Observation of hydrodynamic modulation of gravity‐capillary waves by dominant gravity waves

Abstract

Accurate evaluation of the hydrodynamic modulation of short wind waves by dominant gravity waves is needed for various applications of microwave radar remote sensing of the ocean surface. Such knowledge is also essential for the studies of wind wave coupling and air–sea momentum/energy transfer. Here, we present direct field observations of the hydrodynamic modulation transfer function (MTF) during two field programs (High‐Resolution Remote Sensing Experiment (High‐Res) off Cape Hatteras, June 1993, and Coastal Ocean Processes (CoOP) experiment off the California coast, April and May 1995). Gravity‐capillary wave spectra were obtained using a scanning laser slope gauge (SLSG) for wave numbers between 25 and 800 rad m−1. An array of capacitive wave wires and a motion detection package were used for the measurement of surface gravity waves. These instruments were mounted on a research catamaran, which was towed to the side of a research vessel. The observations were mostly made under low wind conditions. The observed coherence is mostly below 0.1, suggesting that the modulation of short wind waves is not strongly correlated with dominant gravity waves. The coherence is slightly higher for wave numbers 50–100 rad m−1 than for wave numbers above 200 rad m−1. When wind and dominant waves are aligned and the water surface is clean, the magnitude of the hydrodynamic MTF is around 2–4 and its phase is close to 0. When surfactants are present and short wind wave spectra are reduced, the MTF magnitudes become significantly larger for wave numbers above 200 rad m−1. When wind and dominant waves are not aligned, the coherence becomes lower as expected. The results from the two field experiments are roughly consistent with each other. The existing relaxation model may predict the hydrodynamic MTF values that are roughly consistent with the observations if the effect of modulated wind stress over gravity waves is included.