The drag coefficient C_dN is a key parameter for accurately modelling atmospheric and upper ocean dynamics. However, the scatter of C_dN has confused us for several decades. To better understand the mechanism behind the scatter of C_dN, the turbulent theory, which considers the momentum flux carried by the eddies that have vertical size s_z and horizontal size s_x, is used. By adding the wave coherent (WC) stress to the turbulent kinetic energy (TKE) budget, this study models the turbulent eddies reorganized by the waves. The result shows that the reorganized turbulent eddies affect the dimensionless wind gradient, leading to different roughness lengths z₀ derived from different levels of wave-affected wind speed and further resulting in the scatter of C_dN. Two experiments from a fixed platform located in the South China Sea, one is a regular observation, and the other is outside the periphery of typhoons, demonstrate the above finding. The observation shows that ratios of z₀ derived from different levels of wind rely on the proportion of WC stress to total wind stress, and the horizontal integral length scales of eddies are stretched while the vertical sizes of eddies are shrunk. This study indicates that, to reduce the scatter of C_dN, the wave-affected wind should be considered.
 

surface wave,marine atmospheric boundary layer;,turbulence;,momentum flux