北京7.21暴雨低涡演变的湿位涡分析

通过诊断再分析资料和数值模拟结果,从中层湿位涡守恒和低层湿位涡变化的角度分别对北京7.21暴雨过程中中尺度低涡的演变进行分析。结果表明,暴雨前期,对流层中高层高湿位涡的冷空气扩散南下,冷空气到达华北地区上空时,在有利等熵面的引导下从稳定层结向不稳定层结快速下滑,产生了剧烈的正涡度个别变化,使得低涡得到发展加强。另一方面,等熵面上冷暖空气的剧烈交汇在增强雨势的同时,也使得对流层低层至中层产生明显的涡度制造。在不考虑稳定度影响时,低层的非绝热过程引起的湿位涡制造与低涡发展有着很好的正相关,二者在位置上和量级上都有很好的对应。进一步分析表明,非绝热加热的水平不均匀分布是引起湿位涡变化的主要原因。

Moist potential vorticity analysis of the mesoscale vortex in the heavy rainfall on July 21, 2012 of Beijing

By means of reanalysis data and simulation result, the evolution of the mesoscale vortex in the heavy rainfall on July 21, 2012 of Beijing was analyzed through the conservation and evolution of Moist Potential Vorticity (MPV) in middle and low troposphere. Results show that before the heavy rainfall, the southward extension and diffusion of cold air within large MPV produced severe vorticity increment, which enhanced the evolution of meso-scale vortex during downward gliding along the moist isentropic surface from stable to unstable stratification. On the other hand, the strong convergence of cold and warm air not only enhanced the precipitation amount but generated severe vorticity increment in the low and middle troposphere as well. The analysis on vorticity evolution caused by MPV development shows that the development of MPV and the evolution of vorticity match each other very well in the matter of location and magnitude. Further analysis demonstrates that the horizontal inhomogenization of nonadiabatic heating was the main cause of MPV development.