Statistical Characteristics And Numerical Simulation Of Mesoscale Convective System Under Northeast Cold Vortex And The Relation Between Squall Line Evolution And Horizontal Vorticity

Based on the FY-2E and FY-2G geostationary satellite blackbody temperature data,NCEP/NCAR analysis data and hourly precipitation data in summer(from June to August)from 2013 to 2017,the statistical characteristics of the mesoscale convective systems(MCSs)under Northeast Cold Vortex in Northeast China were analyzed.Referring to the definition of MCS by Jirak(2003),the spatial and temporal distribution and large scale environment field of MCSs are researched.Then a squall line process in Bohai Bay on July 25,2016 was simulated using WRF mesoscale numerical model and Doppler radar observation data to study the formation and maintenance of squall line.The results show that:(1)MCS distribution has obvious monthly variation and diurnal variation,the most active month is June;eastward,northeastward and southeastward are the three main moving directions for MCSs.(2)The area of MCS maturing time,the eccentricity and life history of MCS in northeast China are smaller than those in the Jianghuai region and the central and eastern China.And the characteristic of rapid development and slow extinction is consistent with the central and eastern region.(3)Based on the definition of Zheng(2013)(Z standard),the statistical analysis of 2016-2017 MCS is made and compared with the results obtained by Jirak definition(J standard).It is concluded that the environmental field characteristics of the two definitions are basically the same,which mainly show that the MCS are generated in front or rear of 500 hPa trough.MCS is located in the divergence area between the two jet streams,also near the north branch jet at the upper troposphere.At the lower layer,MCS is located on the left side of the low level jet stream,the south and southeast side of the vortex,with strong water vapor and momentum transportation.There is dry warm air and potential vorticity transportation from the vertical reverse loop flow at the middle layer in south of the MCS.MCS which generated in rear of trough,at the same time,had the cold and dry air on the north side of the MCS strengthen the frontal area and the upward movement,which is beneficial to the formation of MCS.(4)There is obvious difference in precipitation between the two recognition criteria,and Zheng's standard is better than Jirak's standard in statistical work of heavy rainfall.The relationship between squall line formation and maintenance and horizontal vorticity are studied by using the results of model simulation.It is found that the anticlockwise bending of the horizontal vorticity vectors at the upper and lower level is of great significance to the formation and maintenance of squall line.Before the squall line occurs,a counter-clockwise rotating center of horizontal vorticity at 200 hPa appears in the west side of the Bohai Bay together with a strong divergence at 900 hPa.The horizontal vorticity in the lower layer bends counterclockwise,which provides a favorable upward motion condition for the formation of the squall line.Then the horizontal vorticity cyclonic vortices of several convection cells in the upper troposphere merge to form a unified large-scale cyclonic vorticity structure,which triggers the ascending motion of the lower layer.At the same time,the uniform cyclonic curvature at 900 hPa in front of the convective region makes the convection cells organize into a banded structure,forming the squall line.During the mature period of squall line,the horizontal vorticity in the upper layer presents a unified large-scale cyclonic vortex structure,while in the lower layer it presents a typical S-shaped curved structure.The horizontal vorticity x component is positive,negative and positive along the convective belt from south to north,and the y component is always positive and the central value of the convective belt is the largest,decreases from the center to both sides of the convective belt,which shows the importance of the rotation direction of the horizontal vorticity vector to the squall line From the analysis of the vertical vorticity equation,it can be concluded that the radar reflectivity echo exhibits obvious rotation around 500 hPa before the formation of MCV(mesoscale convective vortice)in the middle period of squall line development,which is mainly related to the strong positive vorticity horizontal advection term over 500 hPa,the mid-level tilting term and the horizontal divergence term,which makes the middle wind field produce cyclonic rotation and form MCV.