Watching waves

Wind blowing across the sea induces waves of various heights, wavelengths, and speeds. Although the waves’ rich spectrum can be derived from linear theory, nonlinearities are significant, even when the wind is just a breeze. Measuring the wave spectrum is challenging because it entails tracking the height of the sea surface over a range of length and time scales. Fabrice Ardhuin of the French Research Institute for Exploration of the Sea in Plouzané, Brest, France, and his collaborators have met that challenge using high-speed stereoscopic video. Their experiment is set up on a fixed platform situated 500 meters off the southern tip of the Crimean Peninsula. From a vantage 11 meters above the surface, two 5-megapixel cameras monitor the same, roughly 100-square-meter patch of the Black Sea and gather data at 12 frames per second (see figure for two typical frames and the study patch outlined in blue). Correlating the two images—frame by frame, pixel by pixel—yields the surface elevation. Fourier transforming the elevation yields the spectrum. The team now reports the results of an experimental run that took place in October 2011 on a day when the wind was a strong steady breeze of 13 m/s. Thanks to the cameras’ ability to resolve propagation directions, the researchers could measure the amount of wave energy that travels in opposite directions. Because the waves’ interactions contribute to the background noise detected by underwater seismometers, the state of the sea might one day be characterized from seismic data alone. Among their other findings: Waves 1/15 the length of the dominant spectral component tend to travel 70° away from the wind direction. (F. Leckler et al., J. Phys. Oceanogr. 45, 2484, 2015, doi:10.1175/JPO-D-14-0237.1 .)