滇西南一次超级单体持续降雹过程分析

Analysis of a continuous supercell hailfall process in Southwest Yunnan

利用多普勒天气雷达3830/CC和分辨率为1°×1°的NCEP再分析资料,对一次南支槽影响滇西南超级单体雹暴过程进行分析.结果表明:2 h以上深厚持久中气旋、低层辐合高层辐散和和风速随高度增加等中尺度径向风场结构利于低层暖湿空气辐合进入云体和倾斜旋转上升,形成低层弱回波区(Weak Echo Region,WER)、钩状回波和中高层回波悬垂,北侧冷平流入侵形成“V”型槽口,保障超级单体雹暴持续4 h以上,冰雹粒子强烈散射作用形成旁瓣回波.径向风辐合增强并扩展至8 km高度,回波强度、顶高、50 dBz回波高度、−20℃层回波强度、垂直累积液态水含量(Vertically Integrated Liquid,VIL)等参量波动增大,尤其中气旋非常利于超级单体发展和冰雹粒子生长,在中气旋到达强盛阶段(旋转速度≥10 m·s^(−1))26 min内5个特征参量先后出现最大值.南支槽前中层西南气流的汇合提供水汽条件,中低层强垂直风切变提供抬升动力条件,近地层中尺度辐合线创造对流触发条件,低层温湿度锋区提供对流不稳定,而上冷下暖及低层湿度适中、中层高湿、高层干的垂直结构加剧对流不稳定,产生足够上升气流,形成上升气流与下沉气流共存的垂直环流系统,导致超级单体雹暴持续发展并在近地层中尺度辐合线和温湿度锋区附近降雹.

By using Doppler weather radar 3830/CC and NCEP 1°×1°reanalysis data,a supercell hailstorm process in Southwest Yunnan was analyzed under the influence of the south branch trough.The results showed that the mesoscale radial wind field structures,such as a stable and lasting mesocyclone for more than 2 hours,the lowlayer convergence and high-layer divergence,and the wind speeds increasing with heights,were conducive to the low-layer warm and humid air’s converging into the cloud and tilting and rotating up,leading to the formation of a weak echo area(WER)and a hook echo in the low layer,a mid-high layer echo overhang,and a V notch formed by the invasion of the cold advection on the north side.All of these combined ensured the supercell hailstorm lasting more than 4 hours,and the hail particles were strongly scattered to form sidelobe echoes.Radial wind convergence gradually enhanced and extended to a height of 8 km.Echo parameters,such as intensity,top height,50 dBz height,−20℃layer intensity,vertical integrated liquid water content(VIL),increased fluctuatingly.Especially the mesocyclone was very conducive to the development of the supercell and the growth of hail particles.The previous five characteristic parameters had their maximum values within 26 minutes after the mesocyclone reached the strong stage(rotation speed≥10 m·s^(−1)).The confluence of southwestern airflow in front of the south branch trough provided water vapor conditions.The strong vertical wind shear in the middle-low layer provided uplifting dynamic conditions.The mesoscale convergence line in the near-surface created convective trigger conditions,and the low-layer temperature and humidity front area led to the unstable development of convection.Meanwhile,the vertical structure,with the cold upper layer and the warm lower layer,and with moderate humidity at the lower layer,high humidity at the middle layer and low humidity at the upper layer,aggravated convective instability to generate enough updraft,forming a vertical circulation system in which tilted updraft and downdraft coexisted,and causing the continued development of the supercell hailstorm.As a result,hails occurred near the mesoscale convergence line and around the humidity and temperature front in the near-surface layer.