Optical determination of the phase velocity of short gravity waves

Abstract

Images of short wind‐driven gravity waves were taken from an offshore platform, using a charge coupled device television camera recording diffuse sky radiance reflected from the ocean surface. A two‐dimensional power spectrum was calculated from nine statistically independent images. The resultant ensemble‐averaged spectrum exhibited good statistical stability and provided information on the angular spread and direction of the wave components present. One‐dimensional sampling of each image in a sequence allowed a space‐time image to be constructed which clearly shows the effects of wave dispersion as well as the modulation of the phase velocities of the short wavelength waves by the long wavelength components. An ensemble‐averaged space‐time spectrum, when combined with the directional parameters, is compared with the predictions of linear gravity wave dispersion theory. Two distinct wave systems were present: the local wind driven system showed a space‐time spectrum in agreement with linear theory out to ∼1 cyc/m, but with excess phase velocity at higher spatial frequencies. The second wave system, which was presumably generated by a distant wind field, showed a deficiency in phase velocity when compared to linear theory.