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See detailEnsemble smoother for optimizing tidal boundary conditions and bottom roughness by assimilation of High-Frequency Radar surface currents
Barth, Alexander ULg; Alvera Azcarate, Aïda ULg; Staneva, J. et al

Conference (2009, September)

High-Frequency (HF) radars measure the ocean currents at various spatial and temporal scales. These include tidal currents, wind-driven circulation, density-driven circulation and Stokes drift. Sequential ... [more ▼]

High-Frequency (HF) radars measure the ocean currents at various spatial and temporal scales. These include tidal currents, wind-driven circulation, density-driven circulation and Stokes drift. Sequential assimilation methods updating the model state have been proven successful to correct the density-driven currents by assimilation of observations such as sea surface height, sea surface temperature and in-situ profiles. However, the situation is different for tides in coastal models since these are not generated within the domain, but are rather propagated inside the domain through the boundary conditions. For improving the modeled tidal variability it is therefore not sufficient to update the model state via data assimilation without updating the boundary conditions. The optimization of boundary conditions to match observations inside the domain is traditionally achieved through variational assimilation methods. In this work we present an ensemble smoother to improve the tidal boundary values so that the model represents more closely the observed currents. To create an ensemble of dynamically realistic boundary conditions, a cost function is formulated which is directly related to the probability of each perturbation. This cost function ensures that the perturbations are spatially smooth and that the structure of the perturbations satisfies approximately the harmonic linearized shallow water equations. Based on those perturbations an ensemble simulation is carried out using the full three-dimension General Estuarine Ocean Model (GETM). Optimized boundary values are obtained using all observations within the assimilation period using the covariances of the ensemble simulation. [less ▲]

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See detailEnsemble smoother for optimizing tidal boundary conditions and wind forcing by assimilation of High-Frequency Radar surface currents measurements of the German Bight
Barth, Alexander ULg; Alvera Azcarate, Aïda ULg; Staneva, Joanna et al

Conference (2010)

An ensemble smoother scheme is presented to assimilate HF radar surface currents to improve tidal boundary conditions and wind forcings of a circulation model of the German Bight. To create an ensemble of ... [more ▼]

An ensemble smoother scheme is presented to assimilate HF radar surface currents to improve tidal boundary conditions and wind forcings of a circulation model of the German Bight. To create an ensemble of dynamically realistic tidal boundary conditions, a cost function is formulated which is directly related to the probability of each perturbation. This cost function ensures that the perturbations are spatially smooth and that the structure of the perturbations satisfies approximately the harmonic linearized shallow water equations. Based on those perturbations an ensemble simulation is carried out using the full three-dimensional General Estuarine Ocean Model (GETM). Optimized boundary values are obtained using all observations within the assimilation period using the covariances of the ensemble simulation. The approach acts like a smoother scheme since all observations are taken into account. Since the scheme aims to derive the optimal perturbation, it might be called Ensemble Perturbation Smoother. The final analysis is obtained by rerunning the model using the optimal perturbation to the boundary conditions. The analyzed model solution satisfies thus the model equations exactly and does not suffer from spurious adjustments often observed with sequential assimilation schemes. Model results are also compared to independent tide gage data. The assimilation did also reduce the model error compared to those sea level observations. The same scheme has also been used to correct surface winds. Surface winds are crucial for accurately modeling the marine circulation in coastal waters. The method is validated directly by comparing the analyzed wind speed to in situ measurements and indirectly by assessing the impact of the corrected winds on sea surface temperature (SST) relative to satellite SST. [less ▲]

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See detailEnsemble-based assimilation of high-frequency radar surface currents in regional ocean models
Barth, Alexander ULg; Alvera Azcarate, Aïda ULg; Beckers, Jean-Marie ULg et al

Conference (2010)

The results of coastal ocean models depend critically on the accuracy of boundary and initial conditions and atmospheric forcing. The precision of coastal ocean models is limited among others by ... [more ▼]

The results of coastal ocean models depend critically on the accuracy of boundary and initial conditions and atmospheric forcing. The precision of coastal ocean models is limited among others by uncertainty in those forcing fields. Since high-frequency (HF) radar installations provide measurements over a relatively large area, the assimilation of these data has a high potential to reduce the errors in ocean models and to provide a dynamically consistent estimation of the ocean circulation. The assimilation of HF radar data is not without its own challenges: the spatial variation of the surface currents uncertainty, the high temporal resolution of HF radar data, the simultaneous presence of a wide range of processes with distinct spatial and temporal scales (tides and other surface gravity waves, mesoscale and wind-driven circulation), and the generally strong sensitivity of regional models to errors in the boundary conditions and atmospheric forcings. These processess are important aspects to consider in the application of data assimilation methods to HF radar measurements. The results of two data assimilation experiments on the West Florida Shelf (WFS) and the German Bight are presented. HF radar currents are assimilated in a nested West Florida Shelf based on an ensemble of model realizations with different wind forcings. The model is sequentially updated and a filter is implemented to reduce spurious surface-gravity waves. Results of the WFS model assimilating surface currents show an improvement of the model currents not only at the surface but also at depth compared to independent ADCP observations. This West Florida Shelf assimilation experiment does not include tides. Tides are not generated within the domain, but are rather propagated inside the domain through the boundary conditions. The potential of using HF radar data to reduce errors in tidal boundary conditions is shown in a model setup of the German Bight. For improving the modeled tidal variability it is not sufficient to update the model state without updating the boundary conditions. An ensemble smoother to improve the tidal boundary values is presented and validated with independent HF radar measurements and tide-gage data. The ensemble-scheme is also applied to improve the wind forcing by assimilation of surface currents. The improvement of the analyzed wind forcing is assessed by using in-situ wind measurements. [less ▲]

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See detailLes ensembles de données comptables centralisés: un exemple d'analyse, un débat sur leur aide potentielle en gestion de PME
De Bruyn, Christian; Corhay, Albert ULg

in Wilkin, Jacques; Dubois, Daniel (Eds.) L'informatique de gestion à la portée des P.M.E. (1981)

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See detailEnsembles of extremely randomized trees and some generic applications
Wehenkel, Louis ULg; Ernst, Damien ULg; Geurts, Pierre ULg

in Proceedings of Robust Methods for Power System State Estimation and Load Forecasting (2006)

In this paper we present a new tree-based ensemble method called “Extra-Trees”. This algorithm averages predictions of trees obtained by partitioning the inputspace with randomly generated splits, leading ... [more ▼]

In this paper we present a new tree-based ensemble method called “Extra-Trees”. This algorithm averages predictions of trees obtained by partitioning the inputspace with randomly generated splits, leading to significant improvements of precision, and various algorithmic advantages, in particular reduced computational complexity and scalability. We also discuss two generic applications of this algorithm, namely for time-series classification and for the automatic inference of near-optimal sequential decision policies from experimental data. [less ▲]

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See detailEnsembles on Random Patches
Louppe, Gilles ULg; Geurts, Pierre ULg

in Machine Learning and Knowledge Discovery in Databases (2012)

In this paper, we consider supervised learning under the assumption that the available memory is small compared to the dataset size. This general framework is relevant in the context of big data ... [more ▼]

In this paper, we consider supervised learning under the assumption that the available memory is small compared to the dataset size. This general framework is relevant in the context of big data, distributed databases and embedded systems. We investigate a very simple, yet effective, ensemble framework that builds each individual model of the ensemble from a random patch of data obtained by drawing random subsets of both instances and features from the whole dataset. We carry out an extensive and systematic evaluation of this method on 29 datasets, using decision tree-based estimators. With respect to popular ensemble methods, these experiments show that the proposed method provides on par performance in terms of accuracy while simultaneously lowering the memory needs, and attains significantly better performance when memory is severely constrained. [less ▲]

Detailed reference viewed: 325 (60 ULg)
See detailEnsembles reconnaissables de rationnels
Charlier, Emilie ULg

Scientific conference (2012, April)

In this talk I will introduce the recognizable sets of rationals, following the formalism of Jeffrey Shallit. I will point out several open questions regarding this topic.

Detailed reference viewed: 6 (0 ULg)
See detailLa ensenanza de la geografia
Merenne-Schoumaker, Bernadette ULg

in Hiernaux, Daniel; Lindon, Alicia (Eds.) Tratado de Geografia Humana (2007)

Cet article inséré dans un manuel universitaire mexicain cherchant à dresser un bilan de la situation de la géographie humaine aujourd'hui se veut une réflexion générale de ce qu’est et devrait être ... [more ▼]

Cet article inséré dans un manuel universitaire mexicain cherchant à dresser un bilan de la situation de la géographie humaine aujourd'hui se veut une réflexion générale de ce qu’est et devrait être l’enseignement de la géographie au niveau du secondaire et ce au départ de près de 30 ans de recherche en didactique de la géographie. Cette réflexion s’articule autour de quatre questions fondamentales : quels objectifs ? quelles acquisitions de base ? comment enseigner ? comment évaluer les acquis ? [less ▲]

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See detailLa ensenanza del espanol asistida por ordenador
Denooz, Joseph ULg

in Cable, revista de didactica del espanol como lengua extranjera (1988), 2

Detailed reference viewed: 24 (2 ULg)
See detailEnsilage de maïs plante entière: Comparaison de 4 variétés. I. Production, composition et digestibilité
Gielen, Marc ULg; Dufrasne, Isabelle ULg; Van Eenaeme, Christian ULg et al

in Revue de l’Agriculture = Landbouwtijdschrift (1990), 43

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See detailL’ensoleillement et la structure commerciale des places publiques au centre de Liège
Cornélis, Bernard ULg

in Publications de l'Association Internationale de Climatologie (1999), 11

For many years, shop location has been studied at different scales from a socio-economical point of view. Sunshine, on the other hand, is a local phenomenon well-known especially for climatologists and ... [more ▼]

For many years, shop location has been studied at different scales from a socio-economical point of view. Sunshine, on the other hand, is a local phenomenon well-known especially for climatologists and architects. This case-based contribution shows the relationships existing between sunshine and shop location on squares. [less ▲]

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See detailEnt-Trachyloban-3beta-ol, a new cytotoxic diterpene frm Croton zambesicus
Block, Sebastien; Stevigny, Caroline; Gillet, Marie-Claire ULg et al

in Planta Medica (2002), 68

The dichloromethane extract of leaves of Croton Zambesicus (Euphorbiaceae) showing in vitro cytotoxicity against huma cervix carcinoma cells was investigated in order to identify its active compounds. A ... [more ▼]

The dichloromethane extract of leaves of Croton Zambesicus (Euphorbiaceae) showing in vitro cytotoxicity against huma cervix carcinoma cells was investigated in order to identify its active compounds. A bio-guided fractionation by HSCCC followed by MPLC led us to isolate a trachylobane diterpene, ent-trachyloban-3beta-ol, with cytotoxic properties (IC50 on HeLa cells = 7.3 ùg/ml). This is the first report on the cytotoxicity of a trachylobane diterpene. [less ▲]

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See detailEntanglement and Localization of Wavefunctions
Giraud, Olivier; Georgeot, Bertrand; Martin, John ULg

in Complex Phenomena in Nanoscale Systems, NATO Science for Peace and Security Series B: Physics and Biophysics, Volume . ISBN 978-90-481-3118-1. Springer Netherlands, 2009, p. 51 (2009)

We review recent works that relate entanglement of random vectors to their localization properties. In particular, the linear entropy is related by a simple expression to the inverse participation ratio ... [more ▼]

We review recent works that relate entanglement of random vectors to their localization properties. In particular, the linear entropy is related by a simple expression to the inverse participation ratio, while next orders of the entropy of entanglement contain information about e.g. the multifractal exponents. Numerical simulations show that these results can account for the entanglement present in wavefunctions of physical systems. [less ▲]

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See detailEntanglement equivalence of N-qubit symmetric states
Mathonet, Pierre ULg; Krins, Stéphanie ULg; Godefroid, M. et al

in Physical Review. A (2010), 81

We study the interconversion of multipartite symmetric N-qubit states under stochastic local operations and classical communication (SLOCC). We demonstrate that if two symmetric states can be connected ... [more ▼]

We study the interconversion of multipartite symmetric N-qubit states under stochastic local operations and classical communication (SLOCC). We demonstrate that if two symmetric states can be connected with a nonsymmetric invertible local operation (ILO), then they belong necessarily to the separable, W, or Greenberger-Horne-Zeilinger (GHZ) entanglement class, establishing a practical method of discriminating subsets of entanglement classes. Furthermore, we prove that there always exists a symmetric ILO connecting any pair of symmetric N-qubit states equivalent under SLOCC, simplifying the requirements for experimental implementations of local interconversion of those states. [less ▲]

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See detailEntanglement of localized states
Giraud, O.; Martin, John ULg; Georgeot, B.

in Physical Review. A (2007), 76

We derive exact expressions for the mean value of Meyer-Wallach entanglement Q for localized random vectors drawn from various ensembles corresponding to different physical situations. For vectors ... [more ▼]

We derive exact expressions for the mean value of Meyer-Wallach entanglement Q for localized random vectors drawn from various ensembles corresponding to different physical situations. For vectors localized on a randomly chosen subset of the basis, <Q> tends for large system sizes to a constant which depends on the participation ratio, whereas for vectors localized on adjacent basis states it goes to zero as a constant over the number of qubits. Applications to many-body systems and Anderson localization are discussed. [less ▲]

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See detailEntanglement of localized states
Martin, John ULg; Giraud, Olivier; Georgeot, Bertrand

Conference (2007, June)

Detailed reference viewed: 4 (1 ULg)