云微物理对一次吉林暖区降水过程的影响

Impact of Cloud Microphysics on a Process of Warm-Sector Precipitation over Jilin,Northeast China

本文运用WRF3.9区域数值模式模拟了2017年7月13日吉林省永吉县暖区暴雨,较好地再现了此次暴雨过程的中尺度对流系统的单体触发、线状对流群触发、组织化发展以及弓状回波等典型阶段的细致过程;在此基础上,分析了造成暖区降水的中尺度对流系统的云微物理特征,探讨了其影响暖区降水的可能机制。结果表明:吉林永吉暖区降水发生在东北冷涡主导的有利的多尺度环境配置下,引发暖区降水的中尺度系统主要是冷云系统,暖区范围大,过冷水分布位置高,冰晶粒子与过冷水并存,并存区的“播种”效应使得其下方生成大量霰。雨水质量收支及热量收支分析表明:暖区降水系统的触发及组织化阶段,雨水的主要来源是云滴碰并增长,主要汇项是冰晶对雨水的收集;而弓状回波阶段,降水的主要源项除了云滴碰并增长之外,霰融化作用也起到关键的作用,降水主要汇项在低层为雨水蒸发,高层为霰对雨滴的收集;暖区降水的主要热源是水汽凝结潜热释放,主要冷却项是雨水和云水的蒸发。弓状回波阶段,其前部的入流与地面冷垫上方的后向入流汇合后将水汽带入高层;“播种”效应使距地面8 km高度附近的霰粒子含量显著增多,该高度与水汽凝结释放大量潜热形成的高温区重合,故霰粒子大量融化为雨水,产生强降水过程。

A warm-sector rainstorm occurring in Yongji,Jilin Province on July 13,2017 was simulated utilizing the regional model WRF3.9,and the storm development process was reproduced,which included the stages such as the initiation of mesoscale convective cells and linear convective clusters,organized system development,and the formation of bow echos.Based on the simulation data,the characteristics cloud microphysices for the mesoscale convective systems were analyzed,and their impact on the warm-sector precipitation was discussed.The results show that the precipitation process in the Yongji occurred under favorable conditions in which multi-scale environmental structure was dominated by the northeast cold vortex.The mesoscale systems was mainly a cold cloud systems,in which there was a wide warm zone and a high location of supercooled water,with the coexistence of ice and supercooled water.The"seeding"effect of the coexisting area caused a large amount of graupel.The budget analyses of the mass-and heat-hydrometeors showed that during the triggering and organizing stage of the precipitation system showed that the main source of rainwater was the accretion growth of cloud droplets,and the main sink was the collection of raindrop by ice.During the bow-shaped echo stage,besides the accretion growth of the cloud droplets,the melting of graupel also served as the main source,while the main sinks were the evaporation of rainwater in the lower layer and the collection of rainwater by the graupel in the upper layer.The main heat source of warm-sector precipitation was the latent heat released from condensation of water vapor,and the main heat sink(cooling)was the evaporation of rain and cloud water.Furthermore,during the bow-shaped echo stage,the confluence of the inflow at the frontend with the air inflow at the backend above the cold pad on the ground brought water vapor into the upper layer.The"seeding"effect significantly increased the content of the graupel particles around the height of 8 km above the ground,which coincided with the high temperature area as generated by the releasing of a large amount of latent heat through the condensation of water vapor,causing the melting of a large amount of the graupel and resulting in a strong precipitation system.