Observations of the Directional Spectrum of Ocean WavesUsing a Cloverleaf Buoy

Abstract

Analysis of the directional spectra of typical sets of surface wave data obtained in the open sea as well asa bay using a cloverleaf buoy system are reported. It is shown that the directional wave spectrum can be approximated by the product of the frequencyspectrum and a unimodal angular distribution with mean direction approximately equal to that of thewind, and that various forms of frequency spectra exist, even in relatively simple wave systems, dependingon their generating conditions. Ocean waves at fairly short dimensionless fetches show spectral forms withvery narrow spectral width, which are similar to those of laboratory wind waves. On the other hand, thespectral forms for ocean waves at very long dimensionless fetches are quite similar to the Pierson-Moskowitzspectra, which are considered, within our present data, to be the wave spectra with the largest spectral width.Finally, there exist many ocean waves at moderate dimensionless fetches, which show spectral forms with interminate spectral widths lying between the above two extremes. However, a definite relationship betweenthe spectral width and the dimensionless fetch has not been obtained in the present study. Concerning the angular distribution, it is shown that the shape of the angular distribution is dependenton the frequency of the spectral component even in a simple wave system in a generating area, althoughthe mean directions ot the spectral components are independent of the frequency and approximately equalto the wind direction. The angular distribution is very narrow for frequencies near the dominant peak of thefrequency spectrum, whereas it widens rapidly toward high and low frequencies. Thus, the major energy-containing frequency components propagate in almost the same direction as the wind with the least angularspreading. Finally, it is shown that a similarity law is satisfied for the angular distributions, and an idealized formof the angular distribution function is derived for practical purposes.