Evolution and development mechanisms of a rare strong arc-shaped squall line that occurred in northern Beijing in 2017

Based on the NCEP (National Centers of Environmental Prediction) analysis data and various observations from automatic weather stations (AWS), cloud-to-ground lightning positioning system, stationary meteorological satellites and weather radars, the present study uses a "ingredient-based method" to comprehensively analyze environmental conditions, evolution characteristics, trigger and development mechanisms of a rare strong arc-shaped squall line with maximum instantaneous wind on nearly F12 scale and large hails. The squall line originated on the southwestern side of a 500 hPa cold vortex and swept northwestern Hebei Province and central and northern Beijing during the period from afternoon to midnight on 7 July 2017. The strong low-level jet, large convective available potential energy (CAPE) above 2000 J/kg and intense 0-6 km and 0-3 km vertical wind shears were very favorable for the development and maintenance of the squall line. A supercell storm embedded in the squall line produced large hails and high winds. The lower level of 3.8 km altitude for the wet bulb temperature 0℃ was in favor of producing large hails. Dew-point deficit that was up to 30℃ in the middle troposphere and large vertical temperature lapse rate caused large downdraft convective available potential energy (DCAPE), which promoted the formation of 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 hails 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 the strong jet in the middle of the troposphere and high dew-point 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 strong vertical wind shear.