Evolution and Development Mechanisms of a Rare, Strong Arc-Shaped Squall Line Occurring in Northern Beijing in 2017

Based on NCEP (National Centers of Environmental Prediction) analyses data and various observations from automatic weather stations, cloud-to-ground lightning positioning system, stationary meteorological satellites, and weather radars, this paper uses "ingredient-based method" to have comprehensively analyzed environmental conditions, evolution characteristics, trigger and development mechanisms of a rare strong arc-shaped squall line with maximum instantaneous wind of nearly Beaufort Scale 12 and large hail, which swept the northwestern Hebei Province and central and northern Beijing during the period from afternoon to midnight of 7 July, 2017. The squall line occurred in the southwest side of a 500-hPa cold vortex. Strong low-level jet, larger convective available potential energy (CAPE) above 2000 J/kg, intense 0–6 km and 0–3 km vertical wind shear were very favorable for the development and maintenance of the squall line and a supercell storm embedded in the squall line, which produced large hail and high winds. Level of the wet bulb temperature 0 °C at 3.8 km altitude was in favor of producing large hail. Dewpoint deficit of the middle troposphere up to 30 °C and larger vertical temperature lapse rate caused larger downdraft convective available potential energy (DCAPE), which favored very much bow echoes and high winds. The convection initiation of the squall line was triggered near a surface convergence line between northwesterly winds and southwesterly winds. High winds and large hail were mainly located in the low TBB and active positive cloud-ground lightning area. Radar observations show that the squall line developed from a linear convection system to a cluster supercell storm, and finally to an arc-shaped squall line with significant overhang echoes, weak echo regions, mesocyclone (or mesovortex), and strong rear inflows which caused rear inflow notches. Intense downdraft induced by strong jet in the middle of troposphere and high dewpoint deficit was the main cause for the formation of the bow echoes in the squall line. The mechanisms of maintaining the squall line and bow echoes include: large CAPE and DCAPE, and intense vertical wind shear.