The Formation Mechanism And Structure Of A Squall Line On 19th March 2013 In South Of China

Based on several kinds of observational data and numerical simulation, the formation mechanism of a strong squall line on 19 March 2013 in South of China is investigated. The results are as follows.(1) The squall line lies in front of 500 hPa trough, In 700 hPa, there is a shear line and a low level jet. There is also a Southwest vortex in 850 hPa, a cold front and a transverse trough on the ground. Environment wind is mainly the southwest airflow, and its cross-line component vertical shear is smaller than along-line component, this is much different from shears of classic TS mode squall lines. Vertical shear of environment wind is moderate.(2) The squall line is composed by two parts of convective cells. The east part convective cells emerge as broken-areal mode on the north of transverse trough, and the west cells form along mesoscale surface convergence line. Convective cells in west part are initiated by gravity waves.The bowing structure breaks from east part afterwards and the east part echo of the squall line decreased; new stratiform cloud then came out behind of bowing structure, but it is apparently nearer from bowing echo than that when squall line wasn’t broken.(3) The Change of cold pool is obvious. Cold pool becomes strongest when the squall line is the phase of mature stage. The vertical structure of the squall line is influenced by the cold pool. The vorticity torsion and advection are very important to the balance of positive vorticity and negative vorticity in the squall line.(4) The initiation of convection is closely related to mesoscale shear line and pre-front jet cores. According to WRF simulation, it is believed that perturbation of horizontal wind caused by gravity wave is positive at an altitude of 2~3km, and the amplitude is about 4~6m/s. When these positive centers locate on the southwest wind, low level jet cores emerge and are in favor of appearance and development of convective cells.