Spatio-Temporal 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

Mean Square Slope Measurements with the Reflective Stereo Slope Gauge

<p>An optical instrument for the measurement of surface ocean small-scale wave statistics has been developed. This reflective stereo slope gauge (RSSG) is capable of simultaneous measurements of height and slope statistics of the water surface in the field and in the laboratory.</p> <p>Here, first laboratory validation measurements are presented.</p

Heat Transfer and Wave Measurements in the Baltic Sea: Principle, Setup and Plan for SOPRAN II

<p>Heat is used as a proxy tracer for gases to study the transport processes<br> across the sea-surface interface to obtain a detailed insight into<br> the diffusive and turbulent processes controlling the transport. A carbon<br> dioxide laser forces a periodically varying heat flux density onto<br> the water surface and the amplitude damping and phase shift of the<br> sea surface temperature is measured from infrared image sequences.<br> The transport process can be treated by linear system theory and the<br> relation between the input signal (periodically varying surface flux density)<br> and the output (surface temperature) is estimated. Within the<br> framework of the SOPRAN initiative three field experiments in the<br> Baltic Sea were conducted. The locally derived heat transfer rates are<br> scaled to gas transfer rates, which are in good agreement with empirical<br> gas transfer wind speed relationships for moderate winds speeds.<br> At high wind speed, the transfer rates are lower, which is explained<br> by the fact that heat transport is insensitive to bubble-mediated gas<br> transfer, i.e. it measures only a part of the transfer process directly at<br> the water surface.</p