Boundary layer characteristics during the process of a South China squall line

Investigating the changes in boundary layer turbulence during the growth of squall lines and their underlying causes is crucial for enhancing the understanding of squall line intensification mechanisms. This study uses the Weather Research and Forecasting (WRF) model to simulate a squall line event in South China from May 11 to 12, 2020. Based on the agreement between simulated and observed data, high-resolution model output is used to analyze the boundary layer structure and turbulence flux characteristics of the squall line process, with turbulence kinetic energy (TKE) features analyzed through the turbulence kinetic energy budget equation. The results are as follows. (1) The squall line process is divided into four stages—formation, development, maturation, and dissipation—based on radar echo evolution and observational data. During the formation stage, there is significant accumulation of environmental instability energy. In the development stage, a steep gradient of surface equivalent potential temperature (θ_se) is observed, surface cyclogenesis intensifies, triggering the release of instability energy, leading to convective initiation and an increase in squall line horizontal scale. In the maturation stage, the squall line moves southeast and merges with dispersed convection, with mid-level dry and cold air influx strengthening subsidence and forming a cold pool at the surface. As the middle of the squall line breaks, it enters the stage of extinction. (2) The boundary layer turbulence during the squall line event is intense, with TKE accumulating during convective initiation and subsequently decreasing; as the squall line merges with dispersed convection and reaches maturity, TKE rapidly increases to an unusually high value, then decreases to a minimum during nighttime weakening. (3) During the formation stage, strong turbulence facilitates substantial latent heat flux upwards, creating a highly unstable environment; by the maturation stage, latent heat flux reaches its peak in the stratiform cloud region behind the squall line, with the squall line intensifying further. (4) TKE variations are primarily influenced by wind shear and buoyancy terms. During convective initiation, ground vortices result in wind shear being 2-3 times the negative buoyancy term, with terrain uplift contributing to increased TKE. After squall line formation, the cold pool reduces TKE, while in the maturation stage, the squall line moves into complex terrain where both positive wind shear and negative buoyancy terms peak, with wind shear exceeding buoyancy terms by more than ten times, resulting in maximum TKE and enhanced turbulence flux.