梅雨锋中尺度涡旋内微型超级单体龙卷的形成研究

ANALYSIS OF THE FORMATION OF MINI-SUPERCELLS AND ASSOCIATED TORNADOGENESIS WITHIN A MESOSCALE VORTEX EMBEDDED IN A MEIYU FRONT

对一次梅雨锋中尺度涡旋上发展的系列龙卷过程的形势背景、近风暴环境、龙卷演变过程和形成机理进行了研究。在高空强辐散区、中低层西南风急流的强风速辐合背景下,北侧干空气卷入后静止锋上形成了具有弓状结构的“逗点”状中尺度对流系统(MCS),MCS北部γ中尺度涡旋明显发展,产生多个龙卷。相较于S波段多普勒天气雷达,高分辨率(30m距离库、30秒体扫描)相控阵雷达探测到上海崇明陈家镇龙卷及其母体“微型”超级单体的精细结构和形成演变过程,及涡旋速度对、反射率空洞、高谱宽等典型标记;正、负径向速度对的结构和演变揭示涡旋源于近地面,发展后其高度最高达到2.0km;龙卷接地前2~3分钟,收缩拉伸的龙卷涡旋又呈现出从上向下接地的过程。分析表明:极低的抬升凝结高度和极强的低层垂直风切变为龙卷形成提供了环境条件,龙卷发生前低层垂直风切变显著加强;在中尺度涡旋东部降水回波后缘,西南干急流与东南湿急流汇集激发形成“微型”超级单体,其指状回波快速向单体主体靠近、收缩逐渐演变成钩状回波,形成具有空洞特征的龙卷涡旋区。该龙卷形成的可能机制是:指状回波包围区内上升运动与外围下沉运动分别作用于由低层垂直风切变形成的水平涡管上,使水平涡管倾斜向上逐渐转为垂直涡管,在上升运动的作用下不断拉伸加强,涡度迅速加大形成龙卷。针对该次过程,建立了中尺度涡旋上“大涡套小涡”的系列龙卷分布概念模型和“微型”超级单体上龙卷涡旋形成的概念模型。

The synoptic situation,near storm environment,and tornado evolution process and formation mechanism of a series of tornadoes within a mesoscale vortex embedded in a Meiyu front are studied.In the aloft strong divergence zone and the strong wind speed convergence of low-level southwesterly jets,a mesoscale vortex on the stationary front developed after the dry air on the north side was involved,forming a comma-like mesoscale convective system(MCS)with a bow echo.A meso-γ-scale vortex in the MCS’s northern part developed,producing several tornadoes.Compared with the S-band Doppler weather radar,the high-resolution(30m range bin,30-second volume scan)phased array radar at Chenjiazhen,Chongming District,Shanghai,detected the fine structure and evolution of the Chenjiazhen tornado and its parent mini-supercell which was much lower and smaller than classic supercells.Typical characteristics can be found,such as radial velocity pairs,reflectivity holes,high spectral width,etc.The positive and negative radial velocity pair structure and evolution process revealed that the vortex originated near the ground,and then its height reached a height of up to 2.0 km.Only 2~3 minutes before the tornado touched down,the contracted and stretched tornado vortex dropped rapidly from as high as 1.5~2.0 km to near surface.The analysis shows that the extremely low lifted condensation level(LCL)and the extremely strong low-level wind shear provided suitable conditions for the formation of tornadoes.Before the tornado formed,the low-level wind vertical shear was significantly strengthened at the rear edge of the precipitation echo in the eastern part of the meso-γ-scale vortex.The southwestern dry flows and the southeast wet flows converged to form a mini-supercell.The finger-shaped echo quickly approached the parent storm,shrank,and gradually evolved into a hook-shaped echo,forming a tornado vortex area with a hole echo area.It is hypothesized that the ascending motion and the peripheral sinking motion in the encircled area by the hook echo acted on the horizontal vortex formed by the low-altitude vertical wind shear,respectively.As a result,the horizontal vortex tube tilted upward and gradually turned into a vertical vortex,which was continuously pulled under the action of the ascending motion.Finally,the vorticity rapidly increased to form a tornado near Chenjiazhen.A conceptual model of the distribution of tornadoes in the mesoscale vortex and a conceptual model of tornado vortex formation in mini-supercells are established.