The Impact of Nonbreaking Waves on Wind‐Driven Ocean Surface Turbulence


A comprehensive study on the properties of upper ocean turbulence and its response to a variety of wind and wave forcing conditions was conducted at the Surge Structure Atmosphere INteraction (SUSTAIN) facility, University of Miami, RSMAS. The dimensions of SUSTAIN wind‐wave‐current tunnel are 18 m long, 6 m wide, and 2 m high, uniquely allowing for freely forming turbulence unconstrained by side walls. The grid of input parameters covered a range of wave steepness (ak : 0–0.27) within each investigated wind speed (U 10: 2–20 m/s), allowing for the isolation of wave effects on the turbulence driven by both wind and waves. The response of turbulence to each pair of wind‐wave parameters was measured in steady‐state conditions (after 20+ min of settling time) by a suite of nonintrusive turbulence visualization techniques, located at 10 m fetch. These included 3‐D visualization of water tracing dye entrainment by turbulence, underwater particle image velocimetry, and passive and active thermal imagery resolving surface skin temperature and velocity fluctuations. The resulting data set includes both qualitative 3‐D view of subsurface turbulent structures and precise quantitative turbulent kinetic energy (TKE) measurements mapped out in response to the grid of input parameters. In lower winds, TKE was found to grow substantially in response to increasing wave steepness, in line with the expected effect of increasing wave forcing. However, in higher winds the effect of increasing wave steepness on TKE was found to be negative. It is hypothesized to be due to the rise of airflow separation, effectively sheltering much of the water surface from turbulence production by wind friction.
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