Reduced-order 4D-Var: a preconditioner for the Incremental 4D-Var data assimilation method

This study demonstrates how the incremental 4D-Var data assimilation method can be applied efficiently preconditione d in an application to an oceanographic problem. The approach consists in performing a few iterations of the reduced-order 4D-Var prior to the incremental 4D-Var in the full space in order to achieve faster convergence. An application performed in the tropical Pacific Ocean, with assimilation of TAO temperature data, shows the method to be both feasible and efficient. It allows the global cost of the assimilation to be reduced by a factor of 2 without affecting the quality of the solution

Assessment of the ocean circulation in the Azores region as predicted by a numerical model assimilating altimeter data from Topex/Poseidon and ERS-1 satellites

International audienceTwo years of altimetric data from Topex/Poseidon (October 1992-September 1994) and ERS-1 (October 1992-December 1993) were assimilated into a numerical model of the North Atlantic. The results of these simulations are analysed in the Azores region to assess the performance of our model in this particular region. Maps of instantaneous dynamic topography and transports show that the model performs well in reproducing the velocities and transports of the Azores Front. Drifter data from the Semaphore experiment are also used to study the correlation between the drifter velocities and the corresponding model velocities. Some interesting oceanographic results are also obtained by examining the seasonal and interannual variability of the circulation and the influence of bathymetry on the variability of the Azores Front. Thus, on the basis of our two year experiment, it is possible to confirm the circulation patterns proposed by previous studies regarding the seasonal variations in the origin of the Azores Current. Moreover, it is shown that the Azores Current is quite narrow in the first year of assimilation (1992-1993), but becomes much wider in the second year (1993-1994). The role of the bathymetry appears important in this area since the mesoscale activity is shown to be strongly related to the presence of topographic slopes. Finally, spectral analyses of sea-level changes over time and space are used to identify two types of wave already noticed in other studies: a wave with (300 km)-1 wave number and (120 days)-1 frequency, which is characteristic of mesoscale undulation, and a wave with (600 km)-1 wave number and (250 days)-1 frequency which probably corresponds to a Rossby wave generated in the east of the basin

Le rôle des collectivités territoriales dans l’adaptation des enjeux environnementaux globaux à l’échelle locale

Les enjeux environnementaux tels qu’ils sont définis par les politiques publiques aboutissent à des aménagements dans l’espace rural, confrontant les sociétés locales à une transformation de leur environnement immédiat. L’objectif de cet article est de montrer le processus par lequel les enjeux globaux, tels que la lutte contre le changement climatique et la protection de la biodiversité, sont convertis à une échelle locale à travers l’action des structures politiques locales (municipalité, communauté de communes, etc.). La réception de ces aménagements, déterminant leur nature, est liée à deux facteurs principaux : la structure de la société locale et le degré d’adaptabilité du projet en fonction des objectifs attribués au débat public.Cette communication repose sur un travail de terrain où différents aménagements ont été comparés (deux parcs éoliens et deux itinéraires pédestres). Dans tous les cas, il a été observé que le débat public a été organisé par les structures politiques locales pour adapter les dispositifs en fonction de la stratification de la société locale et des enjeux du pouvoir local.Environmental stakes, as defined by public policies, result in developments in rural space, confronting local societies to transformations of their close environment. The article aims at explaining the process through which global stakes, such as the struggle against climatic change and the protection of biodiversity, are converted into local stakes through the action of local political structures. Two main factors determine the perception of these stakes by the population : the structure of local societies, and the degree of adaptability of these developments.The communication is based on the comparison of different developments (two Aeolian parks and two pedestrian paths). In all cases, the public debate has been organised by local political structures to adapt the developments to the stratification of local societies and to the local power stakes

Efficient interface conditions for the coupling of ocean models

International audienceAlthough the two-way nesting is the most usual coupling method in the ocean community, it does not address the correct problem but an approximation, and does not ensure enough regularity through the interface between the two models1. The correct approach consisting in a full coupling is more difficult and expensive than the two-way nesting, since it requires to find and implement an algorithm ensuring that the solutions in each domain satisfy the regularity conditions through the interface. The global-in-time non-overlapping Schwarz algorithm is particularly well suited for such a coupling, and can lead to improved physical results. Our work thus aims at improving the ocean coupling by determining efficient interface conditions for the usual ocean equations (the so-called 3-D primitive equations). These ones are composed of advection-diffusion equations for tracers such as temperature and salinity, and dynamics equations, which can be approximated in the 2-D case by the shallow-water equations

A reduced-order strategy for 4D-Var data assimilation

This paper presents a reduced-order approach for four-dimensional variational data assimilation, based on a prior EO F analysis of a model trajectory. This method implies two main advantages: a natural model-based definition of a mul tivariate background error covariance matrix $\textbf{B}_r$, and an important decrease of the computational burden o f the method, due to the drastic reduction of the dimension of the control space. % An illustration of the feasibility and the effectiveness of this method is given in the academic framework of twin experiments for a model of the equatorial Pacific ocean. It is shown that the multivariate aspect of $\textbf{B}_r$ brings additional information which substantially improves the identification procedure. Moreover the computational cost can be decreased by one order of magnitude with regard to the full-space 4D-Var method

Coupling 3D Navier-Stokes and 1D shallow water models

International audienceThe present work addresses the problem of coupling hydrodynamical models with different spatial dimensions, which can be used in order to reduce the computational cost of river numerical models. We show that this problem can be tackled quite efficiently by designing a simple algorithm using techniques borrowed from domain decomposition theory. This algorithm is non intrusive, i.e. allows using existing numerical models with very few modifications. The method is illustrated on an academic test-case, namely a free surface flow in a bend-shaped channel. A 3-D Navier-Stokes model is coupled with a 1-D shallow water model, and results are compared to those obtained in a fully 3-D case. It is shown that the coupling algorithm provides an accurate solution, which can be improved thanks to an iterative algorithm (Schwarz method). This study is performed using the Mascaret-Telemac system

Toward a data assimilation system for NEMO

International audienceIn this note, we discuss the project that has been conceived and the first achievement steps that have been carried out to set up a data assimila-­ tion system associated to NEMO. Of specific interest here are applications to operational oceanography. This data assimilation system is sche-­ matically made of three subcomponents: Interface Components, Built-in Components and External Components. Several elements of this NEMO data assimilation system have already been developed by various groups in France and in Europe and several of them could be introduced in the system (the linear Tangent and Adjoint Model, TAM, is one of the most important of them as far as variational assimilation is concerned), some others will require specific developments. Finally, we introduce the SEABASS reference configuration that is proposed to be the NEMO data as-­ similation demonstrator and the experimentation and training platform for data assimilation activities with NEMO. These various thoughts take advantage of the advances and discussions that have been carried out by the NEMOASSIM working group

Numerical representation of internal waves propagation

Similar to surface waves propagating at the interface of two fluid of different densities (like air and water), internal waves in the oceanic interior travel along surfaces separating waters of different densities (e.g. at the thermocline). Due to their key role in the global distribution of (physical) diapycnal mixing and mass transport, proper representation of internal wave dynamics in numerical models should be considered a priority since global climate models are now configured with increasingly higher horizontal/vertical resolution. However, in most state-of-the-art oceanic models, important terms involved in the propagation of internal waves (namely the horizontal pressure gradient and horizontal divergence in the continuity equation) are generally discretized using very basic numerics (i.e. second-order approximations) in space and time. In this paper, we investigate the benefits of higher-order approximations in terms of the discrete dispersion relation (in the linear theory) on staggered and nonstaggered computational grids. A fourth-order scheme discretized on a C-grid to approximate both pressure gradient and horizontal divergence terms provides clear improvements but, unlike nonstaggered grids, prevents the use of monotonic or non- oscillatory schemes. Since our study suggests that better numerics is required, second and fourth order direct space-time algorithms are designed, thus paving the way toward the use of efficient high-order discretizations of internal gravity waves in oceanic models, while maintaining good sta- bility properties (those schemes are stable for Courant numbers smaller than 1). Finally, important results obtained at a theoretical level are illustrated at a discrete level using two-dimensional (x,z) idealized experiments

Modelling crisis management for improved action and preparedness (CRISMA): Modelling submersion on the Charente-Maritime coast

International audienceCRISMA is a Research and Development project financed by the European Community. Its objective is to develop a simulation-based decision support system, in different domains of the natural or industrial risk (flood, snowstorm, seism, forest fires, accidental pollution, urban accidents). The application in France, coordinated by ARTELIA Eau & Environnement, is devoted to the submersion risk on the Charente-Maritime coast and based on the experience feedback of the storm Xynthia in February, 2010. In this framework, a specific work is in progress through a thesis realized in collaboration between ARTELIA and lNRIA. Its objective is to elaborate a methodology of multi-model coupling which should be effective and applicable for the CRISMA project. These models may differ in several ways, related either to the physics and/or to the numeric concepts. The developed methodology may allow to taking into account more specific areas (urban zone, bridge in charge, …), but also should be able to simplify the simulation by dimension changes of model parts (for example, a 1D model for rivers, and a 2D one for sea). The present work addresses more specifically the problem of coupling models with different spatial dimensions