Effect of variable winds on current structure and Reynolds stresses in a tidal flow: analysis of experimental data in the eastern English Channel

Wind and wave effects on tidal current structure and turbulence throughout the water column are examined using an upward-looking acoustic Doppler current profiler (ADCP). The instrument has been deployed on the seafloor of 18-m mean depth, off the north-eastern French coast in the eastern English Channel, over 12 tidal cycles, and covered the period of the transition from mean spring to neap tide, and forcing regimes varied from calm to moderate storm conditions. During storms, we observed gusty winds with magnitudes reaching 15 m s(-1) and wave heights reaching up to 1.3 m. Analysis of velocity spectra revealed a noticeable contribution of wind-induced waves to spectral structure of velocity fluctuations within the subsurface layer. Near the surface, stormy winds and waves produced a significant intensification of velocity fluctuations, particularly when the sustained wind blew against the ebb tide flow. As during wavy periods, the variance-derived Reynolds stress estimates might include a wave-induced contamination, we applied the Variance Fit method to obtain unbiased stresses and other turbulent quantities. Over calm periods, the turbulent quantities usually decreased with height above the seabed. The stresses were found to vary regularly with the predominantly semidiurnal tidal flow. The along-shore stress being generally greater during the flood flow (similar to 2.7 Pa) than during the ebb flow (similar to -0.6 Pa). The turbulent kinetic energy production rate, P, and eddy viscosity, A(z), followed a nearly regular cycle with close to a quarter-diurnal period. As for the stresses, near the seabed, we found the maximum values of estimated quantities of P and A(z) to be 0.1 Wm(-3) and 0.5 m(2) s(-1), respectively, during the flood flow. Over the storm periods, we found the highest unbiased stress values (similar to -2.6 Pa) during ebb when tidal currents were opposite to the southwesterly winds while, during the flood, the surface stresses slightly exceeded those estimated for a calm period. A comparison of obtained results gives a good agreement with those of other researchers working on direct measurements of turbulence in tidal flows

Surface currents in the Alderney Race from high-frequency radar measurements and three-dimensional modelling

Two weeks of high-frequency radar measurements collected at the Alderney Race are compared with the results of a three-dimensional fully coupled wave–current model. Spatial current measurements are rare in this site, otherwise well investigated through modelling. Thus, the radar measurements offer a unique opportunity to examine the spatial reliability of numerical results, and can help to improve our understanding of the complex currents in the area. Comparison of observed and modelled surface current velocities showed a good agreement between the methods, represented by root mean squared errors ranging from 14 to 40 cm s−1 and from 18 to 60 cm s−1 during neap and spring tides, respectively. Maximum errors were found in shallow regions with consistently high current velocities, represented by mean neap and spring magnitudes of 1.25 m s−1 and 2.7 m s−1, respectively. Part of the differences between modelled and observed surface currents in these areas are thought to derive from limitations in the k-epsilon turbulence model used to simulate vertical mixing, when the horizontal turbulent transport is high. In addition, radar radial currents showed increased variance over the same regions, and might also be contributing to the discrepancies found. Correlation analyses yielded magnitudes above 0.95 over the entire study area, with better agreement during spring than during neap tides, probably because of an increase in the phase lag between radar and model velocities during the latter

The impact of high-frequency current variability on dispersion off the eastern Antarctic Peninsula

We present observations of high-frequency current variability on the continental shelf and the slope of the Antarctic Peninsula using Lagrangian surface drifters deployed as part of the Antarctic Drifter Experiment: Links to Isobaths and Ecosystems (ADELIE) project. Here we focus on high-frequency processes such as tides and inertial oscillations that are typically smoothed out of large-scale spatially averaged, and/or temporally averaged, observed current fields. We investigate the role that this class of motion plays in the transport of physical or biogeochemical properties. Lateral displacements on the shelf and slope are found to be larger than displacements in deeper waters where tidal currents are negligible. We apply this result in a parameterization of the lateral dispersion during an off-line drifter modeling study. The outcome is an improvement on the modeling of Lagrangian drifting particles compared with a standard random walk scheme

Progress towards a french high frequency ocean surface wave radar network

(IF N/A; Q4)International audienceThe use of High Frequency ocean Radars (HFR) as a research and monitoring tool for the observation of the coastal ocean has steadily developed throughout the world over the last decades, especially in the US where more than one hundred sites are currently maintained in a multi-purpose operational network (https://ioos.noaa.gov/project/hf-radar/) all over the country coasts. Such a networking effort is currently being built at the EU level through the JERICO-Next H2020 program in link with Euro-GOOS, with the objectives to share expertises, best practice, quality control procedures and data dissemination tool and to carry the existing EU HFR systems to the level of a unified operational HFR network. The LEFE/GMMC working group ReNHFOR (Research and Networking for High Frequency Oceanographic Radar) acts at the French level to structure the French contribution to this PanEuropean HFR network and assists the French oceanography community in the development of new applications both for academic and operational purposes. This paper presents an overview of the technology, its state of development in France, its potential as a supporting tool for ocean modeling and monitoring, and the integration of French HFR activities in the EU context