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Impact of Shoaling Waves on Coastal Wind Stress

Under uniform wind conditions, shoaling surface waves systematically enhance wind stress and the drag coefficient in shallow coastal waters, with maxima occurring near the onset of depth-limited breaking. Outside of the surf-zone, our idealized experiments show that shoaling fetch-dependent wind waves increase the wind stress by up to 25~40% relative to the associated deep water wind stress.

These results demonstrate that air–sea momentum exchange in coastal regions depends strongly on interactions between surface waves and water depth. At the time of this research, Anctil and Donelan (1996) observations in the Lake Ontario remains the only Cd observations over shoaling waves as they collected measurements at multiple water depths simultaneously. Our idealized simulations are qualitatively consistent with their observations: 1) Cd increases with decreasing water depth up to the onset of depth-limited breaking; 2) the enhancement of Cd is larger over younger than older seas.

Cd under uniform wind

In hurricane conditions, the response of the complex wave fields and hence the wind stress to water depth variations are quadrant-dependent, increasing the spatial-temporal variability of wind stress during landfalling hurricanes. Compared to its deep-water value, wind stress magnitude is enhanced in the right and left quadrants of the hurricanes due to shoaling fetch-dependent waves and opposing-wind swells, respectively. However, wind stress is reduced in the front/rear quadrants due to weaker wind seas. The misalignment between the wind stress and wind speed directions is enhanced in shallow water.

Cd under landfalling TC

Under both uniform wind and hurricane conditions, the effect of shoaling surface waves on wind stress is sensitive to the bottom slope, with more pronounced effect over steeper bottom slope. This is an environmental factor previously underappreciated in the literature and should be taken into account in the parameterization of Cd in shallow water.

Publications

  • Chen, X., Hara, T., Ginis, I. (2020). Impact of shoaling ocean surface waves on wind stress and drag coefficient in coastal waters: 1. Uniform wind. Journal of Geophysical Research: Oceans. [DOI]
  • Chen, X., Ginis, I., Hara, T. (2020). Impact of shoaling ocean surface waves on wind stress and drag coefficient in coastal waters: 2.Tropical cyclones. Journal of Geophysical Research: Oceans. [DOI]

Talks

2021-04-21: Impacts of Shoaling Ocean Surface Waves on Wind Stress and Drag Coefficient, DHS Coastal Resilience Center 6th Annual Meeting, virtual. [watch here]

2018-06-13: Sea-state dependent drag coefficient in shallow water under tropical cyclones, 21st Amer. Meteor. Soc. Conf. on Air-Sea Interaction, Oklahoma City, OK. [watch here]

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