| In the weather events over Central East China,extreme rainfall events caused by mesoscale convective systems(MCS)are often in sudden and heavy.With global climate changes,its impact on people's production and life becomes increasingly prominent,so this type of event has been the focus of China's meteorological research.According to the systems that produced precipitation,this paper first counted and classified the extreme rainfall events over Central East China from 2016 to 2017.The spatial and temporal characteristics of various types of MCS-related events were then analyzed in detail,and the formation environment of multiple-MCS-type events was also studied.Secondly,for the specific event in Hunan,Jiangxi and Fujian during July 17-18,2016,the triggering and maintenance mechanisms of MCS at each stage were analyzed.The main conclusions are as follows:(1)During 2016-2017,a total of 204 events were collected over Central East China.Based on the rainfall-producing systems,they were divided into four types:synoptic type,MCS type,SSS type and unclassified type.MCS-type events were further divided into the subclassifications of leading stratiform MCS,parallel stratiform MCS,trailing stratiform MCS,training line/adjoining stratiform MCS,back building MCS,unorganized MCS,and multiple MCSs.Generally,the synoptic events had the highest frequency,among which vortices and shear lines were more likely to produce extreme rainfall than the pre-front environments.In MCS type,multiple-MCS events were most commonly observed.(2)MCS-type events were mainly concentrated in the coastal areas of South China,south of the Yangtze River,the lower reaches of the Yangtze River,and the Huang Huai Basin,in which multiple-MCS events were the most common type in South China and the middle reaches of the Yangtze River.The events happened mostly from April to October with the peak in July.From spring to summer,the areas where rainstorms occurred shifted from the south to the north,returning to southern China in autumn.From diurnal perspective,lots of precipitation dropped from afternoon to evening,reached the peak around noon and ended before dark.Comparatively,multiple-MCS type had a wide distribution of the onset time,peak rainfall time and end time.In each event,the extreme precipitation of multiple-MCS events lasted longer.(3)Among all MCS subtypes,multiple-MCS events had the highest frequency and the longest mean duration.According to when the maximum hourly rainfall was reported during the rainfall process,these events were divided into early-maturing and late-maturing types.The formation environments of these two types were studied respectively,which was found that there was strong low-level jet at 850h Pa,and a positive vorticity advection appeared at the center of the rainfall domain.However,before the occurrence of early-maturing cases,there were obvious convergence and a tighter pseudo equivalent potential temperature(?se)gradient above the most extreme rainfall-occurrence point,but there were more southerly component and higher?se in the late-maturing composite.Additionally,the early-maturing events had more obvious humidity conditions,the atmosphere had a greater potential of instability,and the lifting condensation level(LCL)was lower.It can be seen that the precipitation of early-maturing events was caused by upward motion and warm features of the pre-environment,while the late-maturing precipitation depended on the warm-air transportation after the event onset.(4)Research on the July 17-18,2016 event shows that the rainfall occurred in the environment with the retreating of West Pacific Subtropical High and the high-altitude shortwave trough moving eastward.The convergence between 925h Pa and 500h Pa,and the suction effect of 200h Pa-divergence were also beneficial conditions.During the whole process,there were four main MCS activities.The back building of cells and the dynamic structure of adjoining stratiform were the main organization types of the MCS in the early precipitation period,while the second system was likely a parallel stratiform MCS.During the early-maturing-rainfall stage,the rain belt first strengthened and then showed adjoining stratiform features.In the late period of precipitation,the back building of convections was significant at the very start,and the rain belt split and regenerated later.The uplifting near boundary layer was the main triggering mechanism of the MCS at the first stage.The near-surface cold pool formed by evaporation cooling played an important role in its organization and maintenance.The triggering of the MCS at the late-maturing stage was related to the atmospheric environment,terrain and the cold pool outflow.The ascending motion generated by convergence triggered the release of unstable energy,enabling MCS to be maintained.During the late period of the first two stages,the cold pool shifted and kept the southerly airflow ascending,producing the MCS of the third stage.Thereafter,the outflow boundary of the cold pool moved to the southeast,triggering new cells.The MCS during the last period formed in an environment where the southwest warm and moist air strengthened,the releasing of unstable energy was the key factor for the convection maintenance. |
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