Isaac Ginis University of Rhode Island Graduate School of Oceanography Coauthors Xuanyu Chen CSU Tetsu Hara URIGSO Workshop on PhysicsDynamics Coupling Princeton University June 2 2022 ID: 1027062
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1. Impacts of Shoaling Ocean Waves on Wind Stress and Drag Coefficient in Coastal WatersIsaac Ginis University of Rhode Island Graduate School of OceanographyCo-authors: Xuanyu Chen (CSU) Tetsu Hara (URI/GSO)Workshop on Physics-Dynamics CouplingPrinceton University, June 2, 2022
2. 1Wind Stress & Drag CoefficientWind stress is defined asCommonly approximated by a bulk method in numerical models: where the drag coefficient (Cd) is traditionally a function of 10-m wind speed (U10) bulk Cd formula = Currently used assumption in the ocean and storm surge models: Cd is the same in open ocean and in coastal waters
3. 4solid: deep waterdashed: <30mIs the observed higher Cd entirely due to shoaling waves?other possible causes:limited fetchatmospheric factorsLimitations of Single Point ObservationAttribute to sourceMVCO WHOI ASIS buoyLarger Cd values reported in coastal oceanLarger Cd values have been attributed to shoaling wave effects:steepening of the dominant waves due to the reduction of wave phase speed and increase of wave amplitudechanging wave shapes during shoalingincreased wave breaking during shoalingReferenced open ocean condition unknown!
4. 4T4T2T1T3Anctil and Donelan (1996): Air-Water Momentum Flux Observations over Shoaling WavesLake Ontario(https://commons.wikimedia.org/w/index.php?curid=39662678)Observational towers at different depthsEnhancement of Cd up to 55% due to shoaling relative to Cd at the deep water tower The drag coefficients were simultaneously observed at Towers 1 (D∼3 m), 2 (D∼5 m), and 4 (D∼13 m)
5. 5 WAVEWATCH III wave modelDirectional-wavenumber spectrumWind stress calculationWind stress modules: URI (Reichl et al. 2014) Miami ( Donelan et al. 2012)4 51Reichl et al. (2014)Append spectral tail 23spectral tail is parameterized
6. 200kmShoaling Domain200kmControl Domain (deep water) Uniform wind fieldUniform wind fieldWind speed: 10 ~ 65m/sWind direction: normal to the shoreline.Bottom Slope: 1:100 , 1:200, 1:400, 1:1000, 1:20006Uniform wind experiments: shoaling of fetch-dependent wind waves
7. (1/1000)(1/100)(1/10)Estimated bathymetry slope for water depth < 100m. 1:1001:20007Louisianacoast (1:2000) Lake Ontario (1:100)Bay of Fundy (1:100)
8. 8Variation of Hs and Cd with water depth at 35 m/s wind speedThe maximum Cd value occurs at location where depth-induced breaking starts to be significant. The increase of Cd is sensitive to bottom slopeUniform wind
9. 9Variation of Cd with water depth is qualitatively consistentwith the Lake Ontario studyT2T4Significant Wave Height Drag Coefficient
10. 8Steeper sea floor larger increase of CdYounger incoming waves larger increase of CdIncoming wave age: wave age of the incoming deep-water dominant waves before shoaling Cdsh/Cddeep Ratio
11. 11Why is Cd enhanced by shoaling waves? ExpA: waves steepen without slowing downExpB: waves steepen & slow down Other possible mechanisms that cannot be tested with our method:1) increased wave breaking during shoaling2) changing wave shapes during shoalingDue to Wave steepening and slowing down:
12. Shoaling of wind waves under uniform onshore wind increases the drag coefficient from its fetch-dependent deep-water value.The relative Cd enhancement is within 25% with the Miami method and 40% with the URI method.The enhancement of Cd is sensitive to the bottom slope, with a larger increase on a steeper bottom slope.Increase of Cd is mainly caused by wave steepening and reduction of the wave phase speed in finite depth.12Summary of uniform wind experiments
13. 13Tropical cyclone experiments: Idealized TC experiments:Intensity: Vmax=35m/s (Cat. 1) , 65m/s (Cat 5)Forward Speed: 5m/s, 10m/sBottom Slope: 1:200, 1:2000Radius of Maximum Wind: 70kmTC track direction : normal to the shoreline.Shoaling DomainControl Domain (deep water) 600km4000km
14. 14Wind fields in four idealized tropical cyclones
15. 15Tropical cyclones generate complex sea stateAdapted from Young (2003)wind seasWind seasΘuw ϵ [0, 45o]Cross-wind Swell Θuwϵ (45o, 135o]Opposing-wind Swell Θuwϵ (135o, 180o]Θuwwind dominant wave Significant wave height, Hs
16. 16Spectrum tail is parameterized using GFDL/HWRF Cd formulation Key assumptions:Spectral tail of the unresolved short waves is parameterized as a function of wind speed.Mean of the sea-state dependent Cd (gray scatters) in deep water follow the GFDL/HWRF bulk formula
17. 17Variation of significant wave height Hs with water depth and bottom slopeRight RearLeft FrontVmax= 65 m/sUT=10 m/s
18. 18Variation of significant wave height Hs with water depth and bottom slopeRight RearLeft FrontVmax= 65 m/sUT=10 m/s
19. 19Right RearLeft FrontVariation of drag coefficient Cd with water depth and bottom slopeSignificant differences between cases with different bottom slopesVmax= 65 m/sUT=10 m/s
20. 20Right RearLeft FrontVariation of drag coefficient Cd with water depth and bottom slopeVmax= 65 m/sUT=10 m/s
21. 21Right RearLeft FrontCd is increased in right-rear and left-front TC quadrantCdsh/Cddeep Ratio
22. 22Right-rear quadrantdominant wave wind Fetch dependent wind waves, same as generated under uniform wind conditions
23. 23dominant wave wind Increase of Cd in the right-rear quadrant is due to steepening and slowing down of the fetch-dependent wind seas as in the uniform wind cases.maximum enhancement increases with storm forward speed. Right-rear quadrantCdsh/Cddeep
24. 24dominant wave wind slope=1:200Depth=30mIncrease of Cd in the left-front quadrant is due to shoaling of opposing-wind swells.Left-rear quadrantCdsh/Cddeep
25. 25Increased variability of the Cd at given wind speed at shallower water depth and steeper bottom slope
26. 26Increased variability of the Cd at given wind speed at shallower water depth and steeper bottom slope
27. During tropical cyclone landfall the wind stress and drag coefficient are enhanced in the storm right-rear and left-front quadrants.The effects of shoaling on the wind stress and drag coefficient are stronger with steeper bottom slopes and faster moving tropical cyclones.The variability of the drag coefficient at a given wind speed increases significantly in shallow waters.27Summary of tropical cyclone experiments
28. Chen, X., T. Hara, and I. Ginis, 2020: Impact of Shoaling Ocean Surface Waves on Wind Stress and Drag Coefficient in Coastal Waters: Part I Uniform Wind, J. Geophys. Res., https://doi.org/10.1029/2020JC016222.Chen, X., I. Ginis, and T. Hara, 2020: Impact of Shoaling Ocean Surface Waves on Wind Stress and Drag Coefficient in Coastal Waters: Part II Tropical Cyclones, J. Geophys. Res., https://doi.org/10.1029/2020JC01622328References: