超级单体风暴中大冰雹增长机制的模拟研究

Numerical Investigation on the Growth of Large Hail in a Simulated Supercell Thunderstorm

为调查超级单体中大冰雹的运行增长机制,使用三维冰雹分档对流云模式结合三维粒子运行增长模式,对一例超级单体风暴进行了数值模拟。实测风暴的结构如中气旋、弱回波区、前悬回波等被很好地模拟再现,显示了模式对超级单体具有良好的模拟能力。雹胚在风暴发展阶段由过冷雨滴冻结产生,主要分布在主上升气流区上部,在主上升气流区西北侧中高层也有相当数量的雹胚粒子,冰雹主要分布在主上升气流区东侧。风暴发展阶段产生的雹胚有7%～8%增长到1cm以上,1%左右增长到2cm以上,这些大冰雹绝大多数起源于主上升气流区北侧的高层云区,气旋性进入主上升气流区按照简单的上—下形式增长,少数大冰雹起源于主上升气流区西北侧风暴后部中高层,气旋性地沿着主上升气流区的边缘按照下—上—下形式运动增长,表明了超级单体中大冰雹存在两条增长路径。

A three-dimensional convective cloud model with a hail-bin microphysics scheme and a hail growth model were used to further investigate the growth mechanism of large hail in supercell storms. The observed supercell features such as quasi-steady mesocyclone, a pronounced bounded weak echo region and a forward overhang were reproduced well by the simulation. During the storm developing stage, hail embryos were produced mainly by the freezing process of raindrops and occurred at the top of the main updraft region and the mid- to upper-portion of the storm from the north- west flank of the main updraft region. Approximately 7%-8% of hail embryos forming at the developing stage continued to grow into hail larger than 10 mm in diameter; only 1% of hail embryos ultimately achieved diameters of 20 mm. Most of the largest hailstones grew from millimeter-size embryos that originated in the upper-level region around the north flank of the updraft and thereafter entered the main updraft cyclonically and grew into large hailstones in a single up-down path. Other embryos originated in the upper-level region of the northwest flank around the main updraft and grew into large hailstones in a down-up-down path along the updraft edge cyclonically. These results suggest that there are two growth paths for large hailstones in supercell storms.