Abstract
When the nondimensional aerodynamic roughness parameter for the sea surface (gz 0/u * 2,g being the acceleration of gravity,u * the air friction velocity) is plotted as a function of the wave age, the data points in the diagram are distributed mostly in a triangle area between the Charnock formula and the Toba-Koga formula; the nondimensional roughness perameter is not expressed as a unique function of the wave age, but rather there seem to be multiple regimes. In order to investigate the cause of the data point scattering, a reanalysis was made of the 4.5-hour time series of the wind profile and wind-wave statistics which were obtained at an oceanographic tower station under the conditions of a winter monsoon wind having slightly fluctuating speed and steadily growing wind waves.
It is concluded that the averaged variation ofz 0 is given by the Toba-Koga formula with a constant of value 0.015. However, as a result of the wind fluctuation on the time scales ranging from 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 inz 0 directly reflects the degree of over- or under-saturation in the high-frequency range of the wind-wave spectra. Physical interpretation of these variations is also presented.
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Toba, Y., Ebuchi, N. Sea-surface roughness length fluctuating in concert with wind and waves. Journal of the Oceanographical Society of Japan 47, 63–79 (1991). https://doi.org/10.1007/BF02301483
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DOI: https://doi.org/10.1007/BF02301483