In this study,based on observations and numerical simulations,the squall line in front of the tropical cyclone in Hunan and Jiangxi Provinces of China,which preceded the super typhoon "Rammasun"(2014) was simulated and investigated,and the formation and development mechanism of this pre-TC squall line were discussed,as the vertical wind shear in this case was significantly weaker than the squall line in mid-latitude.The diagnostic analysis results showed the following:
Due to the fact that the typhoon moved northward and the subtropical anticyclone moved westward,the pressure gradient between them increased,which led to the enhancement of the low-level air current.The mid-and low-level wind field was a relatively homogeneous southwesterly flow in the area,which was influenced by the squall line,thus the wind direction and wind speed changes were not significant in the vertical direction.As a result,the low-and mid-level vertical shear before the squall line in front of the tropical cyclone were weaker than the mid-latitude squall line.Based on the mesoscale numerical model,the thermodynamic characteristics of the squall line at different stages under weak shear environment were analyzed emphatically.The results showed that the balance between the relatively weak vertical shear and weak cold pool led to the erect updraft of the squall line at the initial stage of the squall line in front of the tropical cyclone.Significant unstable environmental conditions at the leading edge of the squall line with cold and dry air overlapping warm and moist air were an important thermodynamic factor contributing to its development and enhancement.The horizontal wind shear on the right side of the typhoon inverted trough and the gust front which was formed by the low-level divergent airflow and middle-layer rear inflow jet in front of the squall line were the main dynamical factors related to its development.During the mature period,the gradual widening and strengthening of the cold pool on the rear side of the squall line and the squall line in front of the tropical cyclone away from the typhoon led to the decrease of moisture and energy supply.At the same time,the cold and dry air invasion in the mid-level behind the squall line strengthened and diffused toward the lower layer on the side of warm and moist air,which was associated with the warm and moist air flow updraft over the line tilted toward the cold pool side.Consequently,the vertical lifting weakened,which was not conducive to the formation of a new convective cell along the outflow boundary,and was detrimental to the development and maintenance of the squall line.These two features indicated the weakening of the squall line during this period.At the dissipation stage,the squall line moved too far away from the typhoon,and the influence of the typhoon on the squall line became extremely weak.Insufficient moisture supply and lifting led to the decay of the squall line in front of the tropical cyclone.