冷涡背景下一次微型超级单体龙卷的雷达特征和物理过程探究

A case study on the radar characteristics and physical process involved in the genesis of a mini supercell tornado under the background of cold vortex

2019年8月16日渤海北部沿岸出现了一次冷涡背景下的EF1级龙卷。利用营口S波段双偏振多普勒天气雷达探测资料、5 min间隔的地面自动气象站观测资料、盘锦风廓线雷达探测资料及ERA5再分析资料,研究了该龙卷风暴产生的环境条件、龙卷风暴结构特征及龙卷形成的可能物理过程。结果表明:此次龙卷过程发生在500 hPa冷涡主体控制下,低空位于“利奇马”台风残涡西侧水汽输送带内,环境条件表现为弱的风垂直切变和强低层热力不稳定。营口双偏振雷达位于距龙卷发生地15 km处,探测到产生龙卷的微型超级单体钩状回波、下沉反射率核心(DRC)、弱回波洞(WEH)、龙卷残片特征(TDS)等结构。处于消亡阶段雷暴的阵风锋出流向西传播,而营口附近海风锋缓慢东移,两条边界层辐合线相遇加强,在水平切变不稳定的作用下,辐合线上有γ中尺度涡旋形成。辐合线相遇造成的辐合抬升、低层强热力不稳定导致的环境正浮力以及中层中气旋扰动低压共同作用产生强上升气流,γ中尺度涡旋与上升气流叠置,强拉伸作用增强了垂直涡度,可能是低层微尺度气旋形成的关键机制。微尺度气旋直径收缩至最小伴随旋转速度达到最大时刻,对应龙卷生成,中层中气旋与微尺度气旋分离导致龙卷消亡。

An EF1 tornado occurred in the northern coast of the Bohai under the background of cold vortex on 16 August 2019.Using the Yingkou S-band dual-polarization Doppler weather radar data, surface automatic weather station(AWS) observations at 5 min interval, the Panjin wind profile radar data and ERA5 reanalysis data, the environmental background, the structure and formation of the tornadic storm and the tornadogenesis are studied. The results show that the tornado occurred under the background of a cold vortex at 500 hPa, and it was located in the water vapor conveyor belt on the west side of the residual vortex of typhoon "Lekima".The environmental condition is characterized by weak vertical wind shear and strong low-level thermal instability. The Yingkou dualpolarization radar is located 15 km away from where the tornado originated. The hook echo, the descending reflectivity core(DRC),the weak echo hole(WEH), and the tornadic debris signature(TDS) in mini supercell are detected by the radar. The outflow of the decaying thunderstorm gust front moved westward, while the sea breeze front near Yingkou slowly moved eastward. The two boundary layer convergence lines merged, leading to the forming of γ-mesoscale vortex under the influence of horizontal shear instability. The intersection of outflow boundaries, the large positive ambient buoyancy and the vertical perturbation pressure gradient associated with the low pressure induced by the middle level mesocyclone jointly produced strong updrafts. The collocation of the updrafts and the γ-mesoscale vortex played a critical role for the genesis of misocyclone by strong stretching. The combination of the maximum rotation velocity and the minimum diameter of the misocyclone corresponded to the tornadogenesis, and the separation of the misocyclone and the mesocyclone in the middle level led to the dissipation of the tornado.