Collaborative Research: Temporal and spatial scaling of dissipation under non-breaking surface waves

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Quantifying the rate at which surface waves dissipate energy is necessary to properly model upper ocean mixing, circulation and the waves themselves. A fundamental assumption for much of the work so far is that little to no energy goes from waves into turbulence as long as the waves are not breaking. This study will explore the limits of validity of this assumption using a novel instrument, the Optical Turbulence Sensor (OTS) that can measure turbulence over very short time periods, so that turbulence intensity over different parts of a wave can be resolved in the laboratory. This project will result in a richer understanding of upper ocean turbulence under waves and lead to improved parameterizations to be used in models. The project will also support two graduate students, include outreach activities for the general public through ongoing community programs at both institutions, distribute wave and turbulence visualizations through the student-run video project Waterlust. Breaking of waves has been shown to be the dominant mechanism for dissipation in most circumstances, however it has also become clear that additional dissipation is required to match observations in non-breaking conditions. To address this, equations were derived for the dissipation of swell wave energy due to interaction of the sheared wave-induced Stokes drift with pre-existing turbulence. An alternative approach recognizes that water has finite viscosity and as such there must be turbulence and dissipation induced by the wave orbital motions. Unfortunately, it is essentially impossible to isolate these proposed sources of dissipation in the field where mean flows, wave breaking, sheared wind-drift currents and buoyancy effects co-exist. Some laboratory studies have confirmed the existence of non-breaking wave-induced turbulence. However, flow visualization studies did not see evidence of turbulence until the wave amplitudes were such that microbreaking could be important. Addressing this significant knowledge gap regarding the turbulent dissipation of non-breaking surface waves, will take a comprehensive laboratory study. Established measurement technologies will be used to calibrate and validate the OTS in the first year. The OTS can measure the temperature dissipation spectrum over temporal windows as short as 1 ms. This enables the measurement of turbulence in non-stationary conditions and can resolve wave phase dependence. The next experiments will quantify the temporal and spatial scales at which turbulence occurs when non-breaking waves propagate over a fluid that is initially at rest. Additional instrumentation will be used to observe mean flows, wave heights and wave slopes. The final experiments will quantify turbulence generation and amplification in the more realistic condition when there is significant pre-existing turbulence. First, monochromatic mechanical waves will be propagated over water containing grid generated turbulence. Then waves with energy distributed from ~0.2 to 2 Hz will be included. Finally, the wave fields will be forced with variable winds and will be sampled for longer duration to investigate the turbulence generation when sheared mean flows are present. This study will resolve the role of pre-existing turbulence, address whether there is a wave related threshold for turbulence and determine the time, depth and space scales over which turbulence develops.