华南涡旋暴雨的物理对比分析

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

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_(?)的变化则相反,无论是其绝对值或它占总动能的比值,都不如切变线低涡大,散度风的变化对于后者的发展至关重要。前者在发展过程中有大量位能向散度风动能转换,构成其主要发展能源;而后者则主要靠散度风动能向旋转风动能转换维持其发展。通过本分析,进一步证实由散度风动能向旋转风动能转换是中尺度系统发展的普遍规律;当有大量位能释放时。这种转换就更趋活跃。

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 Kt 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 Kx 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 Kx 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 Ki to Kt is very strong through the whole period. It is verified further from this study that the energy conversion from Kz to K+ 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.