Ocean surface boundary layer dynamics off the southern coast of France in the NW Mediterranean is investigated by using velocity observations by high-frequency (HF) radars, surface drifting buoys and a downward-looking drifting acoustic Doppler current profiler (ADCP). The analysis confirms that velocities measured by HF radars correspond to those observed by an ADCP at the effective depth z f = k −1, where k is wavenumber of the radio wave emitted by the radar. The radials provided by the radars were in a very good agreement with in situ measurements, with the relative errors of 1 and 9 % and root mean square (RMS) differences of 0.02 and 0.04 m/s for monostatic and bistatic radar, respectively. The total radar-based velocities appeared to be slightly underestimated in magnitude and somewhat biased in direction. At the end of the survey period, the difference in the surface current direction, based on HF radar and ADCP data, attained 10°. It was demonstrated that the surface boundary layer dynamics cannot be reconstructed successfully without taking into the account velocity variation with depth. A significant misalignment of ∼30° caused by the sea breeze was documented between the HF radar (HFR-derived) surface current and the background current. It was also found that the ocean response to a moderate wind forcing was confined to the 4-m-thick upper layer. The respective Ekman current attained the maximum value of 0.15 m/s, and the current rotation was found to be lagging the wind by approximately 40 min, with the current vector direction being 15–20° to the left of the wind. The range of velocity variability due to wind forcing was found comparable with the magnitude of the background current variability.

As an extension of previous work in Wang et al. (Ocean Dyn 64:1447–1456, 2014), this article presents significant waveheights of swell inverted from a 13 month dataset of two high-frequency (HF) phased array radars. As an important intermediate variable in the calculation of significant waveheights, relative swell directions obtained by two different methods from a single radar station are also presented. The impact of the inaccuracy of relative swell direction on the calculation of waveheight is investigated and an alternative way of using constant swell direction is proposed. Radar-inverted swell significant waveheights using different ranges of relative swell directions are investigated. Results are assessed by WAVEWATCH III model hind casts. Analysis of the complete database shows that radar-inverted swell significant waveheights agree reasonably well with model estimates with large scatter. Standard deviation of the difference between the two estimations increases with waveheight, whereas the relative standard deviation, normalized by waveheight, keeps nearly constant. The constant direction scheme of waveheight inversion gives good estimations except for energetic swell exceeding the small perturbation assumption. Statistical analysis suggests that radar measurement uncertainty explains a considerable part of the difference between radar and model estimates. Swell estimates from both radar stations are consistent. This enables combined use of both radar spectra at common radar cells. Use of double spectra solves the ambiguity of relative swell direction, i.e., absolute swell direction is obtained, and effectively improves the accuracy of swell direction by the least-squares method.

涌浪在近海海洋环境中起到的重要影响受到了越来越多的关注.高频地波雷达提供了一种准实时观测涌浪的新手段.本文比较了法国Iroise海区2008年2-7月期间的涌浪频率的浮标观测结果和高频雷达反演结果.结果表明,雷达反演涌浪频率与浮标结果总体吻合良好,两种观测方式之间的相关系数为0.92,均方根偏差为0.004 5 Hz.雷达观测结果大部分集中在2-4月中低频涌浪发生的时期,而对5-7月期间的高频涌浪的观测能力显著降低.

This paper documents the study of the radar signature of the sea surface on the images collected by an X-band weather radar and its application to remote sensing of the coastal zone. The main radar parameters considered here are the reflectivity factor [which was converted into a normalized radar cross section (NRCS)], the differential reflectivity, and the mean radial velocity. Measurements of the NRCS in the vicinity of an instrumented offshore buoy allowed for identifying its variations with wind speed and relative direction, which are found to be consistent with the Georgia Institute of Technology (GIT) model and other measurements. A more accurate empirical model for the NRCS is developed. For moderate to strong winds, the mean radial velocity is linearly related to the radial component of the wind speed, showing the potentiality of weather radars to map the radial surface wind speed at the scale of a basin. The influence of surface currents is weak here but should be likely taken into account in other places with stronger currents. Measurements of the differential reflectivity are also consistent with other measurements made elsewhere for the polarization ratio. The analysis of radar data over the entire coverage could also be performed using surface wind estimates from a meteorological model. Similar but less accurate results are obtained for the NRCS and the mean radial velocity. The extra spatial variability not due to wind speed variations is identified. An inversion procedure is proposed to recover the wind vector field from radar measurements after calibration of the NRCS values. Results are promising and pave the way to the concept of weather radar for ocean remote sensing.

Inertial band response of the upper ocean to changing wind is studied both theoretically and by analysis of observations in the northwestern Mediterranean. On the nontraditional f plane, because of the horizontal component of the earth's rotation for waves of inertial band with frequencies slightly below the local inertial frequency f, there is a waveguide in the mixed layer confined from below by the pycnocline. It is argued that when the stratification is shallow these waves are most easily and strongly excited by varying winds as near-inertial oscillations (NIOs). These motions have been overlooked in previous studies because they are absent under the traditional approximation. The observations that employed buoys with thermistors, ADCPs, and two 16.3-MHz Wellen Radar (WERA) HF radars were carried out in the Gulf of Lion in April-June 2006. The observations support the theoretical picture: a pronounced inertial band response occurs only in the presence of shallow stratification and is confined to the mixed layer, and the NIO penetration below the stratified layer is weak. NIO surface magnitude and vertical localization are strongly affected by the presence of even weak density stratification in the upper 10 m. The NIO surface signatures are easily captured by HF radars. Continuous 1.8-yr HF observations near the Porquerolles Island confirm that shallow stratification is indeed the precondition for a strong NIO response. The response sensitivity to stratification provides a foundation for developing HF radar probing of stratification and, indirectly, vertical mixing, including spotting dramatic mixing events and spikes of vertical heat, mass, and momentum exchange.

High-frequency (HF) radars are commonly used for coastal circulation monitoring. The objective of the study is to assess what is the minimum timescale of variability of the geophysical surface currents that are accessible to the radar measurement given the intrinsic noise of this measurement. Noise properties are derived from the power density spectra (PDSs) of radial current records, which are compared to a model of the PDS of idealized currents contaminated by an additive white noise. The data were collected by two radar systems operating in the Northwestern Mediterranean. Periods of 3 weeks to 7 months are considered. Most of measured currents are affected by a white noise effect. Noise properties vary in time and space and are not specific to a particular radar station or to the radar signal processing method used (beam forming or direction finding). An increase of the noise level reduces the effective temporal resolution of radar-derived currents and then increases the minimum observable timescale of variability of geophysical currents. Our results are consistent with results of comparison found in literature between in situ sensors and radar measurements as well as between two radars operating along a same base line. The study suggests a self-sufficient method, requiring no external data, to estimate the minimum sampling period to consider for getting data sets having a minimized contamination by instrumental noise. This period can also be taken for smoothing or filtering measured currents.

We simulate the Doppler spectra that can be obtained under experimental conditions from bistatic high-frequency oceanic radar. For this, we combine the bistatic second-order theory with the characteristics of the full radar system, such as antenna patterns, range attenuation, and postprocessing of the received signal. Because of the sharp variation of the bistatic geometry at short range, we show that it is crucial to take these effects into account for the correct interpretation of the first-order Bragg peaks. The second-order spectrum is more robust to the system characteristics but can under some conditions also suffer from its artifacts, especially in the vicinity of the secondary peaks. A comparison is made with experimental spectra acquired recently with a Wellen radar system on the Mediterranean coast. The Wave Watch III model is used to simulate directional wave height spectra after a preliminary validation with in situ buoy measurements. Experimental and simulated Doppler spectra agree generally well, except in the case where the directional wave spectrum has little energy in the line of sight of the radar.

We present a numerical investigation of horizontally polarized microwave scattering from 1-D sea surfaces at extreme grazing angles. Rigorous electromagnetic calculations are performed with a specific integral formalism dedicated to grazing angles. Sample sea surfaces are simulated using a classical Pierson-Moskowitz elevation spectrum together with weakly nonlinear hydrodynamic models, namely, the Creamer solution, the "choppy wave model," and a recent improved version thereof. For this, the electromagnetic integral formalism is extended to surfaces with irregular sampling. For the different nonlinear surface models and assuming no large-scale current, we evidence a dramatic increase, followed by a saturation of the mean Doppler shift in the last few grazing degrees, with a limiting value depending quasi-linearly on the significant wave height. Our numerical investigations confirm that breaking events are not necessary to produce fast scatterers but tend to show that they are necessary to reproduce the elevated level of backscattered power. The results of this study also support the hypothesis that the blow-up of the mean Doppler shift at grazing angle is associated to an electromagnetic sharp edge effect on the large surface crests rather than geometrical shadowing of the troughs.

A Mediterranean Ocean Observing System for the Environment (MOOSE) has been set up as an interactive, distributed, and integrated observatory system of the North West Mediterranean Sea in order to detect and identify long-term environmental anomalies. In this framework, the Mediterranean Institute of Oceanography (MIO) operates two couple of High Frequency Surface Wave Radars (HFSWR). HFSWRs provide synoptic observations of sea surface currents with high temporal (0.25-1 h) and spatial (1-3 km) resolution and long range (30-100 km). They can therefore be used to study the variability of the current as well as eddy dynamics. One site, based on the WERA (Helzel Messtechnik) technology and installed near Toulon, is operational and monitored in real-time since more than 3 years. It is composed of two WERA systems featuring respectively a non-linear receiving array and a full bistatic configuration, both working in Direction Finding mode. The second site, intended to extend further East the coverage of the Observatory, was set up in 2014 near Nice with two SeaSonde (Codar) systems. We present here an overview of the HFSWR network, the surface current mapping facility offered by the system, and recent observational results and applications.

This article presents long period ocean wave (swell) frequencies inverted from a 13-month dataset of high-frequency (HF) phased array radars and an assessment of these estimates by comparison with WAVEWATCH III model data. The method of swell frequency inversion from high-frequency radar sea echo Doppler spectra is described. Radar data were collected from a two-site HF Wellen Radar (WERA) radar system on the west coast of Brittany (France) operating at 12 MHz. A standard beam-forming processing technique has been used to obtain Doppler spectra of processed radar cells. Swell frequencies are obtained from the frequencies of particular spectral peaks of the second-order continuum in hourly averaged Doppler spectra. The data coverage of effective Doppler spectra considered for swell frequency estimates shows the influence of islands and shallow water effects. Swell estimates from both radar stations are in good agreement. The comparison of radar-derived results to WAVEWATCH III (WW3) estimates shows that radar measurements agree quite well with model results. The bias and standard deviation between two estimates are very small for swells with frequency less than 0.09 Hz (period >11 s), whereas radar estimates are generally lower than model estimates for shorter swells, along with higher standard deviation. Statistical analysis suggests that radar measurement uncertainty explains most of the difference between radar and model estimates. For each swell event, time series of frequency exhibits a quasi-linear frequency increase which is associated with the dispersive property of wave phase velocity. The use of swell frequency estimates from both radars on common radar cells only slightly increases the accuracy of swell frequency measurement.

Abstract. HF radar measurements are used to optimize surface wind forcing and baroclinic open boundary condition forcing in order to constrain model coastal surface currents. This method is applied to a northwestern Mediterranean (NWM) regional primitive equation model configuration. A new radar data set, provided by two radars deployed in the Toulon area (France), is used. To our knowledge, this is the first time that radar measurements of the NWM Sea are assimilated into a circulation model. Special attention has been paid to the improvement of the model coastal current in terms of speed and position. The data assimilation method uses an ensemble Kalman smoother to optimize forcing in order to improve the model trajectory. Twin experiments are initially performed to evaluate the method skills. Real measurements are then fed into the circulation model and significant improvements to the modeled surface currents, when compared to observations, are obtained.

In the framework of the French MOOSE project (Mediterranean Ocean Observing System on Environment), the Mediterranean Institute of Oceanography is operating HF radars on the North Western Mediterranean coast. The surface circulation in this region is characterized by a large-scale flow (Northern Current) and by a broad range of other scales of variability induced by meteorological and tidal forcing. The ability of HF radars is to provide synoptic observation as sea surface current map every hour and over long distances. One site is already operational nearby Toulon for more than two years and a second one is in deployment around Nice. This paper gives an overview of the radars network, of the surface current mapping facility offered by the system, and of recent observation results and applications.

The data from two high-frequency radars (HFR) operating in the Iroise Sea are re-processed by applying an improved version of the direction finding algorithm, removing wave-induced surface currents and the variational interpolation on a regular grid. Combining these processing techniques allowed reconstruction of the surface currents at a level of details that was not previously available. Refined resolution enabled to identify fine-scale structures of surface circulation, to quantify the variability of tidal currents and the residual (time averaged) velocity field, and to explain spatial intermittence in polarization of the tidal current ellipses. The analyzed data span two month-long periods in spring and late summer of 2007. The major findings include (a) a dipole structure in the vorticity field characterized by two oppositely rotating eddies, generated on the leeward side of the Ushant Island at flood (negative polarity) and at ebb (positive polarity); (b) an extremely strong fortnightly variability of tidal currents northwest of the Ushant Island with the highest velocity magnitude of 3.9m/s caused by the interference of the major semi-diurnal tidal constituents; (c) a significant contribution of the higher order nonlinear tidal harmonics to the surface currents in the Fromveur strait, which maintains strong tidal currents and affects the shape of their fortnightly modulation. The residual circulation is characterized by two distinct zones approximately separated by the 100m isobath: in the offshore zone the residual currents have a significant contribution of the wind-driven component, whereas the nearshore zone is characterized by extremely strong (up to 0.4m/s) time-independent residual circulation featuring two permanent anticyclonic eddies: north of the western extremity of the Sein archipelago, and north the Ushant Island. The acquired data and the presented results could be useful for regional model validation and studies of the local eddy dynamics, tidal fronts, and passive tracer transport in the region.

The Northern current is the main circulation feature of the North-Western Mediterranean Sea. While the large-scale to mesoscale variability of the northern current (NC) is well known and widely documented for the Ligurian region, off Nice or along the Gulf of Lions shelf, few is known about the current instabilities and its associated mesoscale dynamics in the intermediate area, off Toulon. Here, we took advantage of an oceanographic cruise of opportunity, the start of a HF radar monitoring programme in the Toulon area and the availability of regular satellite sea surface temperature and chlorophyll a data, to evaluate the realism of a NEMO-based regional high-resolution model and the added value brought by HF radar. The combined analysis of a 1/64° configuration, named GLAZUR64, and of all data sets revealed the occurrence of an anticyclonic coastal trapped eddy, generated inside a NC meander and passing the Toulon area during the field campaign. We show that this anticyclonic eddy is advected downstream along the French Riviera up to the study region and disturbs the Northern current flow. This study aims to show the importance of combining observations and modelling when dealing with mesoscale processes, as well as the importance of high-resolution modelling.

Correction of open boundary conditions (OBC) is attempted to improve surface velocity fields by assimilating HF radar velocities in a North Western Mediterranean (NWM) coastal model nested in a large scale operational model (Mercator Ocean system PSY2) providing IC (Initial Conditions) and OBC. A method based on HF radar velocities assimilation using an Ensemble Kalman Filter (EnKF) to derive the optimal wind forcing had already been validated. The objective of this work is to implement this method to the OBC correction. An ensemble simulation of the NWM sea model is carried out under different OBC to estimate model error covariance and covariance between surface currents and OBC. We evaluate the ability to correct the baroclinic oceanic forcings and to improve the surface current using a distant HF radar system. First, the method is assessed using twin experiments and a NWM sea model based on a Regional Ocean Model System (ROMS) configuration at 1/12°. Next, the method is applied to a high resolution (1/64°) NEMO-based model using a HF radar system operating in the Cote d'Azur. The method evaluation is done in both the eulerian and the lagrangian framework, based on a comprehensive data set (surface radial currents, surface drifter trajectories) obtained during the TOSCA (MedProgram) campaign. TOSCA project intends to optimize the response to marine accidents (oil spill, search and rescue) in Mediterranean sea, and the radar data assimilation may represent a great advantage to describe with more accuracy surface currents.

Currents effects on waves have led to many developments in numerical wave modeling over the past two decades, from numerical choices to parameterizations. The performance of numerical models in conditions with strong currents is reviewed here, and observed strong effects of opposed currents and modulations of wave heights by tidal currents in several typical situations are interpreted. For current variations on small scales, the rapid steepening of the waves enhances wave breaking. Using different parameterizations with a dissipation rate proportional to some measure of the wave steepness to the fourth power, the results are very different, none being fully satisfactory, which points to the need for more measurements and further refinements of parameterizations. For larger-scale current variations, the observed modifications of the sea state are mostly explained by refraction of waves over currents and relative wind effects, that is, the wind speed relevant for wave generation is the speed in the frame of reference moving with the near-surface current. It is shown that introducing currents in wave models can reduce the errors on significant wave heights by more than 30% in some macrotidal environments, such as the coast of Brittany, in France. This large impact of currents is not confined to the locations where the currents are strongest, but also downwave from strong current gradients.

Les modèles numériques d’états de mer décrivent l’évolution du spectre d’énergie de l’état de mer sous l’action conjointe de plusieurs processus physiques qui apportent, dissipent ou transfèrent de l’énergie. Une meilleure prise en compte des interactions non-linéaires entre quadruplets de fréquences de vagues est essentielle à l’amélioration des modèles d’états de mer. Sur la base d’une approche initialement introduite par Lavrenov (2001), nous avons développé et optimisé une méthode de calcul quasi-exacte des interactions non-linéaires vague-vague en grande profondeur d’eau. Cette approche, nommée GQM (pour « Gaussian Quadrature Method »), est basée sur l’utilisation de quadratures de Gauss dans l’évaluation du terme d’interaction, et permet d’obtenir des estimations très précises du terme de transfert non-linéaire à des coûts de calcul raisonnables. A l’aide de cette méthode, nous nous sommes d’abord intéressés à la modélisation de l’évolution temporelle du spectre des vagues dans un cas homogène et sans forçage. Une situation plus proche de la réalité, mais conservant une géométrie simple (cas de fetch limité), a ensuite été modélisée en prenant en compte les termes de forçage par le vent et de dissipation par moutonnement et la propagation spatiale des vagues. Les travaux effectués confirment la nécessité de modéliser de façon précise les interactions non-linéaires dans les modèles d’états de mer et montrent la faisabilité de ces améliorations grâce à la méthode GQM et à l’algorithme de calcul mis au point.