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Large‐scale offshore wind farms are expected to influence surface waves by modifying local wind forcing through wake effects. We use regional coupled ocean‐atmosphere‐wave model simulations to investigate a realistic large‐scale offshore wind development scenario in the northeastern U.S. during boreal summer. Near‐surface wind speeds are reduced by 10% over lease areas and within downstream wake regions, leading to decreases in significant wave height (3%) and wave‐supported momentum flux (30%). This further leads to reductions in surface roughness length (16%) and near‐surface ocean turbulent kinetic energy (20%). Spectral analysis shows a clear reduction in wind‐sea energy, indicating suppressed local wind‐wave growth near the wind farms. Weaker winds favor the development of longer‐period waves, increasing dominant wave phase speed by 3% and suggesting a transition to an older sea state. Modern bulk flux algorithms often parameterize surface roughness using inverse wave age and/or wave slope. This raises the question of whether wake‐driven reductions in inverse wave age and wave height impact air‐sea momentum exchange. To assess this, we compare fully coupled simulations with an atmosphere‐only run excluding wave coupling. Results show that about one‐third of the reduction in roughness length can be attributed to sea state changes, while two‐thirds result from lower friction velocity due to lower wind speeds. However, the impact of sea state on the drag coefficient and momentum flux is negligible (1%), suggesting that wake‐induced wind speed reductions are the primary driver, with sea state changes playing a secondary role.
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Improving Wave‐Based Air‐Sea Momentum Flux Parameterization in Mixed Seas
In winter, the Northwest Tropical Atlantic Ocean can be characterized by various wave age-based interactions among ocean current, surface wind and surface waves, which are critical for accurately describing surface wind stress. In this work, coupled wave-ocean-atmosphere model simulations are conducted using two different wave roughness parameterizations within COARE3.5, including one that relies solely on wind speed and another that uses wave age and wave slope as inputs. Comparisons with the directly measured momentum fluxes during the ATOMIC/EUREC4A experiments in winter 2020 show that, for sea states dominated by short wind waves under moderate to strong winds, the wave-based formulation (WBF) increases the surface roughness length in average by 25% compared to the wind-speed-based approach. For sea states dominated by remotely generated swells under moderate to strong wind intensity, the WBF predicts significantly lower roughness length and surface stress (≈15%), resulting in increased near-surface wind speed above the constant flux layer (≈5%). Further investigation of the mixed sea states in the model and data indicates that the impact of swell on wind stress is over-emphasized in the COARE3.5 WBF, especially under moderate wind regimes. Various approaches are explored to alleviate this deficiency by either introducing directional alignment between wind and waves or using the mean wave period instead of the wave period corresponding to the spectral peak to compute the wave age. The findings of this study are likely to be site-dependent, and mostly concern specific regimes of wind and waves where the original parameterization was deficient.
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- PAR ID:
- 10450614
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 128
- Issue:
- 3
- ISSN:
- 2169-9275
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2022JC019277
