【摘要】 利用常规气象观测资料、新一代天气雷达、自动站、NCEP再分析资料等对西北气流形势下一次局地大暴雨伴多次降雹的强对流天气形成机制和对流系统结构进行了精细化分析。结果表明:(1)局地大暴雨伴冰雹发生在西北气流控制和大气层结极不稳定的形势下,14:00(北京时,下同)CAPE较08:00显著增大,为大暴雨和冰雹提供了不稳定能量;对流层低层的水汽含量大值中心为后向和前向传播新生单体的不断生成提供了充分的水汽条件;地面局地加热不均匀,午后地面温度达到对流温度临界值使地面暖气团自由上升,从而产生初始对流回波。在达到热力对流的条件下,地面中尺度辐合线和露点锋对局地大暴雨伴多次冰雹天气的发生有加强触发作用,地面中尺度低压是辐合维持和水汽集中的重要原因。(2)雷达图上,初始回波在周口附近生成、加强并向东南方向移动的过程中,其后侧和右后侧不断有中γ尺度对流单体生成,新生单体经历了积云生成加强、成熟合并、减弱消散阶段,其传播方向和移动方向近于相反,使周口附近强回波呈准静止动态平衡状态而持续存在。随后,在许昌到太康近东西向带状回波的前侧不断有中γ尺度新对流单体生成,并与周口附近后向传播的对流单体相接,排列成西北—东南向的线状多单体回波带,前向传播和后向传播分别经历了后侧减弱和前侧减弱阶段,中间回波在周口附近发展最旺盛。向前和向后两种传播形式多单体结构中的中γ尺度对流单体形成显著的"列车效应"使周口、西华出现局地大暴雨和多次降雹。在平均径向速度图上有中尺度涡旋,西北—东南向线状对流回波带在中低层有辐合—辐散—辐合相间的结构特征,在高层则与中低层相反,线状雷暴系统的形成和演变与强雷暴下沉气流抬升暖湿空气有较大关系,对流单体生成于低层辐合、高层辐散处。更多还原

【Abstract】 Using the conventional observation,new generation weather radar,AWS,NCEP data,the weather formation mechanism of a local heavy rainstorm accompanied by repeatedly hail under the northwest flow situation and the structure of mesoscale convective system were detailed analysed. The results showed that:( 1) The local heavy rainstorm accompanied by hail occurred in the situation of northwest airflow and atmospheric stratification was very unstable. The value of CAPE at 14: 00( Beijing time,hereafter the same) was significantly increased compared to that at 08: 00,which provided instability energy for heavy rainstorm and hail; The large value center of water vapor content in the lower-troposphere provided adequate moisture conditions for newborn monomers that generated continued by forward propagation and backword propagation. Local heating of the ground was uneven,the ground temperature reached to the threshold of the convective temperature in the afternoon which made the ground warm air mass free up,and resulted in the initial convective echoes,when it reached thermal convection conditions,the ground mesoscale convergence line and dew-point front for local heavy rainstorm and accompanied by many times hail weather had strengthening and triggering effect. Mesoscale low pressure on the ground was the important reason for convergence maintained and water vapor concentration.( 2) On the chart of radar,in the process of the initial echo generated in Zhoukou nearby,strengthened and moved to south-east direction,γ-scale convective cells constantly generated at the rear and right rear of that,the new generated cells experienced the phase of cumulus clouds formed and strengthened,matured and merged,weaked and dissipated. Its direction of propagation and movement closed to opposite,which made the strong echoes near Zhoukou were the state of quasi-stationary dynamic equilibrium and constantly persistent. Subsequently,new meso-γ-scale convective cells were constantly generated in the front side of the band echo that nearly east-west from Xuchang to Taikang,And it linked to the convective cells of backward spread near Zhoukou,and arrayed into northwest-southeast linear multi-cells echo bands. Forward propagation and backword propagation respectively experienced back weakened and front weakened stages. And the middle echo near Zhoukou developed the most productive. The meso-γ-scale convective cells in the multi-cell structure of two forms of forward propagation and back propagation formed a remarkable ‘train effect’,which made local heavy rainstorm with repeatedly hail weather happened in Zhoukou and Xihua. The charts of average radial velocity contained mesoscale vortex,and the northwest-southeast linear convection echo bands had the structural features of convergence,divergence and convergence alternated in the middle and lower layers. The upper layer was opposite with the middle and lower layers. The formation and evolution of linear thunderstorm system were larger related with that downdraft uplifted warm and humid air of severe thunderstorm. The convective cells were generated in the low-level that appearred convergence,and the upper level that appearred divergence.更多还原