Effects of Water Wave Motions on Spectral Characteristics of Wind Fluctuations in the Marine Atmospheric Surface Layer
Journal of the Meteorological Society of Japan
Spectral characteristics of wind velocity fluctuations in the marine atmospheric surface layer were investigated using data collected in field observations mostly at a tower in the sea, with paying special attention to possible effects due to wave motions of the water surface. The data of wind fluctuations and waves measured simultaneously in the cases when both winds and waves traveled toward almost the same directions were selected and analyzed. Two-dimensional wind fields in the plane
... in the plane parallel to the mean horizontal wind direction (u-component) and to the vertical direction (w-component) were discussed. The wind velocity spectra of u-and w-components in the lower frequency region, observed over sea, deviate upward from those model spectra proposed by Kaimal et al. (1972) and by Busch (1973) on the basis of Monin-Obukhov's similarity theory. From the coherence function analyses, it was found that the wind velocity fluctuations over sea contain a fairly good amount of 'wave coherent perturbations' in addition to the regular turbulences in the atmospheric surface layer. A spectrum excluding the spectral contribution of the wave coherent perturbations shows almost to agree with the model spectrum. The perturbation seems to give little contribution to a cospectrum between u and w, and may, therefore, not yield any significant amount of vertical momentum flux. The results of the frequency response function analyses suggest that the pattern of motion of the perturbation may be an ellipse with a horizontal axis a little longer than a vertical one. The results also suggest that the perturbation may be induced effectively by the wave components whose phase speeds are faster than the measured wind speed, and that an attenuation of the perturbation amplitude with height is small in the observed range of height (50-400cm above the mean sea level). From the above mentioned experimental results, the wave coherent perturbations were recognized as an origin to increase the spectral densities in the lower frequency region. The wave effects on spectral characteristics of winds in the marine atmospheric surface layer may have been first described in this study on the basis of the similarity theory.