General Sea State and Drag Coefficient Observed near Shore in Taiwan Strait

Abstract

The general characteristics of wind waves and drag coefficient are studied using the data from the buoy observation near shore in Taiwan Strait. An algorithm of the bulk aerodynamic method using 10-minute mean wind speed and the temperature difference between the air and sea surface was developed to calculate the equivalent wind speed at 10 m in height and the surface friction velocity. The observation shows that the sea states contains a wide range of wave ages driven by the synoptic wind systems, i.e. the strong northeast monsoon in winter and southwest monsoon in summer, mixed with the thermally diurnal variation across the sea-land boundaries. The large-scale winds generate a number of swells with long fetch and the mesoscale circulation perpendicular to the main streams causing wind and wave misalignments. The drag coefficients display considerable scattering around the linear growth formulation along with the increase of neutral wind speed. This is attributed to the dependence of surface roughness on wave ages. The present observation confirms that the drag coefficients are sensitive to the sea state described using the Charnock constant and hence the wave ages.