Characteristics And Mechanism Of Quasi-Linear Convective Systems In North China

June to August is the main flood season in North China.Affected by the belt of westerlies and subtropical high,continental and oceanic monsoon flows converge here.There existing not only the North China plain near the Bohai Sea,but also many mountains such as Lvliang,Taihang and Yanshan mountains.Therefore,the weather is complex and changeable makes it difficult to predict.The QLCSs(quasi-linear convective systems)are the important severe convective systems that affect North China in flood season and often cause strong weather disaster.However,systematic statistical research of QLCSs is still lacking.At the same time,there are few research results on the mechanism of the action of the topography in North China.In order to establish a complete case database of QLCSs in North China,a set of objective identifying algorithm of QLCSs is developed.The concept of skeleton in Computer Graphics is applied to meteorology,such as echo image preprocessing,skeleton pruning and length-width ratio quantization techniques are developed.The QLCSs in radar echo mosaics conform to the meteorological standards can automatically be identified by this method.Firstly,the detailed identifying algorithm is introduced based on a double QLCSs process in Huang-Huai area in 2016.Then,the QLCSs in Anhui Province in June 2016 are objectively identified by this method,and the moving characteristics of QLCSs are quantitatively.The comparison between disastrous weather observation and subjective identification are carried out.The results show that the shape information of meteorological echo is well preserved by using skeleton image identifying algorithm and the effective identification of QLCSs is realized base on accurately quantifying the long and short axes of convection system.Based on radar mosaic data and skeleton recognition algorithm,171 QLCSs in North China from 2013 to 2018 are given.According to the spatial-temporal distribution characteristics and the statistical results of severe convective weather,the result as follows:the spatial and temporal distribution of QLCSs in North China is not uniform,with obvious annual and monthly variations.The diurnal variation characteristics of its formation and extinction are obvious,too.It is easy to generate in the afternoon and weaken at night.QLCSs are less distribution on the plateau,which often occur along mountains or on the plains,and the Taihang Mountains is an important high-frequency area.So it indicates that the daily variation of thermal conditions and topography have an important impact on the occurrence and development of QLCSs in North China.According to the standard of meteorological disaster warning signal,at least two types of QLCSs exist: one type with strong thunderstorm wind gusts and the other with extreme heavy rainfall.Furthermore,the characteristics of circulation patterns,environmental conditions,topographical process and surface cold pool are given for these two types of QLCS.The findings are as follows: The atmospheric baroclinicity of QLCSs with strong thunderstorm wind gusts is relatively obvious.The large value area of BCAPE(best convective available potential energy)and DCAPE(downdraft convective available potential Energy)caused by the dry middle layer and large temperature reduction rate is an important environmental condition for the generation of strong thunderstorm wind gusts.The strong cold pool and the vertical shear of 0-3 km wind play an important role in forward propagation.The extreme precipitation event caused by QLCS with heavy rainfall is more prominent than the previous type and usually occurs in weak weather scale forcing.We also find plentiful vapor are supplied and the properties of warm-sector heavy rainfall.The back propagation maintained by the interaction between the weak cold pool or the windward slope and the low level south wind flow is the main mechanism of development and slow movement,which is also the direct cause of extreme heavy rainfall.It can be seen from the statistical results that topography played an important role in the formation of QLCSs in North China.Two representative QLCSs processes were screened for high-resolution numerical simulation experiments to explore the formation mechanism of the linear structure of QLCSs.During the formation of QLCSs on August 4,2013,the atmospheric environment was relatively dry,and MAULs(moist absolutely unstable layers)caused by uphill winds were an important reason for triggering linear convection.Due to the sunlight during the day,the mountainous area heats up faster than the basin,forming uphill winds.The boundary layer wind field converges significantly along the mountain range.Then the continuous vertical ascending movement causes the low-level water vapor convergence of the mountain range,forming a water vapor saturation zone along the mountain range.Eventually the convection is triggered in the MAULs area,and the echo also shows a band structure along the mountain.During the formation of QLCSs on July 24,2016,the atmospheric environment was wet,and the formation and development of linear echoes were divided into two stages.In the first stage,the mountainous area on the south side of the trumpet-shaped topography continuously triggers new convection and moves into the plain area on the north side of the trumpet-shaped topography under the action of the guided airflow.The convective cells merge and develop behind the main body of the convection,and the linear structure gradually forms.The numerical simulation sensitivity test finds that after removing the mountain on the south side of trumpet-shaped topography,no band formed.In the second stage,when convection develops to a mature stage,a ground cold pool is formed.The warm and humid southerly airflow interacts with the outflow of the cold pool on the south side of the linear echo,triggering a new convection,and the backward propagation maintains the linear form of the echo.