Effects of Swell Waves on Atmospheric Boundary Layer Turbulence: A Low Wind Field Study

The effect of swell waves on atmospheric boundary layer turbulence under low winds was explored using data from a fixed platform located in the South China Sea. The wind spectra, cospectra, and Ogive curve measured at a height of 8 m above the mean sea surface provided direct evidence that wind stress was affected by swell waves. To interpret such phenomena, an improved approach was derived based on the fact that the total wind stress was the vector sum of turbulent stress and wave-coherent stress. Different from the approaches of earlier studies, our approach did not align the turbulent stress with the mean wind speed. The influence of swell waves on the magnitude and direction of the total wind stress was analyzed using our approach. The results showed that the wave-coherent stress derived from our data accounted for 32% of the total wind stress. The magnitude and angle of the wind stress changed by swell waves depended on the relative angle between the turbulent stress and swell direction. Plain Language Summary A better understanding of the interactions between ocean waves and the atmospheric boundary layer (ABL) is crucial for predicting the future state of air-sea interactions and the long-term development of the climate. Many recent studies have shown that the behavior of the ABL under swell waves (ocean waves with old ages) is significantly different from that under young waves. In this study, we focus on the effect of swell waves on ABL turbulence by using observational data from a fixed platform located in the South China Sea. We show that swell waves can not only induce upward momentum but also absorb momentum. The change in the wind stress vector by swell waves also depends on the relative angle between the turbulent stress and swell direction. To interpret these phenomena, an improved approach is given here based on the fact that the total wind stress is the vector sum of turbulent stress and wave-coherent stress.