Formation Mechanisms And Organization Of Mesoscale Convective Systems In Presummer Extreme Rainfall Over South China

Heavy rainfall events over South China frequently occur during the pre-summer rainy season(April-June).With high population density and rapid economic development in South China,physical mechanisms of the inland and coastal heavy rainfall in South China during the pre-summer rainy season arose great attentions around the world.This study investigates formation mechanisms and organization of mesoscale convective systems in extreme rainfall events over South China during the pre-summer rainy season.Firstly,eight rainfall events are selected to analyze the key large-scale dynamic factors contributing to the concurrent double rainbands over inland and coastal South China,respectively.Secondly,the initiation and maintenance of two mesoscale convective systems(MCSs)producing extreme rainfall over coastal South China on 11 May 2014(namely,the “5.11” rainfall event)are studied.Thirdly,the fine structure of the MCSs with multiple mesoscale rainbands in the “5.11” rainfall event is analyzed,focusing on the relation between the MCSs' organization and the extreme rainfall production.Finally,we investigate the formation of the bow echoes and the associated rear inflows,which play important roles in the formation of the mesoscale rainband training in the“5.11” rainfall event.The main results are as follows:(1)Basing on the hourly precipitation data collected at densely distributed surface Automatic Weather Stations(AWSs)during the South China Monsoon Rainfall Experiment(SCMREX),eight precipitation events with both inland and coastal heavy rainfall concurrently occurring during May and June of 2011-2017 are selected.The key large-scale dynamic factors contributing to formation of the double rainbands are analyzed by applying the Ensemble Sensitivity Analysis(ESA)method to global ensemble forecast results.It reveals that the double low-level jets(LLJs),one in the planetary boundary layer(PBL)and the other in the lower-tomiddle troposphere,are the major contributors.The double LLJs are associated with synopticscale weather systems(fronts,cyclonic vortex,or shear line)over South China.The inland rainband is closely related to both low-level jets at 925 and 850 h Pa,especially their meridinal wind component.A more intense cold front or shear line is accompanied by stronger southwesterly jets on the adjacent south side,favoring a large amount of inland precipitation.The forecast errors of inland rainfall are mostly produced by the forecast errors of geostrophic meridional wind component of the jets.The warm-sector coastal extreme rainfall is more closely associated with the boundary-layer jet(BLJ)at 925 h Pa.A more intense meridional wind component in the southwesterly BLJ leads to stronger convergence near the coastline of South China,which can result in stronger coastal rainfall.The forecast errors of coastal rainfall are mainly contributed by the forecast errors of ageostrophic meridional wind component of the BLJ.(2)The extreme rainfall event(maximal amount up to 451 mm in 24 h)that occurred in coastal South China on 11 May 2014 during the SCMREX is investigated using integrated observations from the dual-Doppler radar pair,extensive mesonetwork,and enhanced upperair soundings.Results show the generation of the extreme rainfall by two consecutive MCSs consisting of multiple meso-?-scale rainbands.The two MCSs occurred over the worm sector to the south of the mid-level trough,low-level shear line and surface front.The initiations of the two MCSs are closely related to the lifting effect of mesoscale topography near the coastline.MCS1(MCS2)is initiated by lifting southeasterly oceanic flows over the northeast-southwest oriented Mt.Longgao(Mt.Ehuangzhang)near the coastline,resulting in continuous convective initiations at the windward slope.New convections move toward the northeast along the Mt.Longgao(Mt.Ehuangzhang),and consist into a meso-?-scale ranband,which would develop into MCS1(MCS2)later.Besides,MCS1 is also initiated by lifting southerly oceanic flows over a coastal mesoscale outflow boundary(MOB)to the south of a convectively generated weak cold pool in previous day.New convections are initiated along the MOB,and develop into MCS1 after the merger with the rainband near the Mt.Longgao.The two MCSs are maintained by lifting southerly oceanic flows over a MOB along the coastline that are enhanced by a convectively generated cold pool.Owing to the precipitation evaporative cooling and cloud shielding effect,the cold pool can persistently exist with a MOB lying between the convectively generated northerly outflows and oceanic southerly flows.The integrated analysis of surface observations and dual-Doppler synthesis demonstrate the lifting effect of the shallow MOB at and beyond the surface,which is a clear show of the 3-D dynamic field of such shallow MOB.New convective cells near the MOB favor the maintenance of two MCSs and the production of extreme rainfall,which also in turn favor the maintainence of the cold pool and the associated MOB.This positive feedback mechanism leads to the long lasting of the two MCSs and the production of extreme rainfall.(3)After the convective initiations near the MOB,the northeastward “echo training” of convective cells,under the influence of environmental southwesterly flows,leads to the extreme rainfall over the surface rain gauge.During the development of convective cells in“echo training” process,the diameter of raindrops decreases,and the number concentration increases significantly,resulting in heavy precipitation.Due to the unevenly distributed convergence along the MOB,“echo training” of convective cells leads to the formation of multiple rainbands in each MCS.Their subsequent propagations in a “rainband training” form,together with the “echo training”,account for the extreme rainfall.The variation of the surface rainfall is closely related to the angle between the orientation of the rainband and its moving direction.The second MCS is characterized with a leading bowing rainband(also as bow echo)showing a process of rapid splitting and reestablishment(RSRE)process,which contributes to the formation of the rainband training.The occurrence of the RSRE process requires ample supply of unstable upstream oceanic flows,the quasi-stationary MOB,and a bowing rainband intersecting with the MOB.The second MCS produces more precipitation than the first one as a result of more rainbands,stronger convective intensity,and more moderate-sized raindrops with larger maximal sizes.(4)Dynamic diagnosis is conducted using the dual-Doppler retrieved 3-D wind fileds to better understand the processes contributing to formation of the two bow echoes(BE1,BE2)in MCS2.Results reveal that the formation of the two bowing rainbands are closely related to the formation and development of rear inflows.The anticyclonic vortex at the southwest of BE1 contributes to the formation of the rear inflow.The appearance of the anticyclonic vortex is prior to the formation of BE1,and contributes 54% of the rear inflow.In a few minutes,the bowing structure forms with two vortex couplets.The anticyclonic vortexes contribute to 58%of the rear inflow,with little contribution from the cyclonic vortexes.Vorticity budget shows that the anticyclonic vortexes are mainly formed through the stretching of the horizontal vorticity.Slightly differ from BE1,BE2 forms and develops only with anticyclonic vortexes.The 62% of the rear inflow at the initiation stage of BE2 is contributed by anticyclonic vortex,which is mainly formed through the horizontal advection and tilting of horizontal vorticity.At the mature stage,it is the horizontal advection and tilting of horizontal vorticity forming the anticyclonic vortexes,and contributing about 52% of the rear inflow.