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Gradient-free optimization via integration
No full textWe develop and analyse an approach to optimize functions not assumed to be convex, differentiable or even continuous. The algorithm belongs to the class of model-based search methods. The idea is to fit recursively to a parametric family of distributions, using a Bayesian update followed by a reprojection back onto the chosen family. Remarkably, reprojection in our scenario boils down to computing expectations, which can be simply approximated through Monte Carlo. We show that when the family of distributions is appropriately chosen this approach can be interpreted as an implicit time-inhomogeneous gradient descent algorithm on a sequence of smoothed approximations of , providing a route to establishing convergence. We establish new results for generic inhomogeneous gradient descent algorithms, which we specialise to the model-based search algorithm in the Gaussian scenario. We illustrate the performance of the algorithm on a challenging classification task in machine learning
3D finite element investigation of hyperelastic foam behavior. I. Voronoi closed-cell microstructures
No full textInternational audienceThe mechanical behavior of isotropic, elastomeric, Classical Voronoi closedcell foams is investigated numerically using 3D finite element simulations under finite-strain uniaxial compression, and infinitesimal-strain shear and hydrostatic loading conditions. Foam microstructures with porosities ranging from 77% to 95% and narrow pore volume distributions were generated by subtracting irregular polyhedra, obtained from Voronoi tessellations of well dispersed seed points, from a cubic matrix. Severe finite-strain uniaxial compression, up to 95% nominal strain, was simulated using the Abaqus/Explicit solver. Parametric studies revealed that the compressive response of the foams scales proportionally with the factor (1 -c) 2 , where c denotes the porosity. Moreover, at high porosities (typically 90%), the influence of the base polymer's hyperelasticity reduces to its initial shear modulus µ 0 , since the matrix undergoes only limited strain, whereas the effect of the polymer's large-strain behavior becomes significant at lower porosities (typically 77%). As a result, all stress-strain curves collapse onto a single master curve when the stress is normalized by µ 0 (1 -c) 2 . The effect of internal gas pressure within the closed cells was also assessed numerically and compared against experimental data, validating the simple analytical model proposed by Gibson and Ashby (1997). At small strains, the shear modulus is accurately predicted by the Differential Hollow Sphere Assemblage (DHSA) originally developed for spherical voids. However, the same model was found to overestimate the bulk modulus, and a phenomenological correction is therefore proposed to enable rapid and more accurate estimation of this quantity.</div
Diurnal Temperature Variations and Migrating Thermal Tides in the Martian Lower Atmosphere Observed by the Emirates Mars InfraRed Spectrometer
No full textInternational audienceAbstract The Martian atmosphere experiences large diurnal variations due to the ∼24.6 hr planetary rotation and its low heat capacity. Understanding such variations on a planetary scale is limited due to the lack of observations, which are greatly addressed with the recent advent of the Emirates Mars Mission (EMM). As a result of its unique high‐altitude orbit, instruments onboard are capable of obtaining a full geographic and local time coverage of the Martian atmosphere every 9–10 Martian days, approximately ∼5° in solar longitude ( L S ). This enables investigations of the diurnal variation of the current climate on Mars on a planetary scale without significant local time (LT) gaps or confusions from correlated seasonal variations. Here, we present the results of diurnal temperature variations and thermal tides in the Martian atmosphere using temperature profiles retrieved from the Emirates Mars InfraRed Spectrometer (EMIRS) observations. The data during the primary mission is included, covering an entire Martian Year (MY) starting from MY 36 L S = 49°. The diurnal temperature patterns suggest a dominant diurnal tide in most seasons, while the semi‐diurnal tide presents a similar amplitude near the perihelion. The seasonal variation of the diurnal tide latitudinal distribution is well explained by the total vorticity due to zonal wind, while that of the semi‐diurnal tide following both dust and water ice clouds, and the ter‐diurnal tide following only dust. Comparison with the updated Mars Planetary Climate Model (PCM, version 6) suggests improvements in simulating the dust and water cycles, as well as their radiative processes
Hommage à Peter Lax (1926-2025)
No full textNational audiencePeter David Lax est décédé le 16 mai 2025 à l'âge de 99 ans. C'est une figure historique des mathématiques et du calcul scientifique qui nous a quittés après une vie extrêmement riche et féconde au cours de laquelle il a posé les bases mathématiques de la mécanique des fluides compressibles, du calcul numérique des ondes de choc, de la théorie du scattering, des systèmes intégrables, des solitons et de l'analyse numérique des équations aux dérivées partielles. Son influence a été et est toujours immense, aussi bien en mathématiques que dans d'autres disciplines comme, par exemple, la mécanique des fluides numérique. Après une première section qui donnera quelques repères biographiques et tracera son portrait, les trois sections suivantes essaieront de montrer sur quelques exemples la profondeur de ses idées et leur impact durable qui façonnent les mathématiques appliquées aujourd'hui encore
Integrating Prediction of Precipitation and Hydrology for Early Actions: The InPRHA Project within the World Weather Research Programme
No full textInternational audienceAbstract Despite advancements in science and technology, flood prediction and preparedness remain challenging due to uncertainties in forecasting atmospheric and hydrologic processes, limited real-time data, and communication barriers. The Integrating Prediction of Precipitation and Hydrology for Early Actions (InPRHA) project, a 5-yr initiative under the WMO’s World Weather Research Programme, is the first to bring together meteorology, hydrology, and social sciences within a steering committee to address these challenges. Building on knowledge from the High Impact Weather (HiWeather) project, InPRHA focuses on multihazard flood forecasting across the entire warning value chain from minutes to days, in a rapidly changing world. A key emphasis is understanding flood predictability and how uncertainties cascade through forecasting systems and are perceived, communicated, and acted upon by diverse stakeholders. This includes bridging research and operations, examining socioeconomic, cultural, and environmental challenges that influence risk perception and response. We propose key scientific questions across seven themes that address critical gaps in integrating predictions along the flood warning value chain. Addressing these gaps requires collaboration across disciplines and agencies. The project is structured into four work packages: DEFINE (identifying challenges), CONSTRUCT (gathering case studies), EXPERIMENT (scientific evaluations), and ENGAGE (community collaboration). Research will span rural, urban, and underdeveloped regions as well as countries with established warning systems, ensuring broad applicability. We invite scientists and practitioners from meteorology, hydrology, hydraulics, impacts, communication, human behavior, and economics to collaborate. By integrating disciplines and fostering transdisciplinary research, InPRHA aims to advance the science and practice of flood forecasting and early warnings to better protect vulnerable communities at risk
Stationary wave dynamics in Venus's upper clouds. Morphology and forcing from Akatsuki/LIR and Venus planetary climate model analyses
No full textInternational audienceStationary waves play a crucial role in vertically transporting momentum and energy in Venus’s atmosphere. Their global contributions (∼-0.1 m s−1 day−1 at the upper cloud) are smaller than those of planetary-scale waves and meridional circulation (∼±1.0 m s−1 day−1), but stationary waves exert strong regional control, shaping the longitudinal structure of the super-rotating flow above highlands. Observations have linked wave signatures near the cloud top (∼70 km) to underlying highland regions. However, their vertical propagation characteristics and contributions to the morphology of super-rotation remain poorly understood.Aims. This study aims to characterize the structure, variability, and propagation of stationary waves in Venus’s atmosphere and to evaluate their role in modulating the longitudinal structure of cloud-top super-rotation.Methods. We analyzed eight years of thermal emission data from Akatsuki/LIR to isolate stationary wave components. Simulations were performed using the high-resolution Venus planetary climate model, which incorporates a realistic topography and a hybrid vertical coordinate system.Results. Stationary wave signatures in the brightness temperature and horizontal winds are consistently observed and simulated above highland regions, with a notable local time dependence and long-term variability. The vertical transport of angular momentum and heat dominates the wave-induced momentum and energy budget, leading to zonal wind deceleration and adiabatic heating in the upper cloud layer. Despite filtering by two weak static stability layers in the deep atmosphere, stationary waves can propagate upward and impact cloud-top dynamics.Conclusions. Stationary waves exert a measurable influence on Venus’s upper-cloud super-rotation by vertically redistributing momentum and heat in longitude. Their effects are modulated by both vertical static stability and diurnal variations. These results highlight the crucial role of stationary waves in maintaining the observed longitudinal structure of the super-rotating atmosphere
Rapid emergence of a maths gender gap in first grade
No full textInternational audiencePreventing gender disparities in mathematics is a worldwide preoccupation. In infancy and early childhood, boys and girls exhibit similar core knowledge of number and space. Gender disparities in maths are, therefore, thought to primarily reflect an internalization of the sociocultural stereotype that ‘girls are bad at maths’. However, where, when and how widely this stereotype becomes entrenched remains uncertain. Here, we report the results of a 4-year longitudinal assessment of language and mathematical performance of all French first and second graders (2,653,082 children). Boys and girls exhibited very similar maths scores upon school entry, but a gender gap in favour of boys became highly significant after 4 months of schooling and reached an effect size of about 0.20 after 1 year. These findings were repeated each year and varied only slightly across family, class or school type and socio-economic level. Although schooling correlated with age, exploiting the near-orthogonal variations indicated that the gender gap increased with schooling rather than with age. These findings point to the first year of school as the time and place where a maths gender gap emerges in favour of boys, thus helping focus the search for solutions and interventions.Prévenir les disparités de genre en mathématiques est une préoccupation mondiale. Durant la petite enfance, filles et garçons présentent des connaissances fondamentales similaires des nombres et de l’espace. Les disparités de genre en mathématiques sont donc généralement interprétées comme le reflet de l’intériorisation progressive du stéréotype socioculturel selon lequel « les filles sont mauvaises en maths ». Cependant, le moment, le lieu et l’ampleur de l’ancrage de ce stéréotype restent incertains.Ici, nous présentons les résultats d’une étude longitudinale menée sur 4 ans, portant sur la performance linguistique et mathématique de l’ensemble des élèves français de CP et de CE1 (2 653 082 enfants). À leur entrée à l’école, garçons et filles obtenaient des scores très similaires en maths, mais un écart en faveur des garçons devenait hautement significatif après seulement 4 mois de scolarité, pour atteindre un effet d’environ 0,20 après un an.Ces résultats se sont répétés chaque année, avec des variations très faibles selon la famille, la classe, le type d’école ou le niveau socio-économique. Bien que la scolarisation soit corrélée à l’âge, l’analyse des variations quasi orthogonales montre que l’écart entre les genres augmente avec la scolarité plutôt qu’avec l’âge.Ces conclusions désignent donc la première année d’école comme le moment et le lieu où émerge un écart de genre en mathématiques en faveur des garçons, ce qui permet de mieux cibler les pistes de solutions et les interventions
Tropics-wide intraseasonal oscillations
No full textInternational audienceThe tropical climate variability is characterized by various oscillations across a range of timescales. Oscillations that imprint the tropical mean state are generally attributed to slow processes, such as the seasonal cycle or interannual variability. Here, we identify a pronounced tropics-wide intraseasonal oscillation (TWISO) in satellite observations and reanalyses. This oscillation, with a period of 30 to 60 d, is evident across multiple variables and involves interactions between convection, radiation, surface fluxes, and large-scale circulation. It is primarily manifested as convective perturbations in the tropical Indo-Pacific warm pool accompanied by oscillations in the large-scale tropical overturning circulation. Here, we examine the relationship between TWISO, the Madden–Julian Oscillation (MJO), and the instability of radiative-convective equilibrium. Certain phases of TWISO coincide with specific phases of the MJO, suggesting a potential connection between the two. However, although the MJO can amplify the oscillation amplitude of TWISO, it is not essential for TWISO to occur. Finally, due to its broad manifestation across the tropics, TWISO potentially exerts widespread influence on tropical weather and climate at regional scales
Role of the Junction Voltage on the Overflow Current in Light-Emitting Diodes
No full textInternational audienceQuantum-well (QW) based light-emitters, such as light-emitting diodes (LEDs) and lasers, of various semiconductor materials experience a reduction in their efficiency when operating at higher temperatures, a phenomenon referred to as "thermal droop". Amongst the various claims on the origins of thermal droop, an increased overflow current with increasing temperatures is a common contender. Since overflow of carriers can only occur when the junction voltage, VJunction, approaches the built-in voltage, VBI of any diodes, we develop a simple method relating the difference between VJunction and VBI to approximate the upper limit of overflow occurring in light-emitting QW-based diodes. The measured difference between VJunction and VBI of state-of-the-art commercial blue and green InGaN-based LEDs at various temperatures up to ~450 K suggests negligible overflow is occurring. To further experimentally verify that there is no overflow, we perform temperature dependent electron emission spectroscopy on the same commercial blue and green LEDs and find no evidence of thermally enhanced overflow carriers up to ~450 K. In agreement with our claims that VJunction must approach VBI for overflow to occur, two-dimensional temperature dependent electrical simulations of violet, blue, and green LEDs including alloy disorder and V-defects demonstrate that overflow can be significant in violet LEDs, where the small band offset between the InGaN QW and GaN cladding layers due to the larger QW bandgap requires larger VJunction to reach standard operating current densities, thereby approaching VBI. On the other hand, simulations indicate that overflow is negligible in blue and green LEDs, whose smaller QW bandgaps result in smaller quasi-Fermi levels difference to reach significant carrier injection, resulting in a VJunction much smaller than VBI up to large operating current densities. Considering that overflow is negligible in blue and longer-wavelength LEDs, and our observations of the large thermal droop occurring at low current densities, where Shockley-Read-Hall (SRH) recombination dominates, we conclude that thermally enhanced SRH processes are the most significant contributor to thermal droop. Finally, we also simulate the carrier densities in the different QWs of a multiple QW LED and observe a reduction in the total carrier density at a given operating current density which results in a decrease in the total Auger-Meitner current of the LED from just the thermally enhanced carrier redistribution amongst QWs without taking any possible additional temperature dependence of their recombination coefficients. Taking all this into account, minimizing thermal droop effects in LEDs can be achieved by a reduction in defect density, using wider band gap p-n junction defining cladding layers, and operating at higher currents.</div
Integrating Aggregated Electric Vehicle Flexibilities in Unit Commitment Models using Submodular Optimization
No full textThe Unit Commitment (UC) problem consists in controlling a large fleet of heterogeneous electricity production units in order to minimize the total production cost while satisfying consumer demand. Electric Vehicles (EVs) are used as a source of flexibility and are often aggregated for problem tractability. We develop a new approach to integrate EV flexibilities in the UC problem and exploit the generalized polymatroid structure of aggregated flexibilities of a large population of users to develop an exact optimization algorithm, combining a cutting-plane approach and submodular optimization. We show in particular that the UC can be solved exactly in a time which scales linearly, up to a logarithmic factor, in the number of EV users when each production unit is subject to convex constraints. We illustrate our approach by solving a real instance of a long-term UC problem, combining open-source data of the European grid (European Resource Adequacy Assessment project) and data originating from a survey of user behavior of the French EV fleet.</div