Data assimilation experiments using diffusive back-and-forth nudging for the NEMO ocean model

International audienceThe diffusive back-and-forth nudging (DBFN) is an easy-to-implement iterative data assimilation method based on the well-known nudging method. It consists of a sequence of forward and backward model integrations, within a given time window, both of them using a feedback term to the observations. Therefore, in the DBFN, the nudging asymptotic behaviour is translated into an infinite number of iterations within a bounded time domain. In this method, the backward integration is carried out thanks to what is called backward model, which is basically the forward model with reversed time step sign. To maintain numeral stability, the diffusion terms also have their sign reversed, giving a dif-fusive character to the algorithm. In this article the DBFN performance to control a primitive equation ocean model is investigated. In this kind of model non-resolved scales are modelled by diffusion operators which dissipate energy that cascade from large to small scales. Thus, in this article, the DBFN approximations and their consequences for the data assimilation system setup are analysed. Our main result is that the DBFN may provide results which are comparable to those produced by a 4Dvar implementation with a much simpler implementation and a shorter CPU time for convergence. The conducted sensitivity tests show that the 4Dvar profits of long assimilation windows to propagate surface information downwards, and that for the DBFN, it is worth using short assimilation windows to reduce the impact of diffusion-induced errors. Moreover, the DBFN is less sensitive to the first guess than the 4Dvar

Oscillatory wave-induced boundary layer flow over a rippled bed

The aim of this project is to study the wave-induced flow above a rippled bed. Sea bed ripples do not have a major impact on the main flow pattern, but they strongly influence the structure of the boundary layer. Therefore they are of interest in coastal engineering as they are part of the sediment transport process and have influence on surface wave dissipation and flow friction in coastal areas. By both numerical and experimental means, the flow characteristics were studied. A two-dimensional computer model solving this particular flow was developed and com­pared to other numerical and experimental data and the available theory. Experiments were conducted in a wave tank fitted with a rigid rippled bed, and flow visualizations were done using fluorescent dye filmed by a digital video camera. Different flow regimes were identified and were classified in terms of relevant parameters such as the ripple slope, the wave orbital amplitude to ripple wavelength ratio and the Taylor number. For a weak flow oscillation to ripple wavelength ratio, two-dimensional structures tend to develop in the form of recirculation cells. For a stronger flow oscillation and a medium to steep ripple slope, the flow separates giving rise to vortices ejected from the ripple crests every half-wave period. Three-dimensional instabilities also appear in two different forms. The most common form observed is a structure of &quot;rings&quot;. The other form called brick-pattern is suspected to involve centrifugal instabilities and play a major role in the building of three-dimensional ripple shapes. A final summary of the flow characteristics was done, using the numerical and experimental results.</p

La modélisation océanique: un outil privilégié mais sensible de l'océanographie moderne

Les travaux présentés ici abordent différentes thématiques de recherche relatives à l'océanographie, en particulier la description de la dynamique océanique à différentes échelles. L'angle choisi pour présenter ces travaux porte sur les atouts et limitations de la modélisation numérique océanique, un outil incontournable de l'océanographie moderne pour simuler la circulation océanique. Ces aspects sont illustrés en se focalisant principalement sur les travaux de recherche menés au cours de ma carrière. Une première partie est centrée sur l'assimilation de données (AD) dans les modèles océaniques physiques et couplés physique-biogéochimiques. Un résultat original de mise en œuvre de filtre de Kalman (SEEK) via une procédure IAU (Incremental Analysis Update) est montré puis un exemple d'assimilation de données physiques et biogéochimiques pour un modèle couplé de l'Atlantique Nord est présenté. Les résultats montrés mettent en avant d'une part l'aspect incontournable de l'utilisation de l'AD en océanographie pour l'amélioration de la compréhension des processus, et d'autre part la complexité de manipulation de ces configurations « modèle – système d'AD » de par leur très grand nombre de degré de liberté et de possibilité de paramétrisation. Une seconde partie est focalisée sur les points forts et les limites de la modélisation océanique à haute résolution (HR). Certains atouts de ce type de configuration à HR sont montrés tels que la capacité à mieux représenter les instabilités dynamiques à méso-échelle, le guidage bathymétrique, les échanges côte-large et à être exploitable pour de nombreuses applications sociétales (échouage de méduse, suivi de polluant, etc...). Puis quelques limitations connues sont évoquées telles que les problèmes d'adéquation avec les forçages (océaniques et atmosphériques) et les limites de validité de certaines hypothèses physiques habituellement codées dans ces modèles classiques (hypothèse hydrostatique, paramétrisation des flux air-mer). Une dernière partie vise à discuter les perspectives de travail dans les domaines évoqués telles que les méthodes innovantes d'AD, les modèles à très HR et leur couplage avec des modèles d'atmosphère ou de prévision de vagues ainsi que l'intérêt croissant de ce type de modèle pour les applications sociétales

Wave-induced boundary layer flows over a flat and rippled bed

International audienceThe wave-induced boundary layer flow over a rippled bed is studied. A two-dimensional model aiming to solve the flow in the vicinity of the bed has been developed. First the model is tested for a flat bed case and comparison is made with experimental data obtained by laser Doppler velocimetry (LDV) in two wave flumes presenting different characteristics. As the numerical and experimental results are compared, particular attention is given to the mean Eulerian velocity profiles calculated from the two sources and the theoretical solution from Longuet-Higgins [Philos. Trans. Roy. Soc. Lond. A 245 (1953) 535]. Discrepancies are explained and the use of the available theory is discussed. The numerical model is then adapted to a rippled bed case and compared to experimental data obtained by LDV in a wave flume fitted with a rippled bed. The dynamical consistency of the model is tested and the appearance of a vortex created at each half-wave cycle is shown. Mean velocity profiles are plotted at different locations along the ripple profile. Different types of profile are obtained depending on the location. Mean velocity profiles from the model are studied for other flow and ripple conditions and it is shown that the profiles located at mid-distance between the crest and the trough always featured negative overshoot amplitude while the profiles located above the crest and the trough could be of either sign. An example of flow visualizations is also shown and qualitative comparisons are made with the numerical model run for a similar case

Assessment of the coastal dynamics in a nested zoom and feedback on the boundary current: the North-Western Mediterranean case

International audienceThe Northern Current (hereafter NC), the major current in the North-Western Mediterranean (hereafter NWM) basin, has been largely investigated in the litterature for its mesoscale features. Its behaviour in the Var region can strongly condition the downstream flow along the Gulf of Lions shelf and Spain coast, making this zone a key area. However, the sub-mesoscale dynamics of the area and its potential impacts on the rest of the flow are not well known. This work reveals the potential interest of better simulating high-resolution dynamics in a restricted area and how this could improve the circulation representation in a larger area. To address this question, a very high resolution configuration (1/192∘) nested in an already existing high-resolution configuration (1/64∘) has been developed, using the NEMO model. Comparisons with observations show that the very high-resolution changes only weakly the mean NC characteristics but can significantly modify individual mesoscale events such as eddies and meanders occurring in the zoomed area. Furthermore, the coastal dynamics and episodic intrusions of a NC secondary branch inside a semi-enclosed bay appear to be significantly enhanced. In a second stage, the assessment of the feedback of this improved dynamics on the regional mesoscale dynamics is shown, this being allowed by the two-way coupling option of the embedded configuration using AGRIF

Étude de la dynamique du Courant Nord au large de Toulon, à l'aide de modèle, observations in-situ et données satellites

L objectif général de cette thèse est de contribuer à l avancement de la connaissance de la variabilité du Courant Nord Méditerranéen (CN) et de ses interactions avec la dynamique côtière, en s appuyant principalement sur une configuration numérique réaliste à haute résolution de la façade méditerranéenne française, basée sur le modèle de circulation océanique NEMO et nommée GLAZUR64.La validation de cette configuration avec toutes les observations disponibles sur la période d'étude (CTD, gliders, ARGO, radar HF, ADCP, altimétrie et SST satellite) a permis d'évaluer le réalisme des simulations et leur paramétrisation, et de montrer l'apport de la haute-résolution par rapport aux configurations de bassin au 1/12 utilisées aux frontières de GLAZUR64.Enfin, l'utilisation d'un forçage océanique opérationnel a permis d'utiliser une simulation en complément des données d'une campagne en mer, pour l'étude ciblée d'un tourbillon anticyclonique associé à un méandre du CN au large de Toulon, en avril 2011 [Guihou et al., 2013].The main objective of this work is to improve our knowledge of the Northern Mediterrranean Current (NC) variability and its interactions with coastal dynamics, using high-resolution modelling of the North-Western Mediterranean Sea, based on the NEMO code and nammed GLAZUR64.The configuration has been validated using all available observations for the period of study (CTD, gliders, ARGO, HF radar, ADCP, satellite altimetry and SST), in order to assess the realism of the simulations and their parameterisations. The contribution of the high-resolution for the simulation of the mesoscale dynamics, compared to 1/12 configurations used at the boundaries is shown.Finally, the use of an operationnal oceanic boundary forcing allowed us to use a combined data-model approach to study the generation and advection of an anticyclonic eddy trapped at the coast and associated to a NC meander, flowing off Toulon in April 2011 [Guihou et al., 2013].TOULON-Bibliotheque electronique (830629901) / SudocSudocFranceF

Strong intrusions of the Northern Mediterranean Current on the eastern Gulf of Lion: insights from in-situ observations and high resolution numerical modelling

International audienceThe Northern Mediterranean Current is the return branch of the cyclonic circulation of the northwestern Mediterranean Sea. Because of geostrophic constraints, this warm and oligotrophic current is forced to flow westward along the continental slope of the Gulf of Lion. But, occasionally, it penetrates on the shelf and strongly impacts the local biogeochemistry and in turn the primary production. By combining in situ observations and high-resolution modelling, it is shown that intrusions on the eastern part of the gulf are mainly forced by easterly or northwesterly wind events, through physical mechanisms that are very different in nature. Easterlies induce a piling of water along the Gulf of Lion coast that drives, through geostrophy, an alongshore shelf-intruding current. This intrusive current occurs independently of the stratification and is concomitant with the wind forcing. On the other hand, intrusions due to northwesterlies only occur during stratified conditions and are related to the development of upwellings along the Gulf of Lion coasts. When the upwelling develops, a northwestward alongshore pressure force balances the Coriolis force associated with the onshore flow at depth. When the winds drop, the upwelling relaxes and the onshore flow weakens. Consequently, the Coriolis force no longer counterbalances the pressure force that ultimately dominates the momentum balance, causing the displacement of the Northern Current on the Gulf of Lion shelf approximately 1 day after the wind relaxation. This time lag between the northwesterlies decrease and the intrusions permits to anticipate possible changes in the biogeochemistry of the Gulf of Lion

Impact of representativeness errors in an ocean application for the Back-and-Forth Nudging method

European Geosciences Union General Assembly 2013, 7-12 April, Vienna, AustriaThe goal of data assimilation (DA) is to combine, in the “best” possible way, all available sources of information of a given system to reconstruct its state. The appropriate weight given to observations and models is a function of their respective errors. In the specific case of oceanographic applications, one significant source of errors is discretization as, for instance, it truncates the portion of the spectrum being explicitly resolved. Spectral differences between the observations and the model solutions represent a challenge in data assimilation because: i) The statistical properties of the representativeness errors need to be estimated, and ii) in a nonlinear system, the realism of the resolved spectra depends on the validity of the physical parameterization of the effects of the non-resolved processes. Finding appropriate strategies to deal with these two issues remains one of the major present efforts in ocean data assimilation. In this work, we investigate the problem of assimilating data coming from an eddy resolving ocean simulation into an eddy permitting model.With such a setting, the ocean model cannot be considered as perfect because the observations contain information of a spectral band not accounted by the model, and also because it should be expected that the resolved spectral band slightly differs between the data and the model. Therefore, this experimental setting allows the presence of both errors of representativeness of observations and model errors due to unsatisfactory parameterization of diffusive/viscous processes. This study has been conducted using an idealized simulation of a subtropical double gyre circulation (SQB gyre) using the NEMO model (Madec and the NEMO team, 2008). The observations are derived from a high resolution configuration (1/12º), while the assimilative model has a coarser (1/4º) resolution. The most significant mean difference between both free solutions is the southward displacement of the convergence zone observed in the 1/12º simulation. This difference is probably induced by interactions between small and large scales, better resolved in the higher resolution model. Similar modeling issues were reported for correctly positioning the Brazil-Malvinas confluence and the Gulf Stream separation from the coast. In both cases the differences found between model and data could be reduced thanks to an increase of the model resolution. The data assimilation method used in our experiments is the Back and Forth Nudging (BFN, Auroux and Blum, 2005), that avoids the perfect model hypothesis by adding an additional term into the prognostic equation in order to relax the model towards the observations. We investigate the ability to reconstruct the observed time series at each observation location and correct the position of the frontal structures as well as the time persistence once the nudging term is removed with respect to the spatial and temporal observations sampling (Jason-2, and SWOT). Lastly, the advantage of the back and forth iterations is assessed by comparing the results with the pure straight nudging method for which only the forward model is usedPeer Reviewe

Steady and unsteady flow in wave-induced boundary layers

International audienc

Modeling the transport and accumulation of floating marine debris in the Mediterranean basin

In the era of plastic and global environmental issues, when large garbage patches have been observed in the main oceanic basins, this work is the first attempt to explore the possibility that similar permanent accumulation structures may exist in the Mediterranean Sea. The questions addressed in this work are:can the general circulation, with its sub-basins scale gyres and mesoscale instabilities, foster the concentration of floating items in some regions? Where are the more likely coastal zones impacted from open ocean sources?Multi-annual simulations of advected surface passive debris depict the Tyrrhenian Sea, the north-western Mediterranean sub-basin and the Gulf of Sirte as possible retention areas. The western Mediterranean coasts present very low coastal impact, while the coastal strip from Tunisia to Syria appears as the favourite destination. No permanent structure able to retain floating items in the long-term were found, as the basin circulation variability brings sufficient anomalies