Thermal buoyancy effects on coherent structures and structure functions in the atmospheric surface layer

Multi-point synchronized high-frequency streamwise velocity datasets, measured at distinct heights within the logarithmic region of both laboratory wind tunnel flows and atmospheric surface layer (ASL) flows, were utilized to investigate the effects of atmospheric thermal stability on coherent structures as well as second-order and higher even-order structure functions in the ASL. This study reveals that ASL thermal conditions exert a significant influence on the geometric morphology and energy-containing characteristics of coherent structures, thereby profoundly modulating the logarithmic scaling behavior of second-order and higher even-order structure functions under unstably stratified ASL conditions. More specifically, the thermal buoyancy effect significantly enhances the energy of large-scale energy-containing coherent structures. Consequently, the slopes of second-order structure functions in the inertial range increase with variations in thermal stability. In addition, our analysis demonstrates that the impact of thermal buoyancy stability on the ratios of structure functions across different orders is relatively minor. Statistically insignificant discrepancies were observed in these ratios when derived from wind tunnel experiments, unstably stratified ASL observations, and near-neutral ASL observations. Thus, despite the modulating effect of thermal stability, accurate quantification of these structure function ratios remains feasible. This study may contribute to a more comprehensive understanding of turbulence driven by both shear and thermal buoyancy in the ASL.