Case Analysis of a Squall Line Process in Beijing Area in 2021

From 18:00 June 25, 2021 to 01:00 June 26, 2021 a squall line appeared in Beijing. Synoptic scale environment conditions, physics parameters fields, Beijing Doppler radar products are analyzed. Results are as follows. 1) At 500 hPa, Beijing was in front of trough which from mid-eastern inner Mongolia to central Hebei. Temperature trough lagging behind height trough over the baroclinic zone in the troposphere is favorable to system developing. Surface chart shows high east and low west situation. 2) Beijing located in the center of 850 hPa \(\theta_{se}\) high energy zone, which does benefit to accumulate and trigger of unstable energy for convective weather.3) Sounding analysis at 14:00 June 25 showed that the curve of temperature and dew point temperature was trumpet shape from bottom to top. The distribution form of upper dry and lower wet and the appropriate height of 0° layer and −20° layer were beneficial to the occurrence of large hail. The vertical shear of the wind direction was a clockwise rolling flow, when the low-level water vapor supply was sufficient, convection development was an important indicator of sever convective weather. 4) Reflectivity factor profile showed obvious echo wall, highly suspended strong echo, bounded weak echo region, the obvious jump of VIL, the appearance of ZDR column and KDP 4.3°/km at corresponding time indicated the occurrence of hail. Analysis of radial velocity map showed that the ambiguity of radial velocity is closely related to the extreme wind at ground stations.