Investigation of convection velocities of three velocity components and scalars in the atmospheric boundary layer

In this study, a dataset of synchronously measured wind speed, temperature, and relative humidity acquired by an unmanned aerial vehicle array at multiple streamwise locations is employed to investigate the convection velocities of three velocity components, temperature, and relative humidity in the atmospheric boundary layer at heights up to 150 m. Results show that the global convection velocities of the streamwise (Ucu), spanwise (Ucv), and vertical (Ucw) wind speed components, as well as temperature (Uct) and relative humidity (Uch), are statistically slightly larger than the local mean velocity (U). Statistical analysis of the ratios between each convection velocity and the U reveals that the convection velocities of three velocity components and scalars are roughly 10% higher than U. Scale-dependent convection velocities of streamwise velocity, temperature, and relative humidity show that these values decrease with increasing frequency within the range 0.01 Hz ≤ f ≤ 0.1 Hz, and the convection velocities of large-scale (or low-frequency) flow structures at f = 0.01 Hz are approximately three times larger than U. In contrast to laboratory turbulent boundary layer flows, atmospheric boundary layer flow can receive energy input at its upper boundary, which may drive high-speed flow structures toward the ground. This is consistent with the hypothesis that the enhanced convection velocities may be associated with large-scale turbulent structures and the possible downward transport of high-speed flow structures in the atmospheric boundary layer.