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LEI Lei, SUN Jisong, CHEN Mingxuan, et al. 2021. Organization Process and Thermal Dynamic Structure of a Squall Line in Beijing [J]. Chinese Journal of Atmospheric Sciences (in Chinese), 45(2): 287−299. doi: 10.3878/j.issn.1006-9895.2005.19198
Citation: LEI Lei, SUN Jisong, CHEN Mingxuan, et al. 2021. Organization Process and Thermal Dynamic Structure of a Squall Line in Beijing [J]. Chinese Journal of Atmospheric Sciences (in Chinese), 45(2): 287−299. doi: 10.3878/j.issn.1006-9895.2005.19198

Organization Process and Thermal Dynamic Structure of a Squall Line in Beijing

  • On Aug. 7, 2015, a broken-line convective system appeared in the northwest of North China, then moved to the southeast and collided with a multi-cell system over the plains of Beijing, which eventually organized to form a strong squall line that caused local flash flooding, wind gusts, and large hailstones to fall over the Beijing area. Based on multiple data sources, our analyses indicate that the squall line formation process had three stages: First was the development and movement of a broken-line convective system in the upstream, followed by the regeneration and consolidation of multiple cells over the plains, and then the organization of a squall line once the upstream broken-line convective system had crossed over the mountains and merged to form multi-cell storms over the plains. During the second stage, local convection was triggered just north of the city by the inhomogeneous temperature distribution combined with local convergence. Along with a cold pool and expansion of the inhomogeneous temperature area, the regenerated convection propagated south ward due to the southward intensified temperature gradient. At the squall-line development stage, the dynamic structure was characterized by two strong inflows—a mid-tropospheric rear inflow at a height of 4500–5000 m and another strong inflow at the squall line moving in a low-level direction perpendicular to the orientation of the squall line. These two inflows induced separate vertical clockwise circles in front of and behind the squall line. The vertical circulation in front of the squall line was continuously intensified as the rear and front inflows were enhanced, which corresponded to a strengthening of the vertical wind shear. This dynamic process was advantageous to an ambient vertical shear in the squall-line organization, which was also a significant factor in the rapid movement and development of the squall line. When the rear inflow disappeared, the frontal vertical circulation weakened and the squall line gradually dispersed. Third, regarding the thermodynamic structure, a stronger cold pool appeared with a temperature disturbance of −8°C at a depth of 1.5 km when the upstream convective system merged with the multi-cell system over the plain. As a result, the upward motion was strengthened at the leading edge of the meso-β temperature gradient. This favored the intensification and development of the squall line. Lastly, a long gust front was induced with the strong squall line.
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