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There have been three principal ways for measuring the wind stress over the sea: profile method, eddy correlation method, and inertial method (IDM for brevity). Among them the IDM is the most indirect... and ambiguous, so that it is examined with respect to its validity in particular. First a method（direct regression method）is proposed and tested by which to estimate the spectral level in the inertial subrange of wind velocity, which is the key process of the IDM．Since the direct regression method uses no ensemble average, it is available even when a sufficient number of samples are difficult to obtain，as is the case for the real wind over the sea. The direct regression method proved to yield the true spectral level，when a definite factor is used. Then the property of the IDM is argued by applylng it to the wind turbulence over the sea observed at the Tsuyazaki Ocean Station，2 km offshore of Fukuoka，Japan. There certainly was a distinct frequency region characterized by the slope of－5／3 for all the three components of wind velocity : longitudinal (i.e. leeward)，transverse，and vertical ones. However，the spectral form of the vertical component slightly differed from the other two at high frequencies. The most serious was an evident difference of the spectral level among the three components in contradiction to the assumption of the isotropic turbulence in the inertial subrange; the longitudinal component was the highest，the transverse one was slightly less，and the vertical one was the lowest. Therefore the IDM estimate of wind stress based on the vertical component was only 70％ oft that based on the leeward component. The IDM based on the longitudinal component agreed best with the other two standard methods of the profi1e and eddy-correlation methods. It is necessary to use a correction coefficient if one applies the IDM based on the vertical component of wind velocity. The results suggest that the assumptions of the IDM are not satisfied well by the real wind turbulence over the sea and that one must be careful in applying the IDM. Finally the drag coefficient and roughness of the sea surface thus analyzed were found moderate as a whole; they were a little larger than those in wind flume experiments．続きを見る
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