Book contents
- Frontmatter
- Contents
- Preface
- Acknowledgements
- Contributors
- Frequently Used Symbols
- 1 Overview
- PART 1
- PART 2
- 8 The Influence of Swell on the Drag
- 9 The Influence of Unsteadiness
- 10 The Dependence on Wave Age
- 11 The Influence of Mesoscale Atmospheric Processes
- 12 Wind, Stress and Wave Directions
- 13 The Influence of Surface Tension
- 14 The Influence of Spatial Inhomogeneity: Fronts and Current Boundaries
- 15 Basin Boundaries
- References
- Index
8 - The Influence of Swell on the Drag
Published online by Cambridge University Press: 16 October 2009
- Frontmatter
- Contents
- Preface
- Acknowledgements
- Contributors
- Frequently Used Symbols
- 1 Overview
- PART 1
- PART 2
- 8 The Influence of Swell on the Drag
- 9 The Influence of Unsteadiness
- 10 The Dependence on Wave Age
- 11 The Influence of Mesoscale Atmospheric Processes
- 12 Wind, Stress and Wave Directions
- 13 The Influence of Surface Tension
- 14 The Influence of Spatial Inhomogeneity: Fronts and Current Boundaries
- 15 Basin Boundaries
- References
- Index
Summary
Introduction
Swell is formally defined as old wind sea that has been generated elsewhere. The term “old” is meant to signify that at some past time the swell energy, propagating through a given defined point, had been directly forced by the wind elsewhere. In view of the rather specific notion of “wave age” it might be better to think of swell as “escaped” wind sea. Having come from elsewhere, bearing the imprint of a different storm, swell may propagate at any speed relative to the wind or at any angle to the wind. Indeed, the vector difference in speed of the swell and peak wind sea may provide the only unambiguous criterion for identifying and separating swell from actively growing wind sea. Frequency dispersion separates the components of swell as they propagate away from the source area, and so swell tends to have a narrower spectrum than wind sea; but this provides only a qualitative selection criterion since the bandwidth of wind sea and swell may have considerable variation. For clarity we consider only two clearly defined cases of swell: (1) a distinct peak in the spectrum having peak phase speed greater than the wind component in the direction of propagation of the peak; (2) a distinct peak in the spectrum having peak phase velocity at an angle greater than 90 degrees to the wind.
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- Wind Stress over the Ocean , pp. 181 - 189Publisher: Cambridge University PressPrint publication year: 2001
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