Observed Surface Drag Coefficient Under High Wind Speed Conditions and the Relationship With Coherent Structures

The drag coefficient is a crucial parameter in the interaction between the Earth's surface and the atmosphere because it directly determines the momentum flux. This study examines the variation in the drag coefficient with wind speed over the land surface using observations from two typhoon cases and one cold front case. The analysis shows a significant decrease in the drag coefficient, roughness length, and zero plane displacement when the wind speed is greater than approximately 15 m s(-1). The drag coefficient is proven to be positively related to the coherence of the turbulent structures. The momentum transport modes changed at 15 m s(-1). By regarding the large eddies as scales larger than observation height, it is found that large-scale coherent structures dominate the momentum transfer in strong wind conditions, while small eddies contribute significantly in low to moderate wind conditions. Momentum transport in such large coherent eddies is inefficient, leading to the reduction of the surface drag. The results suggest that the formation of inefficient large coherent eddies is likely due to the existence of inverse energy cascade.