| A severe convective weather with hail and strong winds accompanied by the squall line process occurred in Nantong,Jiangsu Province on 30 April 2021.The maximum wind speed reached 45.4m s-1in Nantong area,which caused certain casualties and property losses.In order to deepen the understanding of the squall line gale weather process and improve the ability to analyze and forecast the severe convective weather,this paper analyzes the weather background and environmental instability conditions of the squall line process by using the reanalysis data of ERA5,the reflectance data of the new generation Doppler radar in East China and the souding data.On this basis,the mesoscale WRF-ARW numerical model is used for high resolution numerical simulation to investigate the causes of the squall line and gale.Then,for this typical case,the synergistic effects of gale generation factors are investigated through controlled tests,cold pool intensity sensitivity tests and cloud condensation nucleus concentration sensitivity tests to understand the mechanisms of cold pool and atmospheric pollution influence on squall line and associated catastrophic gale processes.The main conclusions are as follows:（1）The squall line occurs under the combined effect of high and low level weather systems.The strong cold air moves southward under the action of the upper and middle level cold vortex,with the warm and humid air flowing northward in front of the low level trough.And the sufficient water vapour converge in the area of Jiangsu and Zhejiang.The"post trough type"squall line weather characteristics are very typical,the ground cold front transit triggers the squall line strong convection generation.（2）The gale weather in the Nantong area is generated against the background of environmental potential instability,strong unstable energy accumulation and strong environmental vertical wind shear.The squall line is well developed when the gale process occurs,and the combined effect of strong cold pool outflow,allobaric wind dispersion and momentum transportation downward at high altitude increases the wind speed at ground level.（3）The intensity of the cold pool affects not only the structure and strength of the squall line,but also the generation and strength of the gale.The combination of corresponding dynamic and thermal forces affect the intensity of the downdraft,thus affecting the surface wind speed.The intensity and range of the cold pool are weakened by the increase of middle-level water vapor,the squall line intensity gets weakened and the moving speed gets slow.When the raindrop evaporation is turned off,the resulting convective system has no cold pool,and its structure is different from the squall line.The weakening or disappearance of the cold pool weakens the downdraft,weakens the divergence in the lower layer,and the momentum in the high layer cannot be transmitted downward,leading to the weakening of the surface wind speed.（4）Cloud microphysical processes also influence the effect of cold pools and momentum downward transportation and hence surface wind speed,which decreases with increasing cloud condensation nucleus concentrations.As cloud condensation nucleus concentrations increase,changes in hydromorphic content within the squall line weaken the latent heat release from the middle and upper levels,evaporation cooling at lower levels and the dragging effect of rain water and graupel,leading to a weakening of sinking motions and cold pools,further inhibiting the downward transfer of momentum aloft,which together lead to a reduction in surface wind speed. |
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