Spatiotemporal Measurement of Short Wind-Driven Water Waves

Spatio-temporal measurements of wind-driven short-gravity capillary waves are reported for a wide range of experimental conditions, including wind, rain and surface slicks. The experiments were conducted in a linear wind wave flume and for the water surface elevation eta(x,y,t) both components of the slope field s = grad eta were measured optically. For this the color imaging slope gauge (CISG) was realized, comprising a range of wavenumbers k = sqrt(kx^2 + ky^2) from 60 to 4500 rad/m. The instrument was improved to achieve a sampling rate of 312.5 Hz, which now allows for the computation of 3D wavenumber-frequency spectra S(kx, ky, omega). Using a new calibration method it was possible to correct for the intrinsic nonlinearities of the instrument in the slope range up to ±1. In addition, the Modulation Transfer Function (MTF) was measured and employed for the contrast restoration of the data. The results are generally consistent with former measurements. But, the shape of the saturation spectra in the vicinity of k > 1000 rad/m stands in contradiction to former investigations where a sharp spectral cutoff (propto k^(-2) or k^(-3)) is commonly reported. The new MTF corrected spectra show just a gentle decrease (between k^(-0.5) and k^(-1)) for k >1000 rad/m, which has implications for the modeling of the energy fluxes in the wave field. Concerning the dispersion relation, a first approach for a quantitative evaluation of the wavenumber-frequency spectrum is shown. This includes estimates of the surface tension and the Doppler shift due to the surface shear flow and the wave-wave modulations. The wave measurements were accompanied by synchronized and spatially coinciding measurements of the surface temperature by means of infrared imagery. The temperature data is mapped onto an animated graphical model of the reconstructed surface elevation using a new interactive visualization tool. This allows for an investigation of intermittent small scale processes that are influencing the transfer of heat and gases at the air-water interface, such as microscale wave breaking, small scale Langmuir circulations, and the impact of rain drops.