Abstract
Atmospheric and surface wave data from several oceanic experiments carried out on the Floating Instrument Platform (FLIP) and the Air-Sea Interaction Spar (ASIS) have been analyzed with the purpose of identifying swell-related effects on the surface momentum exchange during near-neutral atmospheric conditions and wind-following or crosswind seas. All data have a pronounced negative maximum in uw cospectra centered at the frequency of the dominant swell np, meaning a positive contribution to the stress. A similar contribution at this frequency is also obtained for the corresponding crosswind cospectrum. The magnitude of the cospectral maximum is shown to be linearly related to the square of the orbital motion, being equal to 1:25Hsd 2np 2, where Hsd is the swell-significant wave height, the effect tentatively being due to strong correlation between the surface component of the orbital motion and the pattern of capillary waves over long swell waves. A model for prediction of the friction velocity u*from measurements of Hsd, np, and the 10-m wind speed U10 is formulated and tested against an independent dataset of ~400 half-hour measurements during swell, giving good result. The model predicts that the drag coefficient CD, which is traditionally modeled as a function of U10 alone (e.g., the COARE algorithm), becomes strongly dependent on the magnitude of the swell factor 1:25Hsd 2np 2 and that CD can attain values several times larger than predicted by wind speed-only models. According to maps of the global wave climate, conditions leading to large effects are likely to be widespread over the World Ocean.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 4777-4795 |
| Number of pages | 19 |
| Journal | Journal of the Atmospheric Sciences |
| Volume | 72 |
| Issue number | 12 |
| DOIs | |
| State | Published - Dec 1 2015 |
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Keywords
- Atm/Ocean Structure/ Phenomena
- Atmosphere-ocean interaction
- Marine boundary layer
ASJC Scopus subject areas
- Atmospheric Science
Cite this
Surface stress over the ocean in swell-dominated conditions during moderate winds. / Högström, Ulf; Sahlée, Erik; Smedman, Ann Sofi; Rutgersson, Anna; Nilsson, Erik; Kahma, Kimmo K.; Drennan, William M.
In: Journal of the Atmospheric Sciences, Vol. 72, No. 12, 01.12.2015, p. 4777-4795.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - Surface stress over the ocean in swell-dominated conditions during moderate winds
AU - Högström, Ulf
AU - Sahlée, Erik
AU - Smedman, Ann Sofi
AU - Rutgersson, Anna
AU - Nilsson, Erik
AU - Kahma, Kimmo K.
AU - Drennan, William M
PY - 2015/12/1
Y1 - 2015/12/1
N2 - Atmospheric and surface wave data from several oceanic experiments carried out on the Floating Instrument Platform (FLIP) and the Air-Sea Interaction Spar (ASIS) have been analyzed with the purpose of identifying swell-related effects on the surface momentum exchange during near-neutral atmospheric conditions and wind-following or crosswind seas. All data have a pronounced negative maximum in uw cospectra centered at the frequency of the dominant swell np, meaning a positive contribution to the stress. A similar contribution at this frequency is also obtained for the corresponding crosswind cospectrum. The magnitude of the cospectral maximum is shown to be linearly related to the square of the orbital motion, being equal to 1:25Hsd 2np 2, where Hsd is the swell-significant wave height, the effect tentatively being due to strong correlation between the surface component of the orbital motion and the pattern of capillary waves over long swell waves. A model for prediction of the friction velocity u*from measurements of Hsd, np, and the 10-m wind speed U10 is formulated and tested against an independent dataset of ~400 half-hour measurements during swell, giving good result. The model predicts that the drag coefficient CD, which is traditionally modeled as a function of U10 alone (e.g., the COARE algorithm), becomes strongly dependent on the magnitude of the swell factor 1:25Hsd 2np 2 and that CD can attain values several times larger than predicted by wind speed-only models. According to maps of the global wave climate, conditions leading to large effects are likely to be widespread over the World Ocean.
AB - Atmospheric and surface wave data from several oceanic experiments carried out on the Floating Instrument Platform (FLIP) and the Air-Sea Interaction Spar (ASIS) have been analyzed with the purpose of identifying swell-related effects on the surface momentum exchange during near-neutral atmospheric conditions and wind-following or crosswind seas. All data have a pronounced negative maximum in uw cospectra centered at the frequency of the dominant swell np, meaning a positive contribution to the stress. A similar contribution at this frequency is also obtained for the corresponding crosswind cospectrum. The magnitude of the cospectral maximum is shown to be linearly related to the square of the orbital motion, being equal to 1:25Hsd 2np 2, where Hsd is the swell-significant wave height, the effect tentatively being due to strong correlation between the surface component of the orbital motion and the pattern of capillary waves over long swell waves. A model for prediction of the friction velocity u*from measurements of Hsd, np, and the 10-m wind speed U10 is formulated and tested against an independent dataset of ~400 half-hour measurements during swell, giving good result. The model predicts that the drag coefficient CD, which is traditionally modeled as a function of U10 alone (e.g., the COARE algorithm), becomes strongly dependent on the magnitude of the swell factor 1:25Hsd 2np 2 and that CD can attain values several times larger than predicted by wind speed-only models. According to maps of the global wave climate, conditions leading to large effects are likely to be widespread over the World Ocean.
KW - Atm/Ocean Structure/ Phenomena
KW - Atmosphere-ocean interaction
KW - Marine boundary layer
UR - http://www.scopus.com/inward/record.url?scp=84950297355&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=84950297355&partnerID=8YFLogxK
U2 - 10.1175/JAS-D-15-0139.1
DO - 10.1175/JAS-D-15-0139.1
M3 - Article
AN - SCOPUS:84950297355
VL - 72
SP - 4777
EP - 4795
JO - Journals of the Atmospheric Sciences
JF - Journals of the Atmospheric Sciences
SN - 0022-4928
IS - 12
ER -