Downward transfer of momentum by wind-driven waves

Nielsen, Peter, Callaghan, David P. and Baldock, Tom E. (2011) Downward transfer of momentum by wind-driven waves. Coastal Engineering, 58 12: 1118-1124. doi:10.1016/j.coastaleng.2011.06.008


Author Nielsen, Peter
Callaghan, David P.
Baldock, Tom E.
Title Downward transfer of momentum by wind-driven waves
Journal name Coastal Engineering   Check publisher's open access policy
ISSN 0378-3839
1872-7379
Publication date 2011-12-01
Sub-type Article (original research)
DOI 10.1016/j.coastaleng.2011.06.008
Volume 58
Issue 12
Start page 1118
End page 1124
Total pages 7
Place of publication Amsterdam, Netherlands
Publisher Elsevier
Collection year 2012
Language eng
Abstract A model for the downward transfer of wind momentum is derived for growing waves. It is shown that waves, which grow due to an uneven pressure distribution on the water surface or a wave-coherent surface shear stress have horizontal velocities out of phase with the surface elevation. Further, if the waves grow in the x-direction, while the motion is perhaps time-periodic at any fixed point, the Reynolds stresses associated with the organized motion are positive. This is in agreement with several field and laboratory measurements which were previously unexplained, and the new theory successfully links measured wave growth rates and measured sub-surface Reynolds stresses. Wave coherent air pressure (and/or surface shear stress) is shown to change the speed of wave propagation as well as inducing growth or decay. From air pressure variations that are in phase with the surface elevation, the influence on the waves is simply a phase speed increase. For pressure variations out of phase with surface elevation, both growth (or decay) and phase speed changes occur. The theory is initially developed for long waves, after which the velocity potential and dispersion relation for linear waves in arbitrary depth are given. The model enables a sounder model for the transfer to storm surges or currents of momentum from breaking waves in that it does not rely entirely on ad-hoc turbulent diffusion. Future models of atmosphere-ocean exchanges should also acknowledge that momentum is transferred partly by the organized wave motion, while other species, like heat and gasses, may rely totally on turbulent diffusion. The fact that growing wind waves do in fact not generally obey the dispersion relation for free waves may need to be considered in future wind wave development models.
Keyword Wind waves
Wave growth
Reynolds stresses
Wind stress
Q-Index Code C1
Q-Index Status Confirmed Code
Institutional Status UQ

Document type: Journal Article
Sub-type: Article (original research)
Collections: School of Civil Engineering Publications
Official 2012 Collection
 
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