080125南方低温雨雪冰冻天气持续降水的数值模拟

Numerical Simulation of 080125 Cold Air,Freezing Rain and Snow in Southern China

利用WRF模式对2008年1月25—29日中国低温雨雪冰冻天气过程进行模拟。结果表明,雨雪期间长江中下游及以南地区长时间存在着高低空急流的耦合形态,且低空急流不断向雨雪区域输送暖湿水汽,使该地区低层的水汽辐合,促进大范围雨雪发生和维持。强高空辐散的抽吸作用,促进低空辐合、整层上升运动加强以及正涡度的维持。干冷空气从对流层高层倾斜南下加强了对流不稳定能量的积累。本次雨雪冰冻天气过程中存在明显的干侵入,降水区北侧对流层高层高位涡干冷空气沿等相对湿度(RH)线密集带侵入低层,促使雨区低层位涡中心迅速增大,促进强降水发生;本次过程表明位涡和降水有很好的对应关系,这对降水预报有很好的指示意义。由于极涡偏强促使冷空气南下,南方近地面浅薄冷空气使雨水结成冰导致灾害发生。

We simulate the low-temperature and freezing-rain/snow event in southern China during 25th--29th January 2008 by using the WRF model, and the results show that there appeared the coupling of lower and upper level jets over the middle and lower reaches of the Yangtze River and the area south of the fiver during the rain/snow episode ;and the low level jet persistently transported warm/wet air towards the rain/snow area, where the low level moisture convergence promoted the occurrence and maintenance of large-scale rain/snow weather. The pumping effect of strong upper level divergence was conducive to the low level convergence, the ascending motion of the whole layer, and the maintenance of positive vorticity in the lower level. Cold air descended from the upper troposphere and moved southwards, enhancing the accumulation of convective unstable energy over the rain/snow area. The cold/dry air with high potential vorticity in the upper troposphere in the north side intruded along the dense HR contour zone into the lower layer of the rain/snow area,rapidly reinforcing the center of potential vorticity there. The close correspondence relation between potential vorticity and precipitation provides a better indicator for precipitation prediction. The stronger polar vortex allowed cold air to plunge southwards to southern China,the surface layer air of very low temperature froze rain water,resulting in the disaster of freezing rain in the episode.