2021年9月通辽一次龙卷形成机理和雷达回波特征分析

Analysis of the Formation Mechanism and Radar Echo Characteristics of a Tornado in Tongliao in September 2021

2021年9月8日午后分别有两个EF1级龙卷对内蒙古通辽市科尔沁区左翼中旗、科尔沁区造成严重影响。为深入认识东北冷涡背景下内蒙古东南部平原地带龙卷的形成机理和特征,利用常规观测资料、ECMWF-ERA5的0.25°×0.25°再分析资料、多普勒雷达探测资料等对本次龙卷过程进行分析。结果表明:龙卷发生于东北冷涡底部、低空冷切南部的西南暖湿气流、地面辐合线南侧偏南气流中,龙卷发生前中低层有干空气入侵,增强大气对流不稳定度;较低的抬升凝结高度,低层位温随高度减小,0~3 km有较强的风垂直风切变、强的垂直上升运动,低层存在正垂直螺旋度大值中心,均是龙卷形成的有利条件;辐合线和干线相伴作用共同触发龙卷。通辽多普勒雷达探测出两龙卷钩状回波、入流缺口、有界弱回波区、气旋性风场辐合、中气旋特征;分析雷达数据反演的风场得出,龙卷发生在近地面辐合线附近的偏南气流中,位置在发展阶段钩状回波移向的东南侧、入流缺口附近;龙卷发生前中低层具有气旋性辐合,气旋性辐合中心逐渐下降,垂直方向上具有较强的垂直风切变,呈现低层辐合高层辐散特征;在低层和中低层的辐合中心接近时,龙卷形成并逐渐接地。

On the afternoon of September 8,2021,two EF1 tornadoes had serious impacts on Zuoyizhong Banner and Horqin District,Tongliao City,Inner Mongolia respectively.The formation mechanism and radar echo characteristics of the tornadoes were analyzed by using conventional observation data,ECMWF-ERA5 0.25°×0.25°reanalysis data,Doppler radar data,etc.The results showed that the tornado occurred at the bottom of the Northeast China cold vortex,with the warm southwest airflow in the southern part of the low-altitude cold cut,and the airflow southerly to the south of the ground convergence line.There was also a dry air intrusion in the middle and lower layers which enhanced the instability of atmospheric convection before it happened.The lower uplift condensation height,low-level temperature decreased with height,strong vertical wind shear of o-3km,strong vertical ascending movement and low-level positive vertical helicity center were all favorable for tornado formation.The tornadoes were triggered by the convergence line and the dryline.Tongliao Doppler radar detected the hook echoes,inflow gaps,bounded weak echo areas,cyclonic wind field convergence,and mid-cyclone features of two tornadoes.Based on the wind field retrieved from radar data,it was concluded that the tornado occurred in the southerly airflow near the near-surface convergence line,and the location was on the southeast side of the hook-shaped echo movement and near the inflow gap in the development stage.Before the tornado occurred,there was cyclonic convergence in the middle and lower layers while the center of cyclonic convergence decreased gradually,and there was strong vertical wind shear in the vertical direction,showing the characteristics of low-layer convergence and high-layer divergence.The tornado would form and touch the ground gradually when the convergent centers of the lower and middle-lower layers were close enough to each other.