A STUDY OF THE WAVE-NUMBER SPECTRA OF SHORT WATER-WAVES IN THE OCEAN

Spatial measurements of capillary-gravity waves in the ocean were obta ined using a scanning slope sensor mounted on a free-drifting buoy int ended to minimize the flow disturbance. The data provide direct calcul ation of the wavenumber spectra of surface curvature in the capillary- gravity wave range. The results indicate that 1) a pronounced peak at the wavenumber k = 9 rad cm(-1) is evident in the curvature spectra fo r wind speeds below 6 m s(-1); 2) the slopes of the curvature spectra are 1 and -1 on the two sides of the spectral peak; 3) the spectral de nsity and mean-square roughness properties increase linearly with wind speed; and 4) these observations suggest a spectral function of the f orm chi(k) = Auc(-2)c(m)k(m)k(-4), which is proportional to u*k(-3) i n the short gravity wave region and uk(-5) in the capillary wave regi on, where u is the wind friction velocity, c(m) the minimum phase vel ocity of surface waves, and k(m) the corresponding wavenumber. Capilla ry-gravity wave wavenumber spectra obtained from the ocean and from la boratory studies are compared. It is found that significant difference s in many important features of short-wave properties exist in the dat asets from these two different environments. A possible reason for the observed differences is attributed to the fluctuation component of th e wind field, which is typically a significant fraction of the mean wi nd speed in the open ocean but much smaller in the laboratory. The ste ady wind in the laboratory produces a surface boundary condition very different from that in the field. This is reflected in the observation that a spectral gap in the vicinity of the minimum phase velocity (an indication of wave blockage by steady surface drift current) was foun d in the laboratory measurements but not in the field data. As a conse quence, the small-scale structures observed in the laboratory and in t he field are significantly different.