Swell and the drag coefficient

Simultaneous measurements of waves and turbulent fluxes were collected from a moored surface buoy in the Philippine Sea. Waves were partitioned into their wind sea and swell components, and the ratio of swell to wind sea energy was used to assign a swell index. The 10-m neutral drag coefficient was calculated using the eddy correlation method. Four hundred hours of data were processed in 30 minute runs for wind speeds 8.5 to 16.5 m s−1 when the peak wave direction was within 90° of the wind direction and included observations during mixed seas, swell dominant, and wind sea dominant conditions. The data were analyzed to explore the influence of swell on the drag coefficient. It was found that when compared to periods of equal wind speed, the drag coefficient was reduced up to 37 % when swell energy was twice that of the wind sea energy. It is believed that this reduction was due to a decrease in the turbulent flux around the swell frequency, suggesting that the swell diminishes the surface aerodynamic roughness.