Effect of the fluctuation of the wind speed and direction on the drag coefficient investigated using tower-based measurements

Drag coefficient C_D is generally expressed as a function of only wind speed U₁₀. However, for a single U₁₀ value there exists a considerable range of observed C_D values. In this study, we observed the wind stress at a coastal tower: the Hiratsuka Offshore Experimental Tower of the University of Tokyo, Japan. Three-axis sonic anemometers were installed atop the tower, which was 20 m above mean sea level. The observation period was from September 15, 2015 to December 31, 2018. The friction velocity was obtained every 10 min. using the eddy correlation method. The pooled data from the observation period revealed a large variation in C_D with respect to U₁₀. The variation of C_D was reduced by excluding data points from periods of large fluctuation of wind speed and/or direction in time series within one hour. Furthermore, the wind speed fluctuation in time series was found to affect the C_D variation in the region of high wind speed, while the wind direction fluctuation influenced the C_D variation in the region of low wind speed. These results show that the C_D variability factors include not only the effects of the wind wave growth, swell, wave breaking, etc., but also those of fluctuation in wind speed and direction. We also investigated the influence of wave effects on the C_D.