inland areas,is an important reason for ‘7·27'rainstorm process.