Mechanism Study Of Torrential Warm-Sector Precipitation Over Beijing Based On Multi-Source Remote Sensing Data Assimilation

Warm sector torrential rainstorms(WSTRs),located in warm zone of cyclone or front of frontal surface,often occurred suddenly in the local area,which can usually caused the destructive flood and endanger people's lives and property.Subsynoptic scale small perturbation(such as ground convergence lines,gust fronts,cold pools etc.)closely related to the formation of WSTRs cannot be distinguished by global operational forecasting models,which limited the precdit ability of WSTRs.There was a torrential rainstorm occurring in Beijing on July 21,2012(“721” torrential rain).It brought a severe flood disaster that occurred only once in 50 years.In just 1-2 days,113 people died and 10 billion yuan lost in the capital and its surrounding areas,which had a great domestic and international impact.The “721”torrential rain can be divided into two stages,an earlier-occurring warm-sector precipitation(WSP)stage and a later-occurring cold-frontal precipitation(CFP)stage.Based on the multisource data assimilation,the formation and evolution processes of the "721" storm had been successfully simulated.Using observered analysis and simulation diagnosis methods,the studies of dynamics,cloud micro-physical and precipitation efficiency of the "721" warm sector precipitation process were carried out.The main conclusions are summarized as follows:(1)The warm sector precipitation had two types of triggering mechanisms which ouccred in Taihang Mountain and Huabei Plain,respectively and also had significant organizational development characteristics.Under the combined action of the Hetao cyclone circulation,the southwest airflow convergence and the local topographic convergence line of the Taihang Mountain,the warm sector convections in mountain area were continuously triggered.Hetao cyclone moved eastward down the mountain,causing the lower atmosphere to oscillate,triggering the gravity wave,which acted together with upward flow of the low-level wind shear's secondary circulation in plain area led the warm sector convections in plain area to trigger.With the southwest flow's steering,convection cells propagated from southwest to northeast along the Taihang Mountain and gradually formed southwest-northeast convective wave trains with obvious backward-forming characteristics.Therefore the convection cells in plain area propagated southeast-northwest convective wave trains with the near-surface southeast wind's steering.The two convection wave trains in different direction converged at the convergence center of the northeast side of the Hetao cyclone(Beijing Fangshan),which significantly enhanced the mesoscale convection system.That maintained locally for long time,forming an extreme value center of WSTRs.(2)The convections of warm sector preicipitation had the characteristics of low-mass center tropical precipitation echoes.The cloud microphysical processes restricted by it had significant feedback effects.The main reasons for strong WSP were that there were abnormally abundant water droplets in the low clouds.On one hand,a large number of low-level water droplets grew into raindrops through coagulation(PRA),then,falling to the ground.On the other hand,the strong upward flow in warm sector took lots of water droplets up to the level above the 0°C,forming a thick supercooled water layer.The upper-level ice-phase particles seeded in the supercooled water layer,and collected by supercooled water(PRACG?g2r,PRACS?s2r),then drops to form lots of raindrops;In addition,a quantity of graupel particles melted near the 0°C layer(PGMLT)which is an important source of rainwater.The generous WSP caused heavy rainwater evaporation(PRE),leading to the strong ground cold pool's formation near the ground.The outflow of the cold pool not only enhanced the MCS,but also easily triggered new convective cells.In CFP stage,the clouds is higher and the ice phase process is stronger.Mass latent heat were released through the upper-level deposition process to enhance the upper updraft.A large number of graupel particles melted into raindrops in the melting layer.However,there was strong dry and cold air intrusion of middle level leading warm and humid air of low level entrained dry and cold air.On the one hand,the rainwater evaporation(PRE)is greatly enhanced,which is not conducive to the formation of large raindrops fallen to the ground.On the other hand,the enhanced low-level downward flow caused the supercooled water droplets difficult to form.And then the collection process of icephase particles by raindrops in melting layer weakened.The important rainwater sources lacked.Therefore CFP is significantly weaker than WSP.(3)The large-scale(LSPE)and the cloud microphysical(CMPE)precipitation efficiency of the WSP stage were significantly higher than the CFP one.Taking the concentration period of WSP and CFP as an example,the average LSPE and CMPE of WSP were 4%-6% higher,and the highest was over 10%.In WSP stage,the lower level was warm and damp advection while the middle level was cold-drying advection in CFP stage,which resulted in higher LSPE in WSP stage.The collection function of supercooled water droplets by snow in WSP stage was significantly stronger than in CFP stage,which resulting in higher CMPE in WSP stage.Based on the above,this paper established three possible physical concept models of the“721” torrential rainfall event.