Numerical Simulation And Diagnostic Analysis Of A Squall Line Process In 4～5 July 2013

A squall line system that occurred during 4～5 July 2013 is analyzed, and a sensitivity numerical simulation study is carried out to investigate the the vertical structural features, inherently unstable mechanisms of squall line, and the effects of latent heating and surface fluxes. Results reveal that:(1)The squall line is located in front of the trough in middle troposphere, coupling with high and low level jets. Squall line system is on the right side of upper jet which is divergence area, the left side of low level jet which is convergence zone. (2)Mature stage squall line has two inflows in the low level of troposphere and two outflows in the high level of troposphere. In addition, the middle level inflow and outflow in front of the system formed a indirect vertical circulation together, causing dry cold air falls to middle level, which enhances the instability ahead of the system. (3) Low level MPV of a y scale convective cell is analyzed, the results reveal that convective instability is more important in development stage, then turns weak in mature stage, while conditional symmetric instability strengthens and maintains convection mature stage caused by enhanced wet baroclinicity and low-level jet. (4)Condensation latent heating contributes to the low-level inflow and high-level outflow of squall line system largely. Indirect vertical circulation before system is promoted by latent heating effect, which results in intensify of high-level momentum’s spreading downward. (5)In mature stage, middle and high level atmosphere is heated by condensation latent heating within cumulus convection, intensifing low-level convergence and high-level divergence, which refuels upward motion largely. Then powerful updraft flow results in stronger latent heating, which forms a positive feedback mechanism with processes above. (6)By affecting the antecedent environmental circulation, the surface heating fluxes also play an important role in the trigger and maintain stage of squall line. Firstly, surface heat flux in daytime enhances the convective boundary layer turbulent mixing, which gradually forms an unstable stratification. Secondly, the boundary layer humidification effect produced by surface latent heating flux stores abundant vapor and unstable energy before system’s outburst.