Diagnostic Analysis And Simulation Of The Convective Initiation Of A Squall Line Over Yellow-Huai River Basin On 14 June 2009

The convective initiation (CI) process has remained a challenging problem in the study of mesoscale meteorology. A severe squall line event occurred over Yellow-Huai River Basin on 14 June 2009, taking this case as an example, based on various observational date and numerical simulation, a detailed analysis of the CI and the formation of the squall line is conducted. It is showed that the severe weather was occurred in a large background with the strong northwest blow with cold advection behind the upper trough associated with a cold vortex over Northeast China.The large convective instability energy, favorable moisture and thermodynamic structure also provided conditions for convection formation. At the initial time some low-level shallow cloud clusters were appeared near the ground convergence line, the deep convective cloud clusters would not develop until the middle or lower level small scale convergence enhanced significantly. The CI over Shandong province was triggered when a small scale discontinuous wind speed disturbance appeared in middle level, where the local convergence strengthen, and then made a rapid development of cloud. The trigger of Henan initial convection is due to pressure-gradient force strengthened during the large scale adjustments which caused wind increasing somewhere then the convergence strengthened, the initial convective clouds developed subsequently at the triple point between the convergence and low-level shear line. At the same time, the boundary outflow from Shandong convection also enhanced the lower layer convergence near Henan. It can be found that the subsequent growing of CI cloud is partly related to the interaction between the CI cloud and environmental wind field. The air flows associated with the Shandong initial convection are similar with those of a mature squall line, which would lead to new convective clouds generation constantly and incorporate into the convective system eventually make the system reinforce into a squall line. During the Henan convective system developing into squall line, the new convective cells generated at the rear of system due to its low-level rear outflow collided with the environmental wind. The squall lines developed from Shandong and Henan merged into as a bigger one at last. During the process the low-level front convergence lines merged firstly. The convergence at the intersection point was enhanced which triggered new convection which adjust the convergence line shape, and made the squall lines in the unity.