"04.12"华北大到暴雪过程切变线的动力诊断

利用地面实测资料和MM5模式输出产品,对2004年12月20~22日发生在华北地区的大到暴雪天气过程的切变线进行了动力诊断分析,结果表明:此次暴雪过程与中尺度切变线的发展东移直接关联。涡度诊断表明:正涡度区的演变与切变线的发展、东移和北抬密切相关,正涡度区内“正涡度核”对预报强降雪的出现有先兆指示意义。涡度、散度垂直剖面图显示,涡度、散度场的空间配置极有利于暴雪切变线发展及暴雪形成与维持。湿相对位涡和涡度变率诊断揭示,涡度变率强度与中低空的条件对称不稳定密切相关;暴雪区上空从低层到高层存在的湿位涡负值中心是造成中低层涡度变率增大及暴雪增幅的重要原因之一;而涡度变率较涡度更能准确反映切变线发生发展的物理机制。

The Dynamical Diagnosis of Snowstorm over North China in December 2004

Based on the products of the mesoscale numerical model MM5 and surface observational data,the dynamical diagnosis of snowstorm over North China during December 20~22,2004 has been analyzed in this paper.The results showed that this snowstorm was directly related to the mesoscale shear line.The diagnosis of vorticity showed that the evolution of positive vorticity area was closely related to the movement towards the east,development,and lifting towards the north.The positive vorticity center can predict the snowstorm in advance.The vertical profile of the vorticity and divergence showed that collocation of vorticity field and divergence field was benefit to the development of snowstorm shear and form and maintaining of snowstorm.The diagnoses of moisture potential vorticity and vorticity variability showed that the intensity of vorticity variability was closely related to the conditional symmetric instability on the mid-low high.The negative center of moisture potential vorticity from low to high levels was one of the important reasons causing vorticity variability and storm increasing.The vorticity variability more accurately reflected physical mechanism of the snowstorm shear than the vorticity.