Analysis of Boundary Layer Structure, Turbulence, and Flux Variations before and after the Passage of a Sea Breeze Front Using Meteorological Tower Data

A detailed analysis of a sea breeze front (SBF) that penetrated inland in the Beijing–Tianjin–Hebei urban agglomeration of China was conducted. We focused on the boundary layer structure, turbulence intensity, and fluxes before and after the SBF passed through two meteorological towers in the urban areas of Tianjin and Beijing, respectively. Significant changes in temperature, humidity, winds, CO₂, and aerosol concentrations were observed as the SBF passed. Differences in these changes at the two towers mainly resulted from their distances from the ocean, boundary layer conditions, and background turbulences. As the SBF approached, a strong updraft appeared in the boundary layer, carrying near-surface aerosols aloft and forming the SBF head. This was followed by a broad downdraft, which destroyed the near-surface inversion layer and temporarily increased the surface air temperature at night. The feeder flow after the thermodynamic front was characterized by low-level jets horizontally, and downdrafts and occasional updrafts vertically. Turbulence increased significantly during the SBF’s passage, causing an increase in the standard deviation of wind components in speed. The increase in turbulence was more pronounced in a stable boundary layer compared to that in a convective boundary layer. The passage of the SBF generated more mechanical turbulences, as indicated by increased friction velocity and turbulent kinetic energy (TKE). The shear term in the TKE budget equation increased more significantly than the buoyancy term. The atmosphere shifted to a forced convective state after the SBF’s passage, with near isotropic turbulences and uniform mixing and diffusion of aerosols. Sensible heat fluxes (latent heat and CO₂ fluxes) showed positive (negative) peaks after the SBF’s passage, primarily caused by horizontal and vertical transport of heat (water vapor and CO₂) during its passage. This study enhances understanding of boundary layer changes, turbulences, and fluxes during the passage of SBFs over urban areas.