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Sampling metastable systems using collective variables and Jarzynski-Crooks paths
No full textInternational audienceWe consider the problem of sampling a high dimensional multimodal target probability measure. We assume that a good proposal kernel to move only a subset of the degrees of freedoms (also known as collective variables) is known a priori. This proposal kernel can for example be built using normalizing flows. We show how to extend the move from the collective variable space to the full space and how to implement an accept-reject step in order to get a reversible chain with respect to a target probability measure. The accept-reject step does not require to know the marginal of the original measure in the collective variable (namely to know the free energy). The obtained algorithm admits several variants, some of them being very close to methods which have been proposed previously in the literature. We show how the obtained acceptance ratio can be expressed in terms of the work which appears in the Jarzynski-Crooks equality, at least for some variants. Numerical illustrations demonstrate the efficiency of the approach on various simple test cases, and allow us to compare the variants of the algorithm
Climate model downscaling in central Asia: a dynamical and a neural network approach
No full textInternational audienceAbstract. High-resolution climate projections are essential for estimating future climate change impacts. Statistical and dynamical downscaling methods, or a hybrid of both, are commonly employed to generate input datasets for impact modelling. In this study, we employ COSMO-CLM (CCLM) version 6.0, a regional climate model, to explore the benefits of dynamically downscaling a general circulation model (GCM) from the Coupled Model Intercomparison Project Phase 6 (CMIP6), focusing on climate change projections for central Asia (CA). The CCLM, at 0.22° horizontal resolution, is driven by the MPI-ESM1-2-HR GCM (at 1° spatial resolution) for the historical period of 1985–2014 and the projection period of 2019–2100 under three Shared Socioeconomic Pathways (SSPs), namely the SSP1-2.6, SSP3-7.0, and SSP5-8.5 scenarios. Using the Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS) gridded observation dataset as a reference, we evaluate the performance of CCLM driven by ERA-Interim reanalysis over the historical period. The added value of CCLM, compared to its driving GCM, is evident over mountainous areas in CA, which are at a higher risk of extreme precipitation events. The mean absolute error and bias of climatological precipitation (mm d−1) are reduced by 5 mm d−1 for summer and 3 mm d−1 for annual values. For winter, there was no error reduction achieved. However, the frequency of extreme precipitation values improved in the CCLM simulations. Additionally, we employ CCLM to refine future climate projections. We present high-resolution maps of heavy precipitation changes based on CCLM and compare them with the CMIP6 GCM ensemble. Our analysis indicates an increase in the intensity and frequency of heavy precipitation events over CA areas already at risk of extreme climatic events by the end of the century. The number of days with precipitation exceeding 20 mm increases by more than 90 by the end of the century, compared to the historical reference period, under the SSP3-7.0 and SSP5-8.5 scenarios. The annual 99th percentile of total precipitation increases by more than 9 mm d−1 over mountainous areas of central Asia by the end of the century, relative to the 1985–2014 reference period, under the SSP3-7.0 and SSP5-8.5 scenarios. Finally, we train a convolutional neural network (CNN) to map a GCM simulation to its dynamically downscaled CCLM counterpart. The CNN successfully emulates the GCM–CCLM model chain over large areas of CA but shows reduced skill when applied to a different GCM–CCLM model chain. The scientific community interested in downscaling CMIP6 models could use our downscaling data, and the CNN architecture offers an alternative to traditional dynamical and statistical methods
New ways of collecting urban freight traffic data and applications for urban freight policies and research
No full textInternational audienceThe current quality of data on urban logistics traffic patterns is poor. Data are scarce, heterogeneous, often old and expensive to collect. This research looks at new ways to collect urban freight data and make that data available to both policy-makers and academia. Three case studies are examined: Rotterdam, the Netherlands; Brussels, Belgium; and Barcelona, Spain. They help identify new sources of data from traffic and parking policies: automated number plate recognition cameras, an on-street delivery smartphone app, and a truck pricing scheme. Both the context in which new data sources of interest to urban logistics are produced, and the level of appropriation of these data by local stakeholders and the research community are examined. We then construct an analysis grid to evaluate the diversity of systems (technical and socio-technical) producing data relevant to urban logistics. This analysis framework can be used to facilitate comparisons between data sources and thus help future case studies. In the three cases, we found that local governments use the data more than researchers do. The issue of data dissemination raises questions about costs (who is to bear the cost of storing, maintaining and extracting data sets?) as well as regulatory questions. We conclude that local administrations should be encouraged to work towards harmonizing data collection methods and data sharing conditions in order to improve the quality and comparability of the data. Cities in countries with little access to new sources of data, such as France for regulatory reasons, should try to find ways to create data source opportunities. In all countries, urban freight research communities should be encouraged to use the now increasingly available dat
Murissement accéléré par effet de confinement dans un milieu poreux
No full textInternational audienceThe coarsening process is driven by the reduction in interfacial energy density, resulting in an enlargement of the size scale within two-phase systems. It plays a crucial role in the stability and the aging of densely packed arrangements of particles, which are commonly found in alloys, foams, emulsions, supersaturated solutions, ... It has been shown previously that, when properly confined, such particles undergoes the so-called anti-coarsening phenomenon, ultimately leading to the one particle-per-pore configuration associated with the arrest of coarsening. In this study, we quantitatively explore the coarsening dynamics of liquid foam confined within the pores of spherical bead packings, where the size of the beads has been adjusted to modify the degree of confinement. We observe that, apart from the final configuration associated with the arrest of coarsening, the more we confine this system within small spaces, the more rapidly it coarsens. Moreover, the broadening of the particle size distribution has an opposite effect with respect to that observed for unconfined systems. This unexpected behavior is shown to arise from the coupling between the liquid trapped at the surface of the grains and the effective liquid volume fraction within the core of the pores.Le processus de murissement est induit par la réduction de la densité d'énergie interfaciale, ce qui entraîne une augmentation de l'échelle de taille dans les systèmes biphasés. Il joue un rôle crucial dans la stabilité et le vieillissement des arrangements denses de particules, que l'on trouve couramment dans les alliages, les mousses, les émulsions, les solutions sursaturées, etc. Il a été précédemment démontré que, lorsqu'elles sont correctement confinées, ces particules subissent le phénomène dit "d’anti-murissement", conduisant finalement à une configuration d'une particule par pore, associée à l'arrêt du processus. Dans cette étude, nous explorons quantitativement la dynamique de murissement d'une mousse liquide confinée dans les pores d'un empilement de grains sphériques, où la taille des grains a été ajustée pour modifier le degré de confinement. Nous observons que, mis à part la configuration finale associée à l'arrêt du murissement, plus ce système est confiné dans de petits espaces, plus le murissement se produit rapidement. De plus, l'élargissement de la distribution de taille des particules a un effet opposé à celui observé pour les systèmes non confinés. Ce comportement inattendu résulte du couplage entre le liquide piégé à la surface des grains et la fraction volumique effective de liquide au cœur des pores
Losing on the home front? Battlefield casualties, media, and public support for foreign interventions
No full textInternational audienceHow domestic constituents respond to signals of weakness in foreign wars remains an important question in international relations. This paper studies the impact of battlefield casualties and media coverage on public demand for war termination. To identify the effect of troop fatalities, we leverage the timing of survey collection across respondents from nine members of the International Security Assistance Force in Afghanistan. Quasi‐experimental evidence demonstrates that battlefield casualties increase the news coverage of Afghanistan and the public demand for withdrawal. Evidence from a survey experiment replicates the main results. To shed light on the media mechanism, we leverage a news pressure design and find that major sporting matches occurring around the time of battlefield casualties drive down subsequent coverage, and significantly weaken the effect of casualties on support for war termination. These results highlight the role that media play in shaping public support for foreign military interventions
The Pearl of the Empire? Private Capital and Concession Rubber in Indochina, 1910-1945
No full textIn this paper, we provide an estimate of "colonial returns" in the French Empire, using a case study on Indochinese rubber plantations between 1910 and 1945. While French colonial capitalism is often distinguished from British and Dutch cases as purely mercantilist and speculative, we show that capital investments and profits were sustained and long-lasting for a number of firms in this context. Relying on an exhaustive list of listed and non-listed companies, along with capital stock, equity prices, surfaces, tonnage, and labor usage, we explain how rubber became, in less than three decades, the colony's primary crop in export value. In doing so, this paper makes three contributions to the economic history of late colonialism. First, we provide a comparison to recent studies of "colonial returns" in South East Asia: as a late-comer to the industry, Indochina benefited from innovations implemented elsewhere, and remained insulated from global limitations on production during the 1920s and 1930s, along with a rise in the global demand. Second, we show that the main driver of capital flows was a new iteration of the concession regime -the mise en valeur -, which provided firms a lenient access to land and unfree, contract labor in exchange for strict equity and cultivation clauses. As a result, Indochinese plantations had much larger surfaces than elsewhere and a more limited share of smallholder production, but also experienced less speculation than in other parts of the Empire. Third, we show that much of the gains happened after 1935, following a massive support from the French government during the Great Depression and a shift of exports from France to the U.S. These gains persisted well into the war, following a wave of concentration at the benefit of a small number of firms; pointing to the long, postcolonial legacies of colonial capital
The Heterogeneous Effects of Government Spending: It’s All About Taxes
No full textInternational audienceHistorically, large changes in U.S. government spending induced fiscal efforts that were not all alike, with some using more progressive taxes than others. We develop a heterogeneous-agent New Keynesian model to analyse how the distribution of taxes across households shapes spending multipliers. The model yields empirically realistic distributions in marginal propensities to consume and labour elasticities, which result in lower responsiveness to tax changes for higher-income earners. In turn, multipliers are larger when spending is financed with higher tax progressivity—that is, when the tax burden falls more heavily on higher-income earners. This result is historically material. We estimate that, on average, tax rates increased more for top-income than for bottom-income earners after a spending shock. Thus, the typical U.S. spending shock was financed with higher tax progressivity. We further exploit the historical variation in the financing of spending to estimate progressivity-dependent multipliers, which we find consistent with the model
A priori and a posteriori analysis of the discontinuous Galerkin approximation of the time-harmonic Maxwell's equations under minimal regularity assumptions
No full textInternational audienceWe derive a priori and a posteriori error estimates for the discontinuous Galerkin (dG) approximation of the time-harmonic Maxwell's equations. Specifically, we consider an interior penalty dG method, and establish error estimates that are valid under minimal regularity assumptions and involving constants that do not depend on the frequency for sufficiently fine meshes. The key result of our a priori error analysis is that the dG solution is asymptotically optimal in an augmented energy norm that contains the dG stabilization. Specifically, up to a constant that tends to one as the mesh is refined, the dG solution is as accurate as the best approximation in the same norm. The main insight is that the quantities controlling the smallness of the mesh size are essentially those already appearing in the conforming setting. We also show that for fine meshes, the inf-sup stability constant is as good as the continuous one up to a factor two. Concerning the a posteriori analysis, we consider a residual-based error estimator under the assumption of piecewise constant material properties. We derive a global upper bound and local lower bounds on the error with constants that (i) only depend on the shape-regularity of the mesh if it is sufficiently refined and (ii) are independent of the stabilization bilinear form
The Making of Civic Virtues: A School-Based Experiment in Three Countries
No full textInternational audienceThis paper shows that schools can foster the transmission of civic virtues by helping students to develop concrete, democratically chosen, collective projects. We draw on a RCT implemented in 200 middle schools in three countries. The program leads students to conduct citizenship projects in their communities under the supervision of teachers trained in the intervention. The intervention caused a decline in absenteeism and disciplinary sanctions at school, alongside improved academic achievement. It also led students to diversify their friendship network. The program has stronger effects when implemented by teachers who are initially more involved in the life of the school
Torrential rainfall with severe flooding associated with a baroclinic disturbance on November 17, 2023, United Arab Emirates (UAE)
No full textInternational audienceThis work examines a severe weather event caused by a baroclinic disturbance with heavy rainfall and thunderstorms, which struck parts of the UAE and caused major flooding on November 17, 2023. A low-pressure trough extending from the Red Sea Trough (RST) towards the Eastern Mediterranean (EM) led to extreme flooding. The unique intensity of this heavy rainfall was correlated to midlatitude disturbance amplification even in the middle and upper tropospheric produced by RST. During the rain event, two intense moisture sources were injected into the region: one from the Indian Ocean, carried by southeasterly near-surface winds at 10m, and the other from the Red Sea and Equatorial Africa, transported at a mid-tropospheric level. A significant temperature gradient with ~8 °C difference in surface temperatures, particularly between the northern areas in contrast with the southern regions, and the wind shear formation over the northern parts of the study area initiated the baroclinicity structure in the borderlines of the cold front and thunderstorms. The findings also revealed an abnormal westerly jet stream intensification at 200 hPa, associated with a negative meridional wind anomaly, signaling the stretching of a Rossby wave over the study area during the heavy rainfall and flooding.</div