arXiv subjects

Charles-Antoine Guérin

Publications and source records attributed to Charles-Antoine Guérin.

12 recordsLinked to original sources

arXiv2026

New features of the sea-surface slope distribution revealed by IASI observations: Directional properties, influence of the wave height, large wave tilts

By analysis of the reflected solar contribution to mid-infrared radiance spectra collected by the IASI instrument, we investigate the influences of both the wind and significant wave height on the wave-slope probability distribution function (PDF). We show that, at equal wind speed, smaller wave heights enhance the directional (upwind-downwind and upwind-crosswind) asymmetries of the probabilities. We also point out that the sea-surface mean square slopes slightly decrease as the wave height increases (by about $5\%$ per meter for winds slower than 6 m/s), an original result for which we propose possible causes (short-scale damping by long waves and/or effect of compounding statistical processes). We also investigate the so far practically unstudied case of steep slopes and find large deviations from the quasi Gaussian behavior of the central part of the PDF, with a quite universal exponential decay of the probabilities with increasing tilt. We here observe larger probabilities in the downwind direction, but similar values in the up- and cross-wind ones. The physical mechanisms responsible for these trends are discussed. Finally, we propose refined parametrizations of the mean square slope versus wind-speed, which departs from the Cox and Munk linear relationships at moderate wind speed (5-8 m/s).

physics.ao-ph↗

arXiv2025

Improved calculation of the second-order ocean Doppler spectrum for sea state inversion

We describe and exploit a reformulation, based on a recently-introduced change of variables, of the double integral that describes the second-order ocean Doppler spectrum measured by High-Frequency radars. We show that this alternative expression, which was primarily designed for improving the numerical inversion of the ocean wave spectrum, is also advantageous for the analytical inversion of the main sea state parameters. To this end, we revisit Barrick's method for the estimation of the significant wave height and the mean period from the ocean Doppler spectrum. On the basis of numerical simulations we show that a better estimation of these parameters can be achieved which necessitates a preliminary bias correction that depends only on the radar frequency. A second consequence of this improved formulation is the derivation of a simple yet analytical nonlinear approximation of the second-order ocean Doppler spectrum when the Doppler frequency is larger than the Bragg frequency. This opens up new perspectives for the inversion of directional wave spectra from High-Frequency radar measurements.

physics.ao-ph↗

arXiv2025

HF radar estimation of ocean wave parameters: second-order Doppler spectrum versus Bragg wave modulation approach

We propose an original technique for the HF radar estimation of the main sea state parameters by exploiting the amplitude modulation of the radar signal time series. While the classical method for ocean wave measurement is based on the second-order ocean Doppler spectrum, this alternative approach uses the slow amplitude modulation of the Bragg wave in the radar signal. Using a nearby buoy and the WWIII model as ground truth, we apply this method to an annual set of HF radar data in the vicinity of Toulon (Mediterranean coast of France) and compare it with \col{the classical formulas proposed by Barrick (1977) to derive the main wave parameters from the Doppler spectrum. Although the modulation approach is physically attractive and easier to implement, we find that the Doppler spectrum-based method is far superior at this stage in calculating the significant wave height and the peak wave frequency, even at long range.

physics.ao-ph↗

arXiv2024

High-Frequency Radar observation of strong and contrasted currents: the Alderney race paradigm

The Alderney Race has been identified as a future site for the development of tidal energy, due to its bidirectional strong current reaching 5 m/s during spring tides. This hydrodynamics is very difficult to measure by in situ or remote sensing means. High-frequency coastal radars can provide a synoptic and near-real-time view of such a complex circulation, but the classical processing algorithms are not adapted to the extreme situation of strongly sheared currents. We propose an improved high-resolution direction-finding technique for the azimuthal processing of such radar data. It uses phased-array systems and combines the advantages of the usual beam-forming technique to eliminate many problems related to the distortion of Doppler spectra by extreme currents. The method is evaluated with a unique data set of radar measurements at two radar frequencies (13 and 24.5 MHz) and three spatial resolutions (200, 750, and 1500 m). The radar-based surface currents are analyzed in the light of a high-resolution numerical model and also compared with in situ measurements. While high azimuthal resolution can be achieved in this way, it is shown that the typical range resolutions of 750 and 1500 m are insufficient to account for the strong spatial variations of the surface current at some specific times and locations.

physics.ao-ph↗

arXiv2023

Revisiting the Cox and Munk wave-slope statistics using IASI observations of the sea surface

We use radiances collected from space by the Infrared Atmospheric Sounder Interferometer (IASI) when looking down at ocean surfaces during the day to remotely determine the probability distribution of wave slopes. This is achieved by using about 300 channels between 3.6 and 4.0 $μ$m and a physically-based approach which properly takes the contribution of the reflected solar radiation into account. Based on about 150 million observations, the same number of wave-slope probabilities are retrieved for wind speeds up to 15 m/s. We revisit and discuss the methodology proposed by Cox and Munk (CM) to derive their celebrated wave-slope probability distribution function (pdf) from photographs of the sun glitter. We propose an original and robust approach for accurate retrievals of the 7 parameters appearing in the Gram-Charlier representation of the pdf. Our results for the mean square slopes are fully compatible with those of CM, and with the more recent results by Bréon and Henriot, but our lower uncertainties enable us to point out departures from the linear wind-speed dependencies and a slight overestimation of the upwind MSS described by the linear fit of CM at moderate wind speed. Our skewness and kurtosis coefficients show clear influences of the wind speed, with a steady decrease of the former and the alongwind kurtosis coefficient being maximal at moderate wind speeds, features that CM could not point out due to the limitations of their measurements. We revisit the renormalization procedure employed by CM to obtain the complete variances from truncated pdf and show that it imposes stringent conditions on the kurtosis coefficients that allow us to determine them accurately, with wind-dependent values specific to the local sea state.

physics.ao-ph↗

arXiv2022

Storm Surges as Seen by Coastal and Spaceborne Radars: Case Studies in British Columbia

Short-term sea level fluctuations prompted by abrupt atmospheric changes can be hazardous phenomena for coastal regions. We report on two such recent storm surges that occurred in 2020 on the shores of British Columbia, Canada. A rare concordance of ground-based and spaceborne sensors made it possible to observe these events with a variety of instruments : (1) a coastal oceanographic radar; (2) the synthetic aperture radar onboard satellite Sentinel-1B; and (3) a network of shoreside tide gauges. In the light of these case studies we show how satellite-based radar data can be used to complement the observation and interpretation of ground-based measurements in the context of ``tsunami-like'' sea level oscillations.

physics.ao-ph↗

arXiv2021

Rapid Scale Wind Profiling with Autoregressive Modeling and L-Band Doppler Radar

Radar Wind Profilers (RWP) are well-established instruments for the probing of the atmospheric boundary layer, with the immense advantage of long-range and all-weather operation capability. One of their main limitations, however, is a relatively long integration time compared to other instruments such as lidars. In the context of L-band RWP we show that the use of Autoregressive (AR) modeling for the antenna signals combined with the Maximum Entropy Method (MEM) allows for a correct estimation of radial wind velocity profiles even with very short time samples. A systematical analysis of performance is made with the help of synthetic data. These numerical results are further confirmed by an experimental dataset acquired near the landing runways of Paris Charles de Gaulle (CDG) Airport, France, and validated using a colocated optical lidar at the Aerological station of Payerne, Switzerland. It is found that the AR-MEM approach can successfully derive reliable wind estimates using integration times as short as 2.5 s where the classical spectral approach can barely provide any measurement. In addition, the technique helps mitigating the impact of uncooperative flyers in the processing of the atmospheric echo, leading to a more successful application of the wind estimation algorithm.

physics.ao-ph↗

arXiv2021

High-Frequency radar measurements with CODARs in the region of Nice: improved calibration and performance

We report on the installation and first results of one compact oceanographic radar in the region of Nice for a long-term observation of the coastal surface currents in the North-West Mediterranean Sea. We describe the specific processing and calibration techniques which were developed at the laboratory to produce high-quality radial surface current maps. In particular, we propose an original self-calibration technique of the antenna patterns, which is based on the sole analysis of the databasis and does not require any shipborne transponder or other external transmitters. The relevance of the self-calibration technique and the accuracy of inverted surface currents have been assessed with the launch of 40 drifters that remained under the radar coverage for about 10 days.

physics.ao-ph↗

arXiv2021

A Reanalysis of the October 2016 "Meteotsunami" in British Columbia with Help of High-Frequency Radars and Autoregressive Modeling

On October 14th, 2016, the station of Tofino (British Columbia, Canada) issued the first ever real-time tsunami alert triggered by a coastal High-Frequency Radar system, based on the identification of abnormal surface current patterns. The detection occurred in the absence of any reported seismic event but coincided with a strong atmospheric perturbation, which qualified the event as meteo-tsunami. We re-analyze this case in the light of a new radar signal processing method which was designed recently for inverting fast-varying sea surface currents from the complex voltage time series received on the antennas. This method, based on an Auto-regressive modeling combined with a Maximum Entropy Method, yields a dramatic improvement in both the Signal-to-Noise Ratio and the quality of the surface current estimation for very short integration time. This makes it possible to evidence the propagation of a sharp wave front of surface current during the event and to map its magnitude and arrival time over the radar coverage. We show that the amplitude and speed of the inferred residual current do not comply with a Proudman resonance mechanism but are consistent with the propagation of a low-pressure atmospheric front and wind vectors as revealed by satellite imagery. This indicates that the event that triggered a tsunami alert is more likely a storm surge than a true meteo-tsunami. Beyond this specific case, another outcome of the analysis is the promising use of oceanographic radars as proxy's for the characterization of atmospheric fronts.

physics.ao-ph↗

arXiv2021

Improved Observation of Transient Phenomena with Doppler Radars: a Common Framework for Oceanic and Atmospheric Sensing

Doppler radars are routinely used for the remote sensing of oceanic surface currents and atmospheric wind profiles. Even though they operate at different frequencies and address different media, they follow very similar processing for the extraction of measured velocities. In particular they both face the challenging issue of capturing geophysical phenomena which vary rapidly with respect to the typical integration time. Recently, the authors applied a non-spectral formalism based on autoregressive processes to model the backscattered time series obtained from High-Frequency oceanic radars. They showed that it allows to calculate Doppler spectra for very short integration times without losing in frequency resolution nor signal-to-noise ratio. We apply this technique to synthetic and experimental data within a common framework and show for the first time the strong potential of the method for the study of transient atmospheric phenomena.

physics.ao-ph↗

arXiv2020

High-Frequency Radar Ocean Current Mapping at Rapid Scale with Autoregressive Modeling

We use an Autoregressive (AR) approach combined with a Maximum Entropy Method (MEM) to estimate radial surface currents from coastal High-Frequency Radar (HFR) complex voltage time series. The performances of this combined AR-MEM model are investigated with synthetic HFR data and compared with the classical Doppler spectrum approach. It is shown that AR-MEM drastically improves the quality and the rate of success of the surface current estimation for short integration time. To confirm these numerical results, the same analysis is conducted with an experimental data set acquired with a 16.3 MHz HFR in Toulon. It is found that the AR-MEM technique is able to provide high-quality and high-coverage maps of surface currents even with very short integration time (about 1 minute) where the classical spectral approach can only fulfill the quality tests on a sparse coverage. Further useful application of the technique is found in the tracking of surface current at high-temporal resolution. Rapid variations of the surface current at the time scale of the minute are unveiled and shown consistent with a $f^{-5/3}$ decay of turbulent spectra.

eess.SP↗

arXiv2020

Self-calibration and antenna grouping for bistatic oceanographic High-Frequency Radars

We propose two concepts for the significant improvement of surface current mapping with bistatic oceanographic High-Frequency Radars. These ameliorations pertain to the azimuthal processing of radar data with linear or quasi-linear antenna arrays. The first idea is to take advantage of the remote transmitter to perform an automatic correction of the complex gains of the receiving antennas based on the analysis of the signal received in the direct path. This direct signal can be found at the zero-Doppler and minimal range cell in the Range-Doppler representation. We term this adjustment as "self-calibration" of the receiving array, as it can be performed in real-time without any specific action from the operator. The second idea consists in applying a Direction Finding technique (instead of traditional Beam Forming) not only to the full array of antenna but also to subarrays made of a smaller number of sequential antennas, a method which we refer to as "antenna grouping". The combination of self-calibration and antenna grouping makes it possible to obtain high-resolution maps with full coverage, thereby combining the respective merits of Direction Finding and Beam Forming techniques. In addition the method is found robust to missing antennas in the array. These techniques are applied to and illustrated with the multistatic High-Frequency Radar network in Toulon.

eess.SP↗