ISSN 1000-0526
CN 11-2282/P
CN 11-2282/P
Archive > Volume 46 Issue 5 > 2020,46(5):643-654. DOI:10.7519/j.issn.1000-0526.2020.05.005 Prev Next
Study on Two Successive Downhill Line Convections Developing on Different Boundary Layer Conditions in Beijing Area
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Abstract:
Two line convections hit Beijing Area successively and caused shorttime heavy rainfalls, gales and small hails on 7 July 2017. In this paper, the two cases were compared based on the high spatiotemporal resolution observations and the Variational Doppler Radar Analysis System (VDRAS) dataset. Both of the line convections were formed on the mountains in Zhangjiakou, located to the northwest of Beijing, and developed at the foothills and on the plains. However, they had different boundary layer conditions, as the second one developed on the cold mass caused by the first one. The results are shown as follows. The first line convection developed into a squall line benefited from both dynamic and thermodynamic conditions of the foothills and the plains, which includes the hot and humid environment in the boundary layer, the convergence line generated by the outflows of the cold pool and the lowlevel environmental southerly wind. At its mature phase, the squall line had the strongest convergence from surface to the height of 1.5 km and an approximately vertical updraft. The second line convection developed into an elevated convection on the cold mass caused by the previous one. The area of strong echo (≥45 dBz) maintained, though the updraft tilted and had a certain decrease. The thermodynamic conditions seemed to be no longer beneficial due to the wide and strong cold mass caused by the first one. The outflows from the two lines formed a convergence line, and the convergence line intensified especially when the cold mass of the second one met the cold mass caused by the first one, which offered dynamic conditions for the second one. The unstable layer above the boundary layer provided convective energy for the elevated convection. Besides, the rebuilt of convective available potential energy (CAPE) after the first convection is possible as warm advection and positive water vapour transport did exist in Beijing Area. Above all, both dynamic and thermodynamic conditions should be 〖JP2〗considered in nowcasting of convective storms, where VDRAS could play a positive role.
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