Optical Determination of Short-Wave Modulation by Long Ocean Gravity Waves

Time series short-wave spectra were produced from photographs acquired during the Joint North Sea Waves Project (JONSWAP). The feasibility of using this surface imagery to quantify the nature of short-wave modulation by long waves is demonstrated. Photographically obtained time series spectra are correlated to the long wave slope. The modulation transfer function of short-wave spectral energy versus long wave slope is measured to have a 9(slope)-1 magnitude with a 90° phase at a wind speed of 5 m/s in the short-wave spectral regime between 3 and 30 cm in wavelength. Comparison of these results with measurements by coherent radars and laser slope gauges indicates good agreement in the measured modulation magnitude but significant disagreement in measured phase. Modulation magnitude and phase predicted from the action balance model are shown to be significantly smaller than empiricaly measured results.