Evaluation of a GPS Wave Buoy for Wind Stress Measurements using Large Wave Tank

There exists considerable disagreement among the observed values of the drag coefficient, C_D. In order to develop a model of C_D, the wind stress has to be calculated from the eddy-correlation method, which is the direct method measuring the horizontal and vertical wind components. A buoy is suitable to measure the wind stress in many sea surface conditions. However, the motion correction is very difficult because the anemometer measures the wind components including the motion of the buoy. In this study, as a first approach, we investigated the motion of a prototype buoy system with installed 3-axis sonic anemometer and 6-axis motion sensor in a GPS observation buoy. The wave tank is in the Ocean Engineering Basin of the Institute of Industrial Science, University of Tokyo, Japan. The imposed conditions were wave period from 1.1 s to 2.5 s, wind speed 0 m/s (only wave), 2 m/s and 5 m/s, and the wave spectrum was regular (monochromatic) or irregular. The motion of the buoy was measured in 120 cases. The comparison between the measured wind speed and the motion velocity of the buoy shows good agreement in any wave period and no-wind cases, though there were small errors. Also, in the condition with wind speed 2 and 5 m/s, the deducted motion velocity of the buoy from the measured wind velocity yielded nearly the true wind speed value. The friction velocity from corrected wind velocity components shows good agreement with the friction velocity measured from a fixed sonic anemometer. It appears that the 6-axis motion sensor accurately measures the motion of the buoy. Consequently, we expect to be able to measure the wind stress in the field using this buoy system.