HAL-Ecole des Ponts ParisTech
Not a member yet
42743 research outputs found
Sort by
Beyond uniqueness: Relaxation calculus of junction conditions for coercive Hamilton-Jacobi equations
No full textInternational audienceA junction is a particular network given by the collection of half lines glued together at the origin. On such a junction, we consider evolutive Hamilton-Jacobi equations with coercive Hamiltonians. Furthermore,we consider a general desired junction condition at the origin, given by some monotone function .There is existence and uniqueness of solutions which only satisfy weakly the junction condition (at the origin, they satisfy either the desired junction condition or the PDE).We show that those solutions satisfy strongly a relaxed junction condition (that we can recognize as an effective junction condition). It is remarkable that this relaxed condition can be computed in three different but equivalent ways: 1) using viscosity inequalities, 2) using Godunov fluxes, 3) using Riemann problems.Our result goes beyond uniqueness theory, in the following sense: solutions to two different desired junction conditions and do coincide if
A year-long observational analysis of atmospheric trace gases and particulate matter in Kathmandu
No full textInternational audienceKathmandu Valley is one of the most densely populated cities in Nepal, facing significant air quality challenges. This study presents a comprehensive analysis based on twelve months of continuous air quality measurement data collected at the Khumaltar Air Quality Monitoring Station (ICIMOD AQMS), which is located in the southern part of the Kathmandu Valley. The study investigates particulate matter (PM10, PM2.5 , PMC (PM10-PM2.5)) alongside trace gases (O3, NOx, SO2, and CO), focuses on their seasonal, diurnal variations, interspecies correlations, potential sources, and the impact of regional atmospheric transport. The results reveal that the annual PM2.5 concentration (49 µg m-3) largely exceeds the WHO air quality guideline of 5 µg m -3 . Pollutant concentrations show clear seasonal variation, with peaks during the winter and pre-monsoon seasons, and a noticeable decline during the monsoon. A strong correlation (r = 0.82, p = 0.00) between PM2.5 and NOx indicates that fossil fuel combustion is a dominant source of fine particulate pollution. During the pre-monsoon season, O3 concentrations occasionally exceed 100 ppb (8-hr running average), with seasonal diurnal concentration observed close to 90 ppb in the afternoon. The study found that temperature and relative humidity significantly influence coarse PM levels, with a strong negative correlation (r = -0.89, p = 0.00) between coarse PM and the relative humidity, suggesting less resuspension under moist conditions. Furthermore, this study highlights the severe air pollution in the Kathmandu Valley throughout the year, driven primarily by local emissions, with regional atmospheric transport further contributing to poor air qualit
Boundary-layer parameterization for assessing temperature and evaporation in floating photovoltaics at the utility-scale
No full textInternational audienceA precursor model for parameterizing the effects of photovoltaic powerplants on the atmospheric boundary layer is developed using computational fluid dynamics. The method allows one to compute the surface roughness lengths, aerodynamical resistances of covered surfaces and convective heat transfer coefficients, adapted for any photovoltaic module layouts and wind directions. It has been applied for two setups: a wind tunnel system and a utility-scale floating photovoltaic installation. In these cases, the altitude-based velocity profiles was reproduced over the arrays; and we found that the turbulence generated by the photovoltaic/atmosphere interaction is greater for head-and tailwinds than sidewinds, therefore affecting the environment and the photovoltaic system. Constructing a digital twin of the floating array using large-scale meteorological fields and the parameters of the precursor model, the temperature of a monitored module was calculated and a spatial variation of 1.3 °C∕km and 5.8 °C∕km was estimated at the utility scale. Moreover, the waterbody evaporation was reduced by 40%-50% due to the photovoltaic panels blocking the vapour removal processes. This result decreased to 14%-20% when considering the flow spatial variations across the waterbody. Further research is necessary to adapt the parameterization to scenarios with low wind velocity
On the spectral radius and the characteristic polynomial of a random matrix with independent elements and a variance profile
No full textInternational audienceIn this paper, it is shown that with large probability, the spectral radius of a large non-Hermitian random matrix with a general variance profile does not exceed the square root of the spectral radius of the variance profile matrix. A minimal moment assumption is considered and sparse variance profiles are covered. Following an approach developed recently by Bordenave, Chafaï and García-Zelada, the key theorem states the asymptotic equivalence between the reverse characteristic polynomial of the random matrix at hand and a random analytic function which depends on the variance profile matrix. The result is applied to the case of a non-Hermitian random matrix with a variance profile given by a piecewise constant or a continuous non-negative function, the inhomogeneous (centered) directed Erdős-Rényi model, and more.</div
What Are Social Norms?
No full textInternational audienceMany theorists tie social norms to attitudes, such as expectations on others, perhaps along with conforming practices. Challenging this view, we instead ground social norms in a social norming process, an often non-verbal social communication process that ‘makes’ the norm through mutual expressions of support. We present the process-based account of social norms and social normativity, and distinguish social norms from social pressures, social practices, and Lewisian conventions. The process-based view brings social norms closer to legal norms, by tying them to ‘expressive acts’, just as laws and contracts arise through acts of voting or signing, not through mere attitudes
Plastic pollution in Leeward, Moorea and Cook islands (South Pacific): A baseline study
No full textInternational audiencePlastic pollution is increasingly affecting the South Pacific, including remote islands and coastal regions of Small Island Developing States (SIDS), yet data remain sparse in many areas. This baseline study presents original data on beach macrolitter and microplastics in surface waters and sediments across six islands of French Polynesia (Moorea, Bora Bora, Tahaa) and the Cook Islands (Aitutaki, Rarotonga), collected during a cruise in 2024. Beach surveys revealed high plastic contamination on most sites, with macroplastics (size >2.5 cm) densities ranging from 18 to 58 items/100 m, exceeding European threshold values (e.g. 22 items/100 m) on majority of sampled beaches. Single-use plastics and plastic fragments dominated litter, with local sources such as tourism and coastal activities clearly identifiable. Surface microplastics (315 μm–5 mm) were found at low densities, ranging from 0 to 4668 particles/km2, confirming relatively low contamination in surface waters. Only 17 particles were detected from 8 manta net tows. Sediment and beach microplastic analysis yielded very limited results, with microplastics found at only two sites (Huahine and Rarotonga), dominated by fragments and colored fibers. These results suggest localized contamination linked to urbanization and tourism rather than widespread oceanic inputs. Despite methodological limitations due to the cruise format, this study contributes rare data from undersampled regions, supporting long-term monitoring efforts and informing future policy and mitigation actions. The results underline the need to improve regional waste management, which will be beneficial for local societies largely based on touris
Governance, Productivity and Economic Development
No full textInternational audienceABSTRACT This paper explores the interplay between transfer policies, R&D, corruption, and economic development using a general equilibrium model with heterogeneous agents and a government. The government collects taxes, redistributes fiscal revenues, and undertakes public investment (in R&D, infrastructure, etc.). Corruption is modeled as a fraction of tax revenues that is siphoned off and removed from the economy. We first establish the existence of a political‐economic equilibrium. Then, using an analytically tractable framework with two private agents, we examine the effects of corruption and evaluate the impact of various policies, including redistribution and innovation‐led strategies
Demonstrating Aeolus capability to observe wind-cloud interactions
No full textInternational audienceModel based studies have shown interactions between wind vertical profiles and cloudiness, but few observational studies corroborate them. The unique observations of Aeolus spaceborne Doppler wind lidar can contribute to fill this gap. In this paper, we merged global Aeolus observations of cloud profiles at full horizontal resolution (3 km along orbit track) with co-located profiles of horizontal winds.We first observed wind-cloud interactions at regional scale over the Indian Ocean. Aeolus captures the strengthening of the Tropical Easterly Jet in early June 2020, with wind speeds exceeding 40 m s -1 in its core, and a simultaneous increase of high cloud fraction up to above 30 %, until the decay of the jet during fall.Secondly, we observed wind-cloud interactions at cloud scale (between 3-100 km) in different regions. Over the Indian Ocean as well as over cumulus and stratocumulus dominated regions, we found that the wind shear inside clouds is smaller than the wind shear in the clear sky surrounding the clouds (statistically significant). In addition, we found that the wind speed difference between the cloud and its surrounding clear sky increases with the clear sky wind shear, especially in cumulus (R = -0.94) and stratocumulus (R = -0.87) dominated regions. This study demonstrated that despite its coarse resolution, Aeolus can capture wind perturbations induced by convective motion.</div
Global transport of stratospheric aerosol produced by Ruang eruption from EarthCARE ATLID, limb-viewing satellites and ground-based lidar observations
No full textInternational audienceThe Atmospheric LIDar (ATLID) instrument of the ESA’s Earth Cloud, Aerosol and Radiation Explorer (EarthCARE) satellite mission launched in May 2024 provides high-resolution vertical profiling of aerosols and clouds at 355 nm. Fully operational since July 2024, ATLID has been witness to a significant perturbation of stratospheric aerosol budget following the eruptions of Ruang volcano (Indonesia) in late April 2024. Using ATLID together with limb-viewing satellite instruments (OMPS-LP and SAGE III), we quantify the stratospheric aerosol perturbation generated by the Ruang eruption and characterize the global transport of volcanic aerosols. To evaluate the ATLID performance in the stratosphere, its data are compared with collocated ground-based lidar observations at various locations in both hemispheres and overpass-coordinated balloon flights carrying AZOR backscatter sonde. The intercomparison with suborbital observations suggests excellent performance of ATLID in the stratosphere and proves its capacity to accurately resolve fine structures in the vertical distribution of stratospheric aerosols. Using various satellite observations, we show that Ruang’s eruptive sequence in April 2024 produced eruptive columns reaching 25 km altitude, and resulted in a doubling of the tropical stratospheric aerosol abundance for several months. The eruption timing in austral Fall and its high-altitude reach fostered efficient poleward transport into the southern extratropics during austral Winter 2024. By the time of the austral Fall 2025, the sulphate aerosols from Ruang have spread across the entire Southern hemisphere and were most probably entrained by the 2025 Antarctic polar vortex, potentially enhancing the polar stratospheric cloud occurrence