Impact of Shoaling Ocean Surface Waves on Wind Stress and Drag Coefficient in Coastal Waters: 1. Uniform Wind

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This study investigates the impact of shoaling wind waves on the drag coefficient in coastal waters. The shoaling wave spectrum is simulated using the WAVEWATCH III (WW3) model with shallow water physics. The high-frequency part (spectral tail), which is unresolved in the wave model, is empirically parameterized as a function of wind speed. The full wave spectrum is then used to estimate the sea-state-dependent wind stress and drag coefficient. Shoaling wind waves are simulated on a sloped bottom under the idealized steady uniform wind. Experimental wind speed spans from 10 to 65 m/s, and the bottom slope is varied from 1:100 to 1:2,000. Our results show that as water depth decreases, the drag coefficient increases gradually to a peak value and then rapidly reduces compared to the deep water value. The maximum Cd value occurs roughly where depth-induced wave breaking starts. The magnitude of Cd enhancement is more significant on a steeper slope and can reach 40%. This Cd enhancement is mainly due to steepening of waves and reduction of the wave phase speed during the shoaling. Our results also suggest significantly larger variability of Cd at a given wind speed in finite-depth waters than in deep water.

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Journal of Geophysical Research: Oceans