MCC转为带状MCSs过程中水平涡度的变化与暴雨的关系

RELATIONSHIP BETWEEN THE VARIATION OF HORIZONTAL VORTICITY AND HEAVY RAIN IN THE PROCESS OF MCC TURNING INTO BANDED MCSS

利用实况资料和WRF中尺度数值模式对2010年6月18—19日的一次MCC转带状MCSs的暴雨过程进行数值模拟与诊断分析。结果表明:850 hPa西南涡和切变线的形成与维持是影响此次暴雨产生的中尺度系统,前期MCC的形成到成熟以低涡降水为主,后期的圆形MCC转为带状MCSs主要为切变线降水。在雨区附近,u、v的垂直切变所形成的强水平涡度造成的旋转,对应垂直环流的上升支可触发暴雨产生,垂直方向上u、v不同的分布可形成不同的垂直环流。低涡与切变线附近的水平涡度有明显差异,这种差异导致暴雨形成的原因不同,低涡暴雨主要由v的垂直切变造成,切变线暴雨主要由u、v的垂直切变共同作用,本次过程中v的垂直切变构成了沿切变线的东西向雨带,u的垂直切变沿纬向的不均匀性引起的垂直运动与切变线上MCSs的生成、发展和多雨团的形成关系密切。低涡、切变线降水中心附近的正倾侧项(水平涡度向垂直正涡度转换)也有类似的差异,低涡的转换主要由v/p<0决定,切变线的转换主要由-u/p>0决定。水平涡度向垂直涡度的转换尺度较小,易在平均状态下被忽略。倾侧项主要有利于暴雨的加强,但对西南涡、切变线的发展贡献较小。

By using real-time data and the mesoscale WRF model, a heavy rain event in the process of MCC （Mesoscale Convective Complex） turning into banded MCSs （Mesoscale Convective Systems） during 18--19 June 2010 is simulated and analyzed in this paper. The result shows that the formation and maintenance of a southwest vortex and shear line at 850 hPa are mesoscale systems. The low-vortex heavy rain mainly happened in the development stage of the MCC, while the shear rainfall occurred in the late stage of the round MCC turning into banded MCSs. It is further found that the vertical shear of u and v fields forms strong horizontal vorticity near the rainfall area. The rotation caused by horizontal vorticity corresponding to the ascending branch of vertical circulation can trigger rainstorms, and the different distribution of u and v fields may produce different vertical circulation. The horizontal vorticity near the low vortex and shear line have obvious differences which may lead to different reasons of the formation of rainstorm. The vertical shear of the v field mainly causes the low-vortex heavy rain, while the shear rainfall forms owing to the vertical shear of both u and v fields. The vertical shear of the v field leads to an east-west oriented rain belt along the shear. Meanwhile, the vertical motion aroused by nonuniformity of the vertical shear of the u field has close relationship not only with the generation and development of MCSs, but also with the formation of multiple rain clusters. The positive twisting term （the conversion of horizontal vorticity to positive vertical vorticity） near the rainfall center has the similar differences between the low vortex and the shear line. The conversion in the low vortex is determined by igv/ap〈O, while that of the shear by -~u/~p〉O. Due to the small conversion scale in the process of horizontal vorticity transforming to vertical vorticity, the twisting term may be ignored on average and it is mainly beneficial for strengthening the precipitation, with less contribution to the development of the southwest vortex and shear line.