Abstract
High-frequency (HF) radar-derived ocean currents are compared with in situ measurements to conclude if the radar observations include effects of surface waves that are of second order in the wave amplitude. Eulerian current measurements from a high-resolution acoustic Doppler current profiler and Lagrangian measurements from surface drifters are used as references. Directional wave spectra are obtained from a combination of pressure sensor data and a wave model. Our analysis shows that the wave-induced Stokes drift is not included in the HF radar-derived currents, that is, HF radars measure the Eulerian current. A disputed nonlinear correction to the phase velocity of surface gravity waves, which may affect HF radar signals, has a magnitude of about half the Stokes drift at the surface. In our case, this contribution by nonlinear dispersion would be smaller than the accuracy of the HF radar currents, hence no conclusion can be made. Finally, the analysis confirms that the HF radar data represent an exponentially weighted vertical average where the decay scale is proportional to the wavelength of the transmitted signal.
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Acknowledgments
This work was funded by the Norwegian Clean Seas Association For Operating Companies (NOFO) and ENI Norge A/S, with contributions from the Research Council of Norway (196438/BIOWAVE and 207541/OILWAVE). The contribution by Ø.B. was carried out as part of the European Union FP7 project MyWave (grant no 284455). The authors would like to thank Svein Sundby and Frode Vikebø (IMR) for helpful discussions and kind assistance during the planning phase, and Erik Berg (IMR), as well as the captain and crew of R/V Johan Hjort, for their kind assistance with the drifter deployments. We would also like to thank Ronald Pedersen (IMR), Tor Gammelsrød (UiB), Erik Kvaleberg, and Anne Hesby (Royal Norwegian Navy) for loan of instruments and assistance with the ADCP rig. Assistance from the captains and crews of the NSO Crusader and R/V Håkon Mosby when deploying and recovering the ADCP rig is also gratefully acknowledged.
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Röhrs, J., Sperrevik, A.K., Christensen, K.H. et al. Comparison of HF radar measurements with Eulerian and Lagrangian surface currents. Ocean Dynamics 65, 679–690 (2015). https://doi.org/10.1007/s10236-015-0828-8
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DOI: https://doi.org/10.1007/s10236-015-0828-8