Study of the evolution characteristics of a long-life squall line with its triggering and maintenance mechanism

Based on various data collected by surface and high conventional observation, FY-4A satellite, dual polarization radar, radar mosaic data and ERA5 reanalysis, this paper analyzed the evolution characteristics, circulation background, environmental conditions, triggering and maintenance mechanism of a long-life squall line in Henan. The results are as follows. (1) The evolution characteristics of the squall line can be divided into four stages. The first one is the initial stage, in which the scattered and isolated convective cells were triggered and then developed into non-organized thunderstorms embedded supercells. The second is organizational development stage, where multiple storms merged and grows rapidly into linear convective system, with significant rear inflow jet and forward propagation, gradually transitioning to a larger bow structure. The third is mature stage, during which the scale of the bow echo further increased, exhibiting a classical bow echo structure. The fourth is weakening stage, the intensity of the squall line weakening and breaking into two segments, the northern segment quickly moved eastward, while the southern segment moved southward slowly and developed slightly. (2) This squall line process occurred under the circulation background of stable Subtropical High, eastward moved low trough and lower shear line. The significant middle-level dry air, high convective available potential energy and moderate vertical wind shear provided favorable conditions in the early stage, strong vertical wind shear played an important role in the later stage. (3) Terrain triggered the scattered and isolated convection cells, which gradually merge and develop into a linear convection system near the surface convergence line. The cold pool outflow and the warm-wet dense zone can form a strong updraft and trigger the new convections, the synchronous enhancement of the cold pool and vertical wind shear also contributed to the maintenance of the squall line. The continuous merging of convections is a key factor for the development and maintenance. In additional, the squall line exhibited a self-organization structure of the coexistence of ambient inflow and system outflow in horizontal direction and the coexistence of lower-level convergence and higher-level divergence in vertical direction, which is also one of the reasons for the long-term maintenance of the squall line.