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Sophisticated measurements of fluid velocity near to an undulating air–water boundary have traditionally been confined to the laboratory setting. Developments in camera technology and the opening of novel modes of analysis have allowed for sensitive measurements of the current profile in the ocean’s uppermost layer. Taking advantage of the Research Platform R/P FLIP as a ‘laboratory at sea’, here we present first-of-their-kind thermal and polarimetric camera-based observations of wave orbital velocities and mean shear flows in the upper centimetres of the ocean surface layer. Measurements reveal a well-defined logarithmic layer as seen in laboratory measurements and described by classical surface layer theory; however, substantial spread of observations is found at low levels of wind forcing, where the Stokes drift of swell may have a substantial impact on the near-surface current profile. A novel application of short time window Fourier transforms allows for the estimation of near-surface wave orbital velocity magnitudes. These are found to be in general agreement with the prescriptions of linear wave theory, although observations diverge from theory at high levels of wind forcing where the interface is subject to surface wave breaking. Finally, the surface gravity wave phase-coherent short wave growth is presented and discussed in the context of hydrodynamic wave and airflow modulation.
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This content will become publicly available on January 1, 2026
Remote sensing of wave-orbital velocities in the surfzone
Wave-orbital velocities are estimated with particle image velocimetry (PIV) applied to rapid sequences of images of the surfzone surface obtained with a low-cost camera mounted on an amphibious tripod. Time series and spectra of the remotely sensed cross-shore wave-orbital velocities are converted to the depth of colocated acoustic Doppler velocimeters (ADVs), using linear finite depth theory. These converted velocities are similar to the velocities measured in situ (mean nRMSE for time series =16% and for spectra =10%). Small discrepancies between depth-attenuated surface and in situ currents may be owing to errors in the surface velocity measurements, uncertainties in the water depth, the vertical elevation of the ADVs, and the neglect of nonlinear effects when using linear finite depth theory. These results show the potential to obtain spatially dense estimates of wave velocities
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- Award ID(s):
- 2318785
- PAR ID:
- 10599410
- Publisher / Repository:
- elsevier
- Date Published:
- Journal Name:
- Coastal Engineering
- Volume:
- 195
- Issue:
- C
- ISSN:
- 0378-3839
- Page Range / eLocation ID:
- 104631
- Subject(s) / Keyword(s):
- Surfzone Wave orbital velocities Particle image velocimetry Surface velocity Remote sensing
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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