Development of a New Technique to Evaluate Surface Winds from Routine Buoy Observation of Ocean Waves

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The objective of this study is to pursue a new technique to evaluate the air friction velocity u_* which is a measure of the sea-surface wind stress, the 10-m wind speed U_<10> and the wind direction from routine observation of ocean waves by using a simple buoy system.From preliminary investigations, it has been found that for the estimation of u_* from wave data, the use of the equilibrium range of the high frequency part of the wave spectra is most promising, and that use of the 3/2-power law together with the significant wave height and period is also available for high wind sea conditions. For the estimation of U_<10> from u_* , it is necessary to establish the most pertinent formula for the drag coefficient or the sea-surface roughness parameter z_0 in which parameters of wind sea conditions are properly included, since the wind stress depends on conditions of the wind sea. Consequently, the first priority has been placed on obtaining the most pertinent formulas for z_<10>.In the conditions when wind waves are developing under nearly steady wind conditions, the averaged variation of z_0 is that given by the Toba-Koga formula z_0 sigma_p/u_* = gamma with gamma = 0.015. However, as a result of the fluctuation of wind of the time scales of several minutes to an hour, data points show a conspicuous fluctuation on the nondimensional roughness parameter -- wave age diagram, in the direction transverse to the averaged variation. The variation of z_0 directly reflects the degree of over - or under-saturation of the high frequency range of the wind-wave spectra. A compromise formula for practical uses to represent the above triangle area is gz_0/u_*^2 = 0.020 (sigma_pu_*/g) ^<-0.5>.