2.1 Wind tunnel experiments
The wind tunnel experiments were conducted at the State Key Laboratory of Earth Surface Processes and Resource Ecology, Beijing Normal University. The wind tunnel is a blowing type with a total length of 71.1 m, the experimental section is 24 m long, 3 m wide, and 2 m tall, and the thickness of the boundary layer was about 70 cm in the experimental section. We installed roughness elements upwind of the experimental section to adjust the wind velocity profiles so they would resemble the profiles observed in the field under different free-stream wind velocities. Details are provided by Cheng et al. (2015).