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

Finite Charges from the Bulk Action

Constructing charges in the covariant phase formalism often leads to formally divergent expressions, even when the fields satisfy physically acceptable fall-off conditions. These expressions can be rendered finite by corner ambiguities in the definition of the presymplectic potential, which in some cases may be motivated by arguments involving boundary Lagrangians. We show that the necessary corner terms are already present in the variation of the bulk action and can be extracted in a straightforward way. Once these corner terms are included in the presymplectic potential, charges derived from an associated codimension-2 form are automatically finite. We illustrate the procedure with examples in two and three dimensions, working in Bondi gauge and obtaining integrable charges. As a by-product, actions are derived for these theories that admit a well-defined variational principle when the fields satisfy boundary conditions on a timelike surface with corners. An interesting feature of our analysis is that the fields are not required to be fully on-shell.Comment: 51 page

From colloidal dispersions to colloidal pastesthrough solid–liquid separation processes

Solid–liquid separation is an operation that starts with a dispersion of solid particles in a liquid and removes some of the liquid from the particles, producing a concentrated solid paste and a clean liquid phase. It is similar to thermodynamic processes where pressure is applied to a system in order to reduce its volume. In dispersions, the resistance to this osmotic compression depends on interactions between the dispersed particles. The first part of this work deals with dispersions of repelling particles, which are either silica nanoparticles or synthetic clay platelets, dispersed in aqueous solutions. In these conditions, each particle is surrounded by an ionic layer, which repels other ionic layers. This results in a structure with strong short-range order. At high particle volume fractions, the overlap of ionic layers generates large osmotic pressures; these pressures may be calculated, through the cell model, as the cost of reducing the volume of each cell. The variation of osmotic pressure with volume fraction is the equation of state of the dispersion. The second part of this work deals with dispersions of aggregated particles, which are silica nanoparticles, dispersed in water and flocculated by multivalent cations. This produces large bushy aggregates, with fractal structures that are maintained through interparticle surface– surface bonds. As the paste is submitted to osmotic pressures, small relative displacements of the aggregated particles lead to structural collapse. The final structure is made of a dense skeleton immersed in a nearly homogeneous matrix of aggregated particles. The variation of osmotic resistance with volume fraction is the compression law of the paste; it may be calculated through a numerical model that takes into account the noncentral interparticle forces. According to this model, the response of aggregated pastes to applied stress may be controlled through the manipulation of interparticle adhesion

Trade and frictional unemployment in the global economy

We develop a multi-country, multi-sector trade model with labor market frictions and equilibrium unemployment. Trade opening leads to a reduction in unemployment if it raises real wages and reallocates labor towards sectors with lower-than-average labor market frictions. We estimate sector-specific labor market frictions and trade elasticities using employment data from 25 OECD countries and worldwide trade data. We then quantify the potential unemployment and real wage effects of implementing the Transatlantic Trade and Investment Partnership (TTIP) or the Trans-Pacific Partnership (TPP), and of eliminating trade imbalances worldwide The unemployment and real wage effects work in conflicting directions for some countries under some trade regimes, such as the US under TTIP. We introduce a welfare criterion that accounts for both effects and splits such ties. Accordingly, US welfare is predicted to decrease under TTIP and increase under TPP

On the assessment of situational intrinsic and extrinsic motivation : the Situational Motivation Scale (SIMS).

The purpose of the present research was to develop and validate a situational (or state) measure of motivation, the Situational Motivation Scale (SIMS). The SIMS is designed to assess the constructs of intrinsic motivation, identified regulation, external regulation, and amotivation (E. L. Deci & R. M. Ryan, 1985, 1991) in field and laboratory settings. Five studies were conducted to develop and validate the SIMS. Overall, results show that the SIMS is composed of 4 internally consistent factors. The construct validity of the scale is also supported by correlations with other constructs as postulated by current theories. Moreover, the SIMS is responsive to experimental induction as evidenced by data gathered through a laboratory study. In sum, the SIMS represents a brief and versatile self-report measure of situational intrinsic motivation, identified regulation, external regulation, and amotivation

Efficient hepatitis C virus particle formation requires diacylglycerol acyltransferase-1.

Hepatitis C virus (HCV) infection is closely tied to the lipid metabolism of liver cells. Here we identify the triglyceride-synthesizing enzyme diacylglycerol acyltransferase-1 (DGAT1) as a key host factor for HCV infection. DGAT1 interacts with the viral nucleocapsid core and is required for the trafficking of core to lipid droplets. Inhibition of DGAT1 activity or RNAi-mediated knockdown of DGAT1 severely impairs infectious virion production, implicating DGAT1 as a new target for antiviral therapy

Building SSPs for climate policy analysis: a scenario elicitation methodology to map the space of possible future challenges to mitigation and adaptation

International audienceThe scientific community is now developing a new set of scenarios, referred to as Shared Socio-economic Pathways (SSPs) that will be contrasted along two axes: challenges to mitigation, and challenges to adaptation. This paper proposes a methodology to develop SSPs with a "backwards" approach based on (i) an a priori identification of potential drivers of mitigation and adaptation challenges; (ii) a modelling exercise to transform these drivers into a large set of scenarios; (iii) an a posteriori selection of a few SSPs among these scenarios using statistical cluster-finding algorithms. This backwards approach could help inform the development of SSPs to ensure the storylines focus on the driving forces most relevant to distinguishing between the SSPs. In this illustrative analysis, we find that energy sobriety, equity and convergence prove most important towards explaining future difference in challenges to adaptation and mitigation. The results also demonstrate the difficulty in finding explanatory drivers for a middle scenario (SSP2). We argue that methodologies such as that used here are useful for broad questions such as the definition of SSPs, and could also be applied to any specific decisions faced by decision-makers in the field of climate change

Use of accelerometry, kinematics and surface electromyography for equine locomotion analysis

Several complementary techniques are used to study the horse’s locomotion. Only a rational combination of these techniques can provide a global and objective approach to the analysis of movement. This paper describes the different methods used in equine locomotion studies : kinetics, kinematics and electromyography. Their advantages and their conditions of use are presented in detail. The combined use of accelerometry, joint kinematics and surface electromyography is then described .This approach requires an evaluation of potential synchronisation errors and their correction. Any conclusion on the exact role of a muscle in the locomotion must take into account the physiology of muscle contraction and the electromechanic delay. Finally, the effects of extending the stride on the hind limbs is described as an example of the potential applications of such methods in the investigation of athletic locomotion. This example shows that velocity changes as small as 0.5 m/s result in significant variations in the period and intensity of muscle activity. Even a small increase in velocity alters the limb mechanics. This study shows how the hind limb generates the velocity increase and adapts to it.L'étude de la locomotion du cheval fait appel à plusieurs techniques complémentaires. Seule leur utilisation conjointe et raisonnée permet une approche globale et objective du mouvement. Ce travail propose tout d'abord une revue des différentes méthodes d'étude de la locomotion employées chez le cheval : cinétique, cinématique et électromyographie. Il présente les apports de chacune et leurs conditions de mise en oeuvre. L'association de l'accélérométrie, de la cinématique articulaire et de l'électromyographie de surface est ensuite détaillée. Cette démarche nécessite d'évaluer et de corriger les éventuelles erreurs de synchronisation. Il faut également considérer la physiologie de la contraction musculaire et le délai électromécanique avant de conclure quant à l'intervention exacte d'un muscle dans la locomotion. Enfin, les applications potentielles de telles méthodes en terme d'étude du geste sportif sont illustrées à partir d'un exemple : l'étude de l'effet des allongements d'allure sur le membre pelvien. Elle montre que des changements de vitesse de l'ordre de 0,5 m.s-1 induisent des variations significatives des périodes et de l'intensité de l'activité musculaire. Une augmentation de la vitesse, même de faible amplitude, entraîne des modifications de l'ensemble de la mécanique des membres. Cette approche permet de comprendre comment le membre pelvien génère l'accroissement de la vitesse et s'y adapte