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Water vapour isotope anomalies during an atmospheric river event at Dome C, East Antarctica
No full textFrom 19 to 23 December 2018, an atmospheric river sourced in the Atlantic hit the French-Italian Concordia station, located at Dome C, East Antarctic Plateau, 3 233 m above sea level. It induced a significant surface warming (+ 18 °C in 3 days), combined with high specific humidity (3 times increase in 3 days) and a strong isotopic anomaly in water vapour (+ 17 ‰ for δ18O). The isotopic composition of water vapour monitored during the event may be explained by the isotopic signature of long-range water transport, and by local moisture uptake during the event. In this study, we used continuous meteorological and isotopic water vapour observations, together with the atmospheric general circulation model LMDZ6iso, to describe this event and quantify the influence of each of these processes. The presence of mixed-phase clouds during the event induced a significant increase in downward longwave radiation, leading to high surface temperature and resulting in high turbulent mixing in the boundary layer. Although surface fluxes are underestimated in LMDZiso, near-surface temperature and specific humidity are well represented. The surface vapour δ18O is accurately simulated during the event, despite an overestimated amplitude in the diurnal cycle outside of the event. Using the LMDZ6iso simulation, we perform a surface water vapour mass budget by decomposing total specific humidity into contributions from individual processes. Our analysis shows that surface sublimation, which becomes significantly stronger during the event compared to typical diurnal cycles, is the dominant driver of the vapour δ18O signal at the peak of the event, accounting for approximately 70 % of the total contribution. The second largest contribution comes from moisture input via large-scale advection associated with the atmospheric river, accounting for approximately 30 % of the total. Consequently, the isotopic signal monitored in water vapour during this atmospheric river event reflects both long-range moisture advection and interactions between the boundary layer and the snowpack. Only specific meteorological conditions driven by the atmospheric river can explain these strong interactions. Given the pronounced imprint of air-snow exchanges on the vapour isotopic signal, improving the representation of local processes in climate models could substantially improve the simulation of the isotopic signal over Antarctica and provide valuable insight into moisture uptake processes
A Unified FPT Framework for Crossing Number Problems
No full textInternational audienceThe basic (and traditional) crossing number problem is to determine the minimum number of crossings in a topological drawing of an input graph in the plane. We develop a unified framework that smoothly captures many generalized crossing number problems, and that yields fixed-parameter tractable (FPT) algorithms for them not only in the plane but also on surfaces.Our framework takes the following form. We fix a surface S, an integer r, and a map κ from the set of topological drawings of graphs in S to ℤ_+ ∪ {∞}, satisfying some natural monotonicity conditions, but essentially describing the allowed drawings and how we want to count the crossings in them. Then deciding whether an input graph G has an allowed drawing D on S with κ(D) ≤ r can be done in time quadratic in the size of G (and exponential in other parameters). More generally, we may take as input an edge-colored graph, and distinguish crossings by the colors of the involved edges; and we may allow to perform a bounded number of edge removals and vertex splits to G before drawing it. The proof is a reduction to the embeddability of a graph on a two-dimensional simplicial complex.This framework implies, in a unified way, quadratic FPT algorithms for many topological crossing number variants established in the graph drawing community. Some of these variants already had previously published FPT algorithms, mostly relying on Courcelle’s metatheorem, but for many of those, we obtain an algorithm with a better runtime. Moreover, our framework extends, at no cost, to these crossing number variants in any fixed surface
Advanced treatment of micropollutants in wastewater: from adsorption to advanced oxidation processes including the emerging disinfectant performic acid
No full textInternational audienc
Erratum: "First absolute wind measurements in Saturn's stratosphere from ALMA observations"
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Combinatoire des algèbres de descentes et revêtements de graphes
No full textWe give a direct combinatorial proof that the product of two descent classes in a symmetric group is a sum of descent classes. The proof is based on the fact that the group product gives a covering map when descent classes are endowed with the graph structure coming from the weak order. The main geometric argument is valid for any Coxeter group, even infinite ones for which the descent algebra does not exist.On donne une preuve combinatoire directe du fait que le produit de deux classes de descentes du groupe symétrique est une somme de classes de descentes. La preuve repose sur un argument géométrique de théorie des graphes, valable dans le cadre général des groupes de Coxeter
A Scalable Nyström-Based Kernel Two-Sample Test with Permutations
No full textTwo-sample hypothesis testing-determining whether two sets of data are drawn from the same distribution-is a fundamental problem in statistics and machine learning with broad scientific applications. In the context of nonparametric testing, maximum mean discrepancy (MMD) has gained popularity as a test statistic due to its flexibility and strong theoretical foundations. However, its use in large-scale scenarios is plagued by high computational costs. In this work, we use a Nyström approximation of the MMD to design a computationally efficient and practical testing algorithm while preserving statistical guarantees. Our main result is a finite-sample bound on the power of the proposed test for distributions that are sufficiently separated with respect to the MMD. The derived separation rate matches the known minimax optimal rate in this setting. We support our findings with a series of numerical experiments, emphasizing applicability to realistic scientific data
Silhouettes and generic properties of subgroups of the modular group
No full textInternational audienceWe show that the probability for a finitely generated subgroup of the modular group, of size n , to be almost malnormal or non-parabolic, tends to 0 as n tends to infinity – where the notion of the size of a subgroup is based on a natural graph-theoretic representation of the subgroup. The proofs of these results rely on the combinatorial and asymptotic study of a natural map, which associates with any finitely generated subgroup of \mathsf{PSL}_{2}(\mathbb{Z}) a graph which we call its silhouette, which can be interpreted as a conjugacy class of free finite index subgroups of \mathsf{PSL}_{2}(\mathbb{Z})
Climate change, natural resources, and conflict
No full textInternational audienceThis paper examines how climate change and natural resource dynamics contribute to conflict, with a focus on the implications of the green transition. It reviews empirical evidence showing that extreme weather events—such as droughts, floods, and heatwaves—are linked to increased violence, particularly through economic disruptions, reduced agricultural productivity, and displacement. The analysis also explores the mechanisms through which climate shocks influence conflict, including opportunity costs, resource competition, and behavioural responses to environmental stress. The discussion then turns to the role of natural resource exploitation, especially in the context of rising demand for minerals essential to low-carbon technologies. The paper highlights how resource price and availability shocks can trigger conflict, often depending on the type of resource, extraction method, and local governance. It also addresses the overlap between climate- and resource-driven conflict risks, emphasizing that their interaction may amplify instability. Throughout, the paper identifies open research questions related to prediction, the effects of long-run environmental changes, and the design of policy responses. These include insurance schemes, climate adaptation strategies, infrastructure investment, and regulatory frameworks for resource governance. The findings point to the need for research that integrates climate and conflict dynamics, with the goal of informing policies that can mitigate the risks associated with environmental change and resource pressures
Who will work on Sunday? The winners and losers of Sunday laws relaxation
No full textInternational audienceIn 2016, a law authorized Sunday working in the retail sector in some thirty French areas. We show that the reform did not coincide with any significant increase in retail trade employment in target areas. However, the increase in the number of days shops are open has led employers to favor employees who are sufficiently experienced to manage a store independently. There has been a significant drop in the employment share of less experienced workers, as well as a sharp decline in the share of single parents, for whom it is difficult to reconcile family responsibilities and Sunday work
Required Research Steps Towards the Realization of CO2-Hydrate Geo-Capsules for Underground Mechanical Energy Storage
No full textInternational audienceRequired Research Steps Towards the Realization of CO2-Hydrate Geo-Capsules for Underground Mechanical Energy Storag