华南涡旋暴雨的物理对比分析

低涡型暴雨是华南地区一种重要的暴雨类型,它常常发生在对流层低层天气尺度切变线上,虽难从气压场上找到,但它所造成的暴雨却很强。我们发现,常规天气图上似乎很弱的这类涡旋,却有较强的流场涡度和典型的散度分布,构成了强降水过程的必要物理条件。本文把它与热带气旋发展过程中的物理结构、能量变化和转换特征进行了诊断、对比分析,指出它们不仅三维物理量场分布差异较大,其发展过程中能量变化也有很大差异。热带气旋的K,值远大于切变线低涡,但K_φ的变化则相反,无论是其绝对值或它占总动能的比值,都不 

THE COMPARISON ANALYSIS OF MESO-SCALE LOW VORTICES IN SOUTH CHINA

By analyzing two kinds of meso-scale low vortices with heavy rainfall, the comparison study of their physical structure, energy variation and conversions has been made. It indicates that for the landing tropical cyclone, the temperature-pressure field is more symmetric with very strong upward motion in its middle. Therefore, its vorticity, correspondingly .the rotational winds kinetic energy K_φ is larger than the one of the baroclinic vortices. However, the latter one occurring on a shear line has a typical structure with strong convergence in lower levels overlapping a divergence centre in the higher levels. The temperature gradient near the vortex centre is rather clear. It is notable that the divergence winds kinetic energy K_x related to the divergent field for the former one is much larger than the one in tropical cyclone during their development period, though the meso-scale low on the shear line seems very weak appearing at the isobar maps. A large amount of potential energy P transfers to K_x and becomes the major energy source during the strengthening period of the tropical cyclone. For the mesoscale low on the shear line, the energy conversion from K_x to K_φ is very strong through the whole period. It is verified further from this study that the energy conversion from K_xto K_φ is an important step for the development of meso-scale low vortex either for the tropical or the baroclinic one. This kind of conversion will be more active when the potential energy P is released in a large amount.