摘要
引入切变风螺旋度和热成风螺旋度,并将其应用于东北冷涡暴雨的诊断分析。理论上,切变风螺旋度定义为风速垂直切变与绝对涡度矢量的点积,表示风速垂直方向的分布不均匀对涡管的扭转效应,由扭转项和垂直涡度的辐合辐散项两部分组成。热成风螺旋度是在切变风螺旋度的基础上利用地转关系和热成风关系得出的简化形式,其强度和符号取决于上升气流和暖湿空气的配置。相对于切变风螺旋度,热成风螺旋度的计算只需要单平面层的资料即可,避免了垂直差分计算,这大大弥补了台站观测中垂直层密度稀疏或者边界层的处理等问题的不足,使得计算大大简化,便于业务应用。在以上定义和理论分析的基础上,选取一次东北冷涡降水过程进行数值模拟,利用模式输出的中尺度资料,诊断分析这次降水过程中的切变风螺旋度和热成风螺旋度。分析表明,降水中心位于切变风螺旋度的正值和负值区的边界,与降水的强度变化一致;而作了热成风近似后的切变风螺旋度中的扭转项(即热成风螺旋度),与切变风螺旋度相似,也能较好地诊断降水和对流(尤其是强降水和强对流)的发展,而且其对暴雨的诊断优于传统的螺旋度。
The shearing wind helicity and thermal wind helicity are introduced and applied to a northeastern cold vortex rainfall event. In theory, shearing wind helicity is defined as the dot product between vertical wind shear and Absolute vorticity vector, which represents the twisting effect of nonuniform distribution of wind vector in the vertical direction on vorticity tube. It includes two terms, torsion and the divergence of vertical vorticity. By use of geostrophic relation and thermal wind relation, the shearing wind helicity is simplified as thermal wind helicity. Its intensity and sign are determined by the collocation between the upward air stream and warm-moist air. Compared with shearing wind helicity, thermal wind helicity is calculated at a single level, which avoids vertical differential and offsets the shortage of data in the boundary and sparse vertical layers. Thus the calculation is simplified largely and conveniently applied to operation. Based on the above definitions and theory analyses, a case study is performed. A northeastern cold vortex rainfall event is simulated. By using the mesoscale model output, shearing wind helicity and thermal wind helicity are analyzed. It is found that, large-value centers of precipitation are located at the interface between positive and negative shearing wind helicity values. The intensity variation of shearing wind helicity is identical with that of precipitation. Similar to shearing wind helicity, by using the thermal wind approximation, the torsion term in the shearing wind helicity formula （i.e., thermal wind helicity） has indicative sense to precipitation and the development of convection, especially for strong precipitation and convection. Furthermore, it corresponds with precipitation and the development of strong convection better than the traditional helicity does.
出处
《大气科学》
CSCD
北大核心
2009年第6期1238-1246,共9页
Chinese Journal of Atmospheric Sciences
基金
国家自然科学基金资助项目40633016
40805001
关键词
切变风螺旋度
热成风螺旋度
东北冷涡
暴雨
shearing wind helicity
thermal wind helicity
northeastern cold vortex
storm rainfall
