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Ocean surface wind wave signatures in single look complex SAR data
No full textInternational audienceSpaceborne synthetic aperture radar (SAR) provides high-resolution sea surface observations to characterize oceanic and atmospheric variability at submesoscales. Despite significant progress in retrieving wind fields from SAR images, challenges remain particularly for single-antenna measurements where wind speed inversion often requires an external wind direction input. This study introduces and evaluates the use of directional features derived from SAR image sub-look cross-spectra to improve wind retrieval. Specifically, we explore the imaginary part of SAR Mean Cross-Spectra (ΔIMACS) and its differential counterpart, ΔIMACS, which capture directional signatures of range-traveling intermediate wind waves. Using systematically collocated Sentinel-1A wave mode SAR images and model winds, we demonstrate that ΔIMACS exhibits a cosinelike dependence on wind direction at given wind speeds at two incidence angles, while ΔIMACS reveals unique sensitivities that complement radar backscattering and can resolve wind direction ambiguities. A novel cost function combining radar backscattering, ΔIMACS, and ΔIMACS is proposed to retrieve wind vectors directly from SAR data without external wind direction inputs. The proposed scheme is evaluated through comparisons with independent model outputs and buoy measurements, showing improved performance. These findings highlight the potential of ΔIMACS and ΔIMACS to enhance self-sufficient wind inversion, offering a promising pathway for single-polarized SAR wind retrieval
Évolution décennale de la composition fonctionnelle du phytoplancton dans la Manche orientale : effets majeurs possibles du changement climatique à venir
No full textInternational audienceGlobal change is known to exert a considerable impact on marine and coastal ecosystems, affecting various parameters such as sea surface temperature (SST), runoff, circulation patterns and the availability of limiting nutrients (like nitrogen, phosphorus and silicon), with each influencing phytoplankton communities differently. This study is based on weekly to fortnightly in vivo fine-spatial-resolution (∼ 1 km) phytoplankton observations along an nearshore–offshore gradient in the French waters of the Eastern English Channel in the Strait of Dover. The phytoplankton functional composition was addressed by automated “pulse-shape recording” flow cytometry, coupled with the analysis of environmental variables over the last decade (2012–2022). This method allows for the characterization of almost the entire phytoplankton size range (from 0.1 to 800 µm width) and the determination of the abundance of functional groups based on optical single-cell signals (fluorescence and scatter). We explored seasonal, spatial and decadal dynamics in an environment strongly influenced by tides and currents. Over the past 11 years, the SST has shown an increasing trend at all stations, with nearshore waters warming faster than offshore waters (+1.05 °C vs. +0.93 °C). Changes in nutrient concentrations have led to imbalances in nutrient ratios () relative to reference nutrient ratios. However, a return to balanced ratios has been observed since 2019. The phytoplankton total abundance has also increased over the aforementioned decade, with a higher contribution of small-sized cells (picoeukaryotes and picocyanobacteria) and a decrease in microphytoplankton, particularly near the coast. Based on an analysis of environmental parameters and phytoplankton abundance, the winters of 2013–2014 and 2019–2020 were identified as shifting periods in this time series. These changes in the phytoplankton community, favoring the smallest groups, could lead to a reduction in the productivity of coastal marine ecosystems, which could, in turn, affect higher trophic levels and the entire food web.Il est connu que le changement global exerce un impact considérable sur les écosystèmes marins et côtiers, affectant divers paramètres tels que la température de surface de la mer (SST), le ruissellement, les schémas de circulation et la disponibilité des nutriments limitants (comme l'azote, le phosphore et le silicium), chacun influençant différemment les communautés phytoplanctoniques. Cette étude est basée sur des observations in vivo hebdomadaires à bimensuelles de phytoplancton à résolution spatiale fine ( ∼ 1 km) le long d'un gradient littoral-hauturier dans les eaux françaises de la Manche orientale dans le détroit du Pas de Calais. La composition fonctionnelle du phytoplancton a été étudiée par cytométrie de flux automatisée « enregistrement de la forme des impulsions », couplée à l'analyse des variables environnementales sur la dernière décennie (2012-2022). Cette méthode permet de caractériser la quasi-totalité de la gamme de taille du phytoplancton (de 0,1 à 800 µ m de large) et de déterminer l'abondance des groupes fonctionnels à partir de signaux optiques unicellulaires (fluorescence et diffusion). Nous avons exploré la dynamique saisonnière, spatiale et décennale dans un environnement fortement influencé par les marées et les courants. Au cours des 11 dernières années, la température de surface de la mer (SST) a affiché une tendance à la hausse à toutes les stations, les eaux littorales se réchauffant plus rapidement que les eaux du large ( + 1,05 °C contre + 0,93 °C). Les variations des concentrations en nutriments ont entraîné des déséquilibres dans les ratios de nutriments () par rapport aux ratios nutritifs de référence. Cependant, un retour à des ratios équilibrés a été observé depuis 2019. L'abondance totale du phytoplancton a également augmenté au cours de la décennie susmentionnée, avec une contribution plus élevée des cellules de petite taille (picoeucaryotes et picocyanobactéries) et une diminution du microphytoplancton, en particulier près de la côte. Sur la base d'une analyse des paramètres environnementaux et de l'abondance du phytoplancton, les hivers 2013-2014 et 2019-2020 ont été identifiés comme des périodes de décalage dans cette série chronologique. Ces changements dans la communauté phytoplanctonique, favorisant les plus petits groupes, pourraient entraîner une réduction de la productivité des écosystèmes marins côtiers, ce qui pourrait, à son tour, affecter les niveaux trophiques supérieurs et l'ensemble du réseau trophique
An Assessment of Stratigraphic Sequence Interpretation from well log Data using Continuous Wavelet Transform
No full textInternational audienceStratigraphic interpretation is an essential task in subsurface studies. However, it is time-consuming and it often results in a small number of scenarios. Some numerical methods exist to propose automatic interpretations, based on either signal processing techniques or machine learning. These methods can be used to propose different acceptable geological scenarios from a same input log. We propose a method to assess the consistency of an automatic solution based on Continuous Wavelet Transform (CWT), with a reference interpretation. The application of this method on a Gamma Ray log highlights the potential of the CWT method for proposing stratigraphic interpretations of well logs
Touristes et scientifiques à bord de navires de croisière en Antarctique, opposition ou coopération ?
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Angular momentum relaxation in models of rotating early-type stars
No full textInternational audienceContext. The rotational evolution of stars remains an open question in stellar physics because numerous phenomena contribute to the distribution of angular momentum.Aims. This paper aims to determine the timescale over which a rotating early-type star relaxes to a steady baroclinic state or, equivalently, the conditions under which its nuclear evolution is slow enough to allow the star's evolution to be modelled as a series of quasi-steady states.Methods. We investigate the damping timescale of baroclinic and viscous eigenmodes that are potentially excited by the continuous forcing of nuclear evolution. We first examine this with a spherical Boussinesq model. Since much of the dynamics is concentrated in the radiative envelope of the star, we then improve the realism of the modelling by using a polytropic model of the envelope that incorporates a realistic density profile.Results. The polytropic model of the envelope highlights the key role of the region at the core-envelope interface. The results of evolutionary models recently obtained with two-dimensional axisymmetric ESTER models appear to arise from the slow damping of viscous modes. Using a vanishing Prandtl number appears to be too strong an approximation to explain the models’ dynamics. Baroclinic modes, previously thought to be good candidates for this relaxation process, are found to be too rapidly damped.Conclusions. The dynamical response of rotating stars to the slow forcing of their nuclear evolution appears as a complex combination of non-oscillating eigenmodes. Simple Boussinesq approaches are not sufficiently realistic to explain this reality. This study underlines the key role of layers near the core-envelope interface in early-type stars as well as the importance of angular momentum transport mechanisms-here represented by viscosity-for early-type stars to reach critical rotation, which is presumably associated with the Be phenomenon
A quantitative assessment of the behavior of metallic elements in urban soils exposed to industrial dusts near Dunkerque (northern France)
No full textInternational audienceIn urban and industrialized areas, soil contamination and degradation caused by industrial dust deposition may pose significant health and environmental risks. Generally, the mobility and thus bioavailability of potentially toxic elements (PTEs) are key factors in these issues. In the Dunkerque agglomeration, one of the most industrialized regions in France, the soils are periodically exposed to metallurgical dust fallout, rich in PTEs. However, no study has reported on the behavior of these PTEs once integrated into the soils. The aim of this study is therefore to assess the fate of PTEs in the urban soils of Dunkerque in terms of vertical migration and potential bioavailability.Four soil short cores were collected in the city of Gravelines (Dunkerque agglomeration) along a gradient from industrial emitters to deposition sites. Each soil core was cut into discrete 1 cm sections for PTE concentration analyses (ICP-AES/MS). Single HCl extractions were performed to evaluate PTE mobility in soils and their behavior according to the current soil parameters. For this purpose, key soil properties were identified, including grain-size distribution, mineralogy, pH, cation exchange capacity (CEC), TOC (total organic carbon), calcium carbonates and water contents in addition to the soil chemical composition (XRF, ICP-AES/MS).The studied soils revealed globally low absorbent capacities for pollutants (CEC averaging ), partially counterbalanced by the buffering effect of calcium carbonates (contents ranging from 8 %–30 %). Near the industrial emitters, minor (1 < EF <3) to moderately severe (5 < EF < 10) enrichment factors (EFs) were highlighted for industrial PTE (Cr, Ni, Mo, Mn, Cd and Zn) in the top 3 cm of soils near the industrial emitters. The contamination profiles of these soils are assigned to atmospheric inputs of metallurgical dust. Using a relatively strong leaching reagent (1 M HCl), we estimated a low vertical mobility for Cr, Ni and Mo (average leached ratios < 25 %) in soils, suggesting their association with refractory phases (natural or anthropogenic). In contrast, Mn, Cd and Zn, which are related to industrial and/or urban sources, present a higher mobility (average leached ratios>60 % for Mn and Cd and about 44 % for Zn).Our study points out the stability of industrial PTEs in soils under the current physicochemical conditions (calcareous soils with a slightly basic pH of 7.8). In this context, the monitoring of industrial PTEs in these urban soils is highly recommended, considering (1) the presence of allotment gardens in the vicinity of emitters and (2) the potential evolution of soil conditions due to increasing flood events
Modélisation expérimentale et numérique de la ségrégation verticale en transport de sédiments par charriage : effet du ratio de taille sur la remontée d'un intrus
No full textVertical size segregation in bedload transport impacts sediment transport and riverbed morphology. To improve sediment transport models, it is essential to understand the grain-scale mechanisms driving segregation. This thesis investigates the upward size segregation of a single intruder particle in a mono-disperse granular bed of smaller beads during bedload transport, with a focus on the impact of the size ratio between the intruder and the smaller particles.Flume experiments were conducted with different size ratios and extensive repetitions to examine the intruder kinematics.Results highlight a regime transition from the passive tracer behavior at size ratio 1 to size ratio around 2: as the size ratio increases, data show less variability and the slope of the intruder spatial trajectory gets steeper. Then, for larger size ratios, the slope stabilizes at a constant value and the time for the intruder to reach the bed surface increases. A kinematic model is developed to predict the segregation duration as a function of intruder depth and size ratio, incorporating a size ratio dependent drift coefficient that reflects the streamwise velocity lag between the intruder and the surrounding particles.Discrete element simulations of the experimental configuration with sidewalls successfully reproduce the experimental trends. Quantitatively, simulations give shorter segregation durations and lower trajectory slopes, attributed to a larger streamwise granular velocity, though the drift coefficients stay consistent with experiments.Simulations without sidewalls were conducted to eliminate boundary effects and provide a more general framework for studying size segregation. Comparisons with the simulations with sidewalls conducted with the same fluid forcing show that the sidewalls slow down the granular flow. As a result segregation durations and slopes of the linear spatial trajectory are smaller without sidewalls. From kinematic considerations, it is shown that the intruder streamwise velocity matches the streamwise velocity of the bed particles found below at a distance that increases linearly with the size ratio, and converges to about of the intruder radius. Based on this result, a more general model for the drift coefficient is developed. Dynamic analysis reveals large force fluctuations, which, when averaged, show that the intruder buoyant weight is balanced by contact forces, with a small residual force acting in the segregation direction. Granular pressure and shear stress computed at the intruder surface are maximum at the lower downstream quarter, with the magnitude increasing linearly with the size ratio.La ségrégation verticale par taille dans le transport de sédiments par charriage impacte les processus de transport sédimentaire et la morphologie du lit des rivières. Pour améliorer les modèles de transport sédimentaire, il est essentiel de comprendre les mécanismes responsable de la ségrégation à l’échelle granulaire. Cette thèse examine la ségrégation verticale d’une particule intruse au sein d'un lit granulaire monodisperse composé de particules plus petites, en mettant l’accent sur l’impact du ratio de taille entre l’intrus et les particules environnantes.Des expériences en canal ont été réalisées pour différents ratios de taille, avec de nombreuses répétitions, afin d’analyser la cinématique de l’intrus. Les résultats révèlent une transition de régime entre un comportement de traceur passif pour un ratio de taille égal à 1, jusqu’à un ratio proche de 2 : à mesure que le ratio de taille augmente, les données montrent une moindre variabilité et la trajectoire spatiale de l'intrus devient plus linéaire avec une pente plus raide. Au-delà, pour des ratios de taille plus élevés, la pente se stabilise à une valeur constante, tandis que le temps nécessaire pour que l’intrus atteigne la surface du lit augmente. Un modèle cinématique a été développé pour prédire la durée de ségrégation en fonction de la profondeur de l’intrus et du ratio de taille, en intégrant un coefficient de dérive dépendant du ratio de taille, qui reflète un décalage de vitesse entre l’intrus et les particules environnantes dans le sens de l'écoulement.Des simulations par éléments discrets reproduisant la configuration expérimentale avec des parois latérales, restituent avec succès les tendances observées expérimentalement. Quantitativement, les simulations donnent des durées de ségrégation plus courtes et des pentes de trajectoires plus faibles, attribuées à une vitesse granulaire plus élevée dans le sens de l'écoulement, bien que les coefficients de dérive restent cohérents avec les expériences.Des simulations sans parois latérales ont été réalisées pour éliminer les effets de bord et fournir un cadre plus général pour l’étude de la ségrégation par taille. Les comparaisons avec les simulations avec parois latérales, réalisées avec le même forçage fluide, montrent que les parois ralentissent l’écoulement granulaire. Par conséquent, les durées de ségrégation et les pentes des trajectoires spatiales sont plus faibles en l’absence de parois. Des considérations cinématiques montrent que la vitesse de l’intrus dans le sens de l'écoulement correspond à celle des particules du lit situées plus bas, à une distance qui augmente linéairement avec le ratio de taille et converge vers environ du rayon de l’intrus. Sur cette base, un modèle plus général du coefficient de dérive a été développé. Une analyse dynamique révèle d’importantes fluctuations de force, qui, une fois moyennées, montrent que le poids apparent de l’intrus est équilibré par les forces de contact, avec une faible force résiduelle agissant dans la direction de la ségrégation. La pression granulaire et les contraintes de cisaillement calculées à la surface de l’intrus atteignent un maximum au niveau de son quart inférieur aval, avec une magnitude augmentant linéairement avec le ratio de taille
Towards home-made electricity ?
No full textAccording to the most pessimistic scenarios, Tunisia's energy demand could triple by 2050. This projection is pushing the development of new technologies to support local electricity production
Connecting the Deep Collection of Sinking Particles With Surface Ocean Signatures
No full textInternational audienceA major pathway in the biological carbon pump is the gravitational sinking of organic particles from the sunlit ocean (0–200 m) to the deep ocean. Variability in particle fluxes measured by sediment traps is often attributed to variability in primary production in the surface ocean. However, particle fluxes are also influenced by physical processes such as mesoscale eddies and fronts. In this study, we assess the impact of upper‐ocean dynamical structures on the variability of particle collection in the deep ocean. This is achieved by forward tracking the trajectories of 51.9 million virtual particles that were homogeneously released at a depth of 200 m with a constant sinking velocity of 50 m in the Northeast Atlantic basin. We found that, despite a homogeneous particle source without biological effects, purely dynamical changes can induce heterogeneity in particle density and origin at depth. The position of sediment traps can thus significantly influence the weekly to seasonal particle collection in the deep ocean. Additionally, we identify and characterize nine particle clusters using a machine‐learning approach. The results show that the seasonality of particle collection at depth can be induced by seasonal variations in upper‐ocean flow structures. Clusters associated with eddy and frontal structures are found to intermittently contribute more than 50% of the particle amount during winter and spring, with smaller secondary peaks in the summer months. This study highlights the connection between mesoscale ocean dynamics and the spatio‐temporal pattern of conservative (non‐biological) particle collection in the deep ocean
On the dynamics of leapfrogging vortex rings
No full textThe evolution of highly concentrated vorticity around rings in the three-dimensional axisymmetric Euler equations is studied in a regime for which the leapfrogging dynamics predicted by Helmholtz is expected to occur. We provide in this paper the first result deriving this phenomenon for a general class of initial data in the suitable regime. The singular interaction of rings requires significant improvements of weak and strong localization estimates obtained in prior works. Our method is based on the combination of a new variational argument and a recently introduced double iterative procedure