Atelier national sur les nuages polaires

24/06/2025 09:00

SIRTA / ICEO : Journée Scientifique 2025

24/06/2025 09:00

Evénement de clôture projet FAIR-EASE

12/06/2025 09:00

Se baigner à Paris ?

22/01/2024 12:00

Cycle « Les grands séminaires IPSL ».

Politique de l’adaptation, enjeu d'action publique national et international pour la France

19/01/2024 14:00

Nouvelle séance du séminaire « Changement Climatique : Sciences, Sociétés, Politique » co-organisé par le Centre Alexandre-Koyré (EHESS-CNRS) et l’ENS (CERES).

Renewables for decarbonizing the electricity sector: which efficient policies

17/01/2024 16:00

Séminaire du cours « Transition énergétique » du CERES (ENS).

Effets urbains sur l'hydroclimat : analyses observées et modélisées

13/12/2024 14:00

La gestion de l’eau urbaine évolue en réponse à l’augmentation de la population urbaine, à la demande croissante en approvisionnement en eau, au changement climatique, et aux modifications régionales de l’hydroclimat induites par la ville. Assurer une approche durable de la gestion de l’eau urbaine nécessite une compréhension approfondie de l’hydroclimat urbain. Cette thèse aborde ce besoin en étudiant les effets urbains sur les précipitations, les influences sur les propriétés des surfaces naturelles, et les interactions entre les systèmes atmosphériques et hydrologiques dans la formation de l’hydroclimat urbain. Une approche multidisciplinaire est adoptée, intégrant des connaissances des sciences atmosphériques et hydrologiques et combinant des analyses empiriques de données observées avec de la modélisation pour offrir des perspectives complémentaires.

L’étude commence par examiner les effets urbains sur les précipitations à travers une combinaison de revue de littérature et d’apprentissage automatique pour évaluer tout consensus. Étant donné le manque d’accord parmi les études utilisant des données de précipitations radar, une méthodologie basée sur le vent et utilisant des précipitations radar est appliquée à un large échantillon de villes aux États-Unis et en Europe. Cette approche vise à déterminer si les zones urbaines ont une influence constante sur les précipitations à travers les régions, abordant le défi de généraliser les effets urbains sur les précipitations. L’aspect de modélisation comprend une revue des modèles existants de surface urbaine, en se concentrant sur leur représentation des processus hydrologiques de surface et des flux d’énergie. Ceci est suivi par le développement d’un nouveau schéma de surface urbaine pour le modèle ORCHIDEE, incorporant des hétérogénéités urbaines et une représentation de l’imperméabilité.

 

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Urban water management is evolving in response to the increasing urban population, rising demand for water supply, climate change, and urban-induced changes in regional hydroclimate. Ensuring a sustainable approach to urban water management requires a deeper understanding of the urban hydroclimate. This thesis addresses this need by investigating urban effects on precipitation, influences on natural surface properties, and the interactions between atmospheric and hydrological systems in shaping the urban hydroclimate. A multidisciplinary approach is adopted, integrating insights from both atmospheric and hydrological sciences and combining empirical analyses of observed data with modeling to provide complementary perspectives.

The study begins by examining urban effects on precipitation through a combination of literature review and machine learning to assess any consensus. Given the lack of agreement among studies using radar precipitation data, a consistent wind-based radar methodology is applied to a broad sample of cities over the USA and Europe. This approach aims to determine if urban areas have a consistent influence on precipitation patterns across regions, addressing the challenge of generalizing urban effects on precipitation. The modeling aspect includes a review of existing urban land surface models, focusing on their representation of surface hydrological processes and energy fluxes. This is followed by the development of a new urban surface scheme for the ORCHIDEE model, incorporating urban heterogeneities and representation of imperviousness.

 

Laboratoires dans lesquels la thèse a été effectuée

• UMR « Milieux environnementaux, transferts et interactions dans les hydrosystèmes et les sols », METIS-IPSL
• Laboratoire Atmosphères et Observations Spatiales, LATMOS-IPSL

Representation and analysis of climate-carbon-nitrogen interactions over permafrost regions in the IPSL Earth system model

12/12/2024 14:00

Permafrost soils, found in cold regions of the globe, contain large amounts of organic carbon. These carbon stocks are threatened by the strong Arctic warming, which causes permafrost to thaw, exposing previously frozen organic matter to decomposition. This results in CO₂ and CH₄ emissions that amplify global warming through the so-called permafrost carbon-climate feedback, with implications for carbon budgets and emission reduction pathways. However, both the timing and magnitude of this feedback remain highly uncertain. In fact, the future dynamics of the permafrost carbon cycle have only been assessed using land surface models (LSMs) or models of intermediate complexity, with contrasting responses.

Among the models of the Coupled Model Intercomparison Project Phase 6 (CMIP6), launched in 2014 to better understand the responses of climate system to anthropogenic forcings, only two Earth System Models (ESMs) include permafrost carbon, and both share the same land component. This thesis sheds new light on the subject by incorporating permafrost mechanisms into another ESM.

As part of my thesis, I developed a new Earth system model, called IPSL-Perm-LandN, building on physical and biogeochemical permafrost processes originally developed at the Institut Pierre-Simon Laplace (IPSL) in the early 2010s. This involved incorporating additional features, such as soil insulation by groundcover, into a version of the land surface model that already included an explicit representation of the land nitrogen cycle.

As a first step towards improving the realism of the representation of the Arctic climate, the latent heat of soil water phase change and soil insulation by soil organic carbon and a surface organic layer (i.e. mosses, lichens, litter) are shown to strongly influence the surface air temperature and snowfall fraction. Their inclusion in the model leads to an improvement of the high-latitude climate simulations and of the thermal state of permafrost, which is consistent with observations and satellite products. IPSL-Perm-LandN is evaluated against observations and data-driven products over the historical period (1850-2014) and consistently simulates much larger permafrost soil carbon stocks than the previous version of the IPSL ESM. The permafrost region is found to be a net carbon sink in recent decades with a net land-atmosphere carbon flux consistent with the upscaling of flux measurements.

Under future increasing atmospheric CO₂ concentrations, the permafrost region remains a carbon sink in IPSL-Perm-LandN despite significant soil carbon losses caused by permafrost thaw, due to counteracting negative feedbacks. In particular, the increased nitrogen availability following permafrost thaw is found to reduce vegetation nitrogen limitation and thus to increase land carbon uptake, although this effect is likely to be overestimated. Finally, the model simulates irreversible land and ocean carbon changes under atmospheric CO₂ overshoot pathways. In aggressive mitigation scenarios, land and ocean turn into carbon sources, partially offsetting mitigation efforts and highlighting the need to minimise temperature overshoots as much as possible.

Are the ocean fine-scales affecting latent heat flux spatial variability in the Northwest Tropical Atlantic?

29/11/2024 16:00

The ocean and the atmosphere are two key components of the climate system playing an essential role in heat redistribution and storage. Heat is exchanged between these components across the air-sea interface in the form of radiative and turbulent heat fluxes. The former are associated with the presence of electromagnetic waves whose ultimate source is a radiating body at a certain temperature. The latter occur because heat is transported from one medium to the other by an imbalance between a given property and a subsequent atmospheric motion. Among the turbulent heat fluxes, we find latent heat flux which results from water evaporation heat release.

Thus, latent heat flux is tightly linked to the evaporation rate and strongly affects cloud formation, precipitation and the large-scale atmospheric and ocean circulations. In this presentation, we will assess the different mechanisms by which the ocean fine-scales affect latent heat flux variations in the Northwest Tropical Atlantic, an a priori relatively quiescent region in terms of air-sea exchanges within the trades. We will perform a multi-dataset approach involving the analysis of remote sensing and in-situ observations, the ERA5 reanalysis and high-resolution coupled simulations. We will focus on three variables: sea-surface temperature, surface currents and sea surface salinity.