| A rare strong arc-shaped squall line with high winds and large hail swept the northwestern Hebei Province and central and northern Beijing during the period from afternoon to midnight of 7 July,2017.Squall line is more likely to occur in the southern Beijing than in the north.The trigger and development mechanisms of this squall line are worth studying.Based on NCEP(National Centers of Environmental Prediction)analyses data,ECMWF(European Center for Medium-Range Weather Forecasts)prediction 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 this squall line.Furthermore,numerical simulation experiment is carried out by RMAPS-ST(Rapid-refresh Multi-scale Analysis and Prediction System—Short Term).The main conclusions are as follows:(1)The initial convection of the squall line occurred near a surface convergence line between northwesterly winds and southwesterly winds.The squall line occurred in the southwest side of a 500 hPa cold vortex.Strong low-level jet,strong high-level jet,larger convective available potential energy(CAPE),intense 0-6 km and 0-3 km vertical wind shear were favorable for the development and maintenance of the squall line,which produced large hail and high winds.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 was very favorable for bow echoes and high winds.The favorable environmental conditions of maintaining the squall line and bow echoes include: large CAPE and DCAPE,and intense vertical wind shear.(2)The evolution of the squall line was rare.Radar observations showed 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.(3)The best numerical simulation scheme is the Thompson microphysical scheme and the YSU boundary layer scheme of simulations at the horizontal resolutions of 9 km and 3 km,and the Morrison(2-moment)microphysical scheme and the MYJ boundary layer scheme of simulation at the horizontal resolution of 1 km.The development of the simulated squall line was about one hour earlier than the actual situation.The simulated large-scale circulation situation,the radar reflectivity and the 1h accumulative precipitation have a good correspondence with the actual situation;the radar reflectivity can reflect the evolution characteristics of the squall line,including the linear convection system,cluster storm,and an arc-shaped squall line.(4)At the mature stage,the rear inflow notches,overhang echoes,weak echo regions,mesovortex and strong rear inflows were successfully simulated.The ground mesoscale pressure field clearly showed the structure of “low pressure-high pressure-low pressure”.The high value region of ice-phase particle mixing ratio had a good correspondence with the actual lightening distribution.At the top of the "arched",the average ice phase particle content was large.A large amount of ice phase particle falling motion enhanced the downdrafts.The mesovortexs had strong convergence,which can enhance the rear inflow and downdrafts,which was conducive to the development and maintenance of the squall line.(5)The internal structure of the simulated squall line at the maturity stage was close to the best mode of the squall line according the RKW theory(cold pool-shear interaction theory)in qualitatively.Quantitatively,the propagation speed of the cold pool and the vertical wind shear component perpendicular to the squall line were compared.At the development stage,the ratio was less than 1.At the mature stage,the ratio was about 1.At the extinction phase,the ratio was more than 1.It showed that the propagation speed of the cold pool and the vertical vertical wind shear component perpendicular to the squall line were approximately balanced,which was close to the best development mode of the squall line according the RKW theory.Therefore,the RKW theory is suitable for explaining the dynamic and thermal mechanisms during the maturity phase of the squall line.The main innovations of this research include: First,the evolution of the squall line is rare,which evolved from a linear convection system to a cluster supercell storm,and finally to an arc-shaped squall line.Second,1-km resolution simulation experiment provided high-resolution data to find the mesovortex at the mature stage of the squall line,which were related to the formation mechanism and maintenance of the squall line.Third,the RKW theory is validated both qualitative and quantitative perspectives,which can explain the maintenance mechanism of the squall line. |
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