1.State Key Laboratory of severe weather, Chinese Academy of Meteorology Sciences;2.Institute of Urban Meteorology, CMA
A broken line convective system, appeared in the northwest of North China, moved to southeast and collided with multi-cells in the plain of Beijing, and eventually formed an organizational strong squall line. It caused local flash rain, gust and big hail over Beijing on Aug. 7, 2015. Based on multiple sources data, the analyzes indicated that, (1) the squall line formation process had three stages: The developing and moving of broken line convective system in the upstream, the rebirth and consolidation of multiple cells in the plain and the organization of the squall line after the upstream broken line convective system crossed over mountains and merged into multi-cells in plain. During the second stage, the local convection was triggered near the urban city by the inhomogeneous temperature distribution together with local convergence；Corresponding with the cold pool, the reborn convection propagated toward south because of the enlarging scale of the inhomogeneous temperature and the southward intensified temperature gradient; (2) At the squall-line development stage, dynamic structure is characterized by two strong inflows. The mid-tropospheric rear inflow was at 4500 to 5000m height, another was the low-level strong inflow perpendicular to squall-line orientation. The two inflows induced two separated vertical clockwise circles at the former and behind the squall line. The vertical circulation in the front of squall line was intensified continuously since the rear and front inflows enhanced together, corresponding with strengthening vertical wind shear. This dynamical process induced the advantageous ambient mesoscale vertical shear for the squall-line organization, which also was a significant factor for the squall-line rapid movement and development. After the rear inflow disappeared, the frontal vertical circulation weakened and squall line dispersed gradually; (3) for the thermodynamic structure, a stronger cold pool appeared with the disturbance temperature -8℃, depth 1.5km, when the upstream convection system merged into the multi-cells system in the plain. As a result, upward motion was strengthened in the leading edge of the β-mesoscale temperature gradient. It was favor to squall line intensifying and developing and induce gust front.