Turbulent characteristics in the atmospheric boundary layer under neutral and unstable thermal stratifications based on LES

Abstract

In the atmospheric boundary layer (ABL), thermal stratification plays a crucial role in influencing turbulent characteristics. However, comprehensive analyses of turbulent characteristics under varying thermal conditions, particularly concerning higher-order statistics, remain limited. Therefore, in this study, one neutral and seven unstable turbulent boundary layers were reproduced by large-eddy simulations. The results show that under unstable conditions, buoyancy effects primarily influence the streamwise velocity in the near-wall region, vertical velocity in regions slightly away from the wall, and the temperature throughout the entire boundary layer height. Turbulence intensity in the vertical direction is significantly affected by the buoyancy effects, while the turbulence intensities in the streamwise and lateral directions are primarily dominated by shear production. As the bulk Richardson number increases, boundary layer height and maximum turbulent kinetic energy decrease. Furthermore, thermal stratification can influence higher-order turbulence statistics near the surface under strongly unstable conditions, but the overall statistical patterns tend to exhibit universal behavior. Within the boundary layer height, the skewness and kurtosis velocities in the unstable cases exhibit generally consistent trends with those in the neutral case, within the range of bulk Richardson numbers from −0.3 to 0.