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

Comparison of reduced-order, sequential and variational data assimilation methods in the tropical Pacific Ocean

This paper presents a comparison of two reduced-order, sequential and variational data assimilation methods: the SEEK filter and the R-4D-Var. A hybridization of the two, combining the variational framework and the sequential evolution of covariance matrices, is also preliminarily investigated and assessed in the same experimental conditions. The comparison is performed using the twin-experiment approach on a model of the Tropical Pacific domain. The assimilated data are simulated temperature profiles at the locations of the TAO/TRITON array moorings. It is shown that, in a quasi-linear regime, both methods produce similarly good results. However the hybrid approach provides slightly better results and thus appears as potentially fruitful. In a more non-linear regime, when Tropical Instability Waves develop, the global nature of the variational approach helps control model dynamics better than the sequential approach of the SEEK filter. This aspect is probably enhanced by the context of the experiments in that there is a limited amount of assimilated data and no model error

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

Analysis of ocean-atmosphere coupling algorithms : consistency and stability

International audienceThis paper is focused on the numerical and computational issues associated to ocean-atmosphere coupling. It is shown that usual coupling methods do not provide the solution to the correct problem, but to an approaching one since they are equivalent to performing one single iteration of an iterative coupling method. The stability analysis of these ad-hoc methods is presented, and we motivate and propose the adaptation of a Schwarz domain decomposition method to ocean-atmosphere coupling to obtain a stable and consistent coupling method

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

Piestsie Feenstra, Les nouvelles figures mythiques du cinéma espagnol (1975-1995). À corps perdus

Piestie Feenstra enseigne en études cinématographiques à l’université Sorbonne Nouvelle-Paris 3 ; il est membre de plusieurs équipes de recherches aux Pays-Bas, en Espagne et en France, notamment du Groupe de recherches sur l’image dans le monde hispanique (grimh) et consacre ses travaux à l’étude du cinéma espagnol. Dans ce livre captivant, qui est l’adaptation de sa thèse de doctorat, l’auteure analyse l’émergence de figures mythiques dans le cinéma espagnol, après la mort de Franco en 1975..

Le ravalement du franquisme par le décor du néo-cinéma historique

La reconstruction du passé récent dans une abondante production de fictions historiques s’effectue surtout à travers le décor et les accessoires d’un récit dont le lien avec l’histoire est parfois ténu. Une certaine tendance esthétisante que l’on peut observer dans nombre de ces films, qui versent souvent dans une nostalgie ambiguë (El Crimen del cine Oriente, Tiempos de azúcar, Secretos del corazón), implique une relecture de l’histoire car le monde imaginaire et fictif ainsi actualisé peut s’imposer lentement comme référence iconique et se substituer peu à peu aux documents authentiques et aux témoignages, qui se retrouvent en décalage avec l’image dominante, dans un processus de ravalement du franquisme qui n’est pas porté par les seuls cinéastes.La reconstrucción del pasado reciente dentro de una abundante producción de ficciones históricas se realiza sobre todo mediante el decorado y el atrezzo de un relato cuyo vínculo con la historia es a veces distendido. Cierta tendencia estetizante observada en muchas de esas películas que a menudo evocan un ambiente de nostalgia ambigua (El Crimen del cine Oriente, Tiempos de azúcar, Secretos del corazón) induce una nueva lectura de la historia porque el mundo imaginario y ficticio así actualizado puede llegar a imponerse paulatinamente como referencia icónica e ir sustituyéndose a los documentos auténticos y a los testimonios que aparecen desfasados con respecto a la imagen dominante, en un proceso que revoque ción del franquismo que no es propio del cine

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