Short wave modulation and breaking, experimental results

Abstract

The small scale roughness of the sea is an important link in the energy flux from the atmosphere to the ocean. It determines growth and limitation of sea waves and its amplitude is modulated by wind, long waves, currents and instabilities.

To study these modulations experiments have been performed on board of the research platform Nordsee. Wind, long and short waves, breakers and surface currents were measured. The analysis shows: Wave generation starts at a wind speed of 1.7 m/s at frequencies around 5 Hz and energy increases with wind up to 3.4 m/s. Short waves are modulated on long waves and spectral decays are near f^-4.5, and f^-2 at frequencies above 8 Hz.

The modulation strength grows with wind speed and wave height and is a function of frequency with a minimum near 8 Hz. Positions of energy maxima are widely scattered on long waves. Second maxima are often developed.

Breaker activities and intensities increase with wind and long wave slope, while the frequency of breaking decreases with wave length.

Short wave modulation and breaking can differ under similar conditions. They cannot be explained by wind or wave height alone. Interpretations have to include the parameters of the interaction system atmosphere-ocean, particularly wave slope and atmospheric stability criteria.