青藏高原东北部一次罕见强对流天气的中小尺度系统特征分析

Analysis of Meso-Small Scale System Characteristics of a Rare Severe Convective Weather in the Northeast Part of Qinghai-Tibetan Plateau

2016年8月17-18日青藏高原东北部出现了罕见的大冰雹、短时暴雨、雷暴大风等强对流天气。运用常规观测资料、NCEP再分析资料、葵花静止气象卫星、多普勒天气雷达等观测资料分析了此次强对流过程的大气环境场和中小尺度对流系统的发生发展和对流传播机制。结果表明:西太平洋副热带高压北抬明显,属于低层暖平流强迫型。水汽输送主要来自南海。维持较长时间的弱冷锋是强对流的地面触发机制;对流云团逐渐演变为MCC,对流传播整体具有沿着河谷往层结不稳定区的正向和往低层入流风的反向传播的特征。河谷地形是影响对流移动和传播路径的关键;强对流风暴单体生命史均较长,强降雹单体为类超级单体和普通多单体,强降水回波属于多单体线状对流。降雹单体整体比降水单体发展得更强,变化幅度更大,尤其是垂直累积液态水含量的变化更剧烈。强对流开始前单体垂直累积液态水含量均是先增后降;几处局地雷暴大风是由雷暴云团内弱降水在较厚的环境干层蒸发而显著降温所产生的较大负浮力或由线状对流中强降水拖曳导致的强下沉辐散气流造成,雷达回波具有质心急剧下降或中层径向速度辐合特征。

A rare severe convective weather process with the large hailstone,local short-time rainstorm,and thunderstorm gale occurred over northeast part of Qinghai-Tibetan Plateau from 17 to 18 August 2016. The genesis and development and propagation mechanism of the meso-small scale convective system,and the atmospheric environment field were all analyzed by using the datum of conventional observation,NCEP reanalysis,himawari-8 satellite,C and X band Doppler radar,etc. And the characteristics between hail echo and rain echo were contrasted too. The results showthat: It belonged to the low-level warm advection forcing type because of the obvious lifting northward of Subtropical High over west Pacific. The Water vapor was transported mainly from the South China Sea,and the Bay of Bengal next. The starting and trigger mechanism of sever convection was the weak cold front on surface with a longer life. Convective clouds gradually evolved into mesoscale convection complex( MCC),and the convection propagated toward unstable Atmospheric stratification zone and backward inflowwind of low-level along Huangshui river valley as a whole,and the convective motion dominated by propagation due to the weak inflowwind. The river valley topography was the key to the moving and propagation path of convection. All severe convective cells had comparatively long life cycle,the strong hail cells were similar-supercells and common multi-cells,and the heavy precipitation cells belonged to the multi-cell linear convection. The strong hail cells generally developed stronger and changed more greatly than the heavy precipitation cells,such as the former had stronger echo intensity,higher organization degree,longer life cycle mesocyclone,higher echo top and echo centroid,and most echo parameters had greater changes,especially VIL( vertical integrated liquid water content). In addition,the strong echo pendency located over the weak echo region in the former cells,etc. But before the beginning of severe convection,the VIL almost increased first and then dropped,and the maximum echo intensity exceeded 60 dBZ but changed relatively small both in the hail and precipitation cells. The four positions of thunderstorm gale were generated by the strongly down-divergence flow,that was produced by greater negative buoyancy due to strong temperature-fall period of weak rain of thunder clouds evaporation in thicker dry air level,or the drag of heavy precipitation in linear convection accompanied with dry air entrainment raindrops. The characteristics in radar echo presented the sharp decline of centroid height and mid-altitude radial convergence( MARC).