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

Overview of my research activities at the BOM (Australia) including Hadley circulation and flash droughts

03/01/2023 11:00

Hanh Nguyen is a climatologist from the Bureau of Meteorology in Melbourne, Australia. She will give a seminar on her research topic on the 3rd of January, 11a.m., at the ENS.

Gestion locale de l'eau dans un contexte de changement climatique

16/12/2022 14:00

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

Modelling and assimilation activities at CNRM on atmospheric composition in support to real time applications

15/12/2022 10:30

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Microphysical and optical characterization of fresh and aged combustion aerosol particles: a simulation chamber study

20/11/2024 14:00

Carbonaceous soot particles are formed during the incomplete combustion of fossil fuels, biofuels and biomass burning and are considered to contribute to a significant part of aerosol emission, especially in polluted areas. Soot particles are known to contain the light-absorbing carbon fractions of Black Carbon (BC) and Brown Carbon (BrC) making them a key species when trying to understand and estimate the interaction between aerosols and atmospheric radiation, i.e. the direct radiative effect (DRE). Current estimations of the DRE of soot and its BC and BrC components remain uncertain due to the difficulties in representing their microphysical and spectral optical properties in models. In particular, gaps persist in describing the variability of the soot optical properties at the source, due to different combustion conditions, and their change during atmospheric lifetime, due to mixing with different aerosol components. Further, differences between laboratory observations and field measurements remain and are not understood.

The present work aims to provide new measurements and descriptions of the physical, chemical, and spectral optical properties of BC- and BrC-containing soot aerosol in order to improve its representation in models. The focus of this work is set on advancing the understanding and description of the variability of the soot properties 1) at generation, from changing combustion conditions, and 2) during atmospheric ageing, in particular, due to the internal mixing with inorganic and organic compounds forming coating at the soot surface. To provide a mechanistic study of soot aerosol properties a coherent set of experiments was set up using the large atmospheric simulation chamber CESAM (French acronym for Multiphase Atmospheric Experimental Simulation Chamber) and a controllable propane-based soot generator.

The experiments were used to determine key optical parameters used in modelling and remote sensing applications like the mass absorption, extinction and scattering cross-section (MAC, MEC, MSC), the single scattering albedo (SSA), and the complex refractive index (CRI). The varying properties of the soot from different combustion conditions allowed the investigation and support of a generalized relationship between the MAC and the particle chemical composition for fresh-emitted particles. Optical calculations based on two descriptions of the particle’s morphology were performed to determine the soot’s CRI and discuss the usability and pertinence of data from different shape representations assumptions.

Soot aerosols were subjected to different simulated atmospheric ageing processes to determine the effects of ageing on their absorbing capacity and physico-chemical properties. Especially, the formation of a coating and the enhancement of the absorption due to this internal mixing were studied using two precursors and coating processes. This enabled the investigation of the relationship between absorption enhancement, particle processing, and coating thickness, relevant across the soot lifecycle.

L’hydroélectricité à l’épreuve du changement climatique : modélisation couplée des systèmes hydrologiques et électriques pour l’adaptation et l’atténuation

18/11/2024 14:00

French hydropower production is expected to undergo major changes in the coming years. On the one hand, global warming intensifies seasonal precipitation contrasts and increases evaporative demand, altering river flows and, consequently, the water resources available for hydropower plants. On the other hand, the growing integration of variable renewable energies, encouraged by CO₂ emission mitigation policies, changes the flexibility requirements of the electrical system. Finally, the management of hydropower reservoirs also depends on the evolution of other water uses.

In this thesis, we propose an integrated modeling approach to simulate and quantify these different effects and their interaction. Our approach is based on the coupling of a land surface model (LSM) and a power system optimization model (PSM), enabling us to jointly represent the constraints related to climate and the electrical system. We represent the multipurpose operation of hydropower reservoirs in the LSM and use the power generation time series simulated by the PSM as a target to guide reservoir operations. Conversely, the hydropower production constraints used in the PSM are defined based on LSM simulations.

We show that this integrated approach enhances both the representation of river flows in the climate model and of production dispatch in the PSM. Most importantly, it allows us to simulate the response of hydropower production to different scenarios of climate change and power system configurations. Regarding the future of hydropower production in France, we find a limited impact of climate change at the annual scale but more pronounced seasonal contrasts, with increased production in winter and decreased production in summer.

Besides, the integration of variable energy modifies the production pattern of reservoir power plants and increases the value of the flexibility provided by hydropower reservoirs. However, our study also highlights significant uncertainty in future production levels and electricity prices, linked to uncertainties in climate projections and the future cost of decarbonized thermal power plants.

Compréhension des processus liés à la production primaire brute pour la modélisation des surfaces continentales

20/11/2024 14:00

Les surfaces continentales constituent la composante la plus incertaine du cycle du carbone. De multiples processus contribuent en effet à la très grande variabilité spatiotemporelle du flux de production primaire brute (gross primary production, GPP), dont la photosynthèse, le transfert radiatif dans la canopée et la phénologie.

Les modèles de surfaces continentales synthétisent notre compréhension de leur fonctionnement, de l’échelle du site à celle du monde. Des données in situ et satellitaires variées nous permettent d’évaluer les variables simulées ou d’étalonner les paramètres des processus représentés, via des techniques d’assimilation de données. Je présente une douzaine d’études auxquelles j’ai contribué, d’abord dans la continuité de ma thèse sur la phénologie, pour des types de plantes fonctionnels décidus comme sempervirents. J’ai transitionné de la phénologie foliaire vers celle de la GPP, et je me suis intéressée à des proxys de la GPP, sachant qu’elle n’est plus directement mesurable à partir de l’échelle de l’écosystème. J’ai d’abord étudié la fluorescence induite par le soleil (sun-induced fluorescence, SIF), puis l’absorption de l’oxysulfure de carbone (carbonyl sulfide, COS) par la végétation. J’ai pour perspective de mieux caractériser la réponse de la GPP aux événements de stress abiotiques (sécheresses et températures élevées) qui vont s’intensifier dans le cadre du changement climatique, mettant en danger le fonctionnement des écosystèmes.

 

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Understanding processes related to gross primary production for land surface modelling Land surfaces are the most uncertain component of the carbon cycle. Multiple processes indeed contribute to the very high spatiotemporal variability of the gross primary production (GPP) flux, including photosynthesis, radiative transfer in the canopy and phenology. Land surface models synthesize our understanding of how they work, from the site scale to the global scale. Various in situ and satellite data allow us to evaluate the simulated variables or calibrate the parameters of the represented processes, via data assimilation techniques.

I present a dozen studies to which I contributed, first in the continuation of my thesis on phenology, for plant functional types both deciduous and evergreen. I transitioned from the leaf phenology to the phenology of GPP, and I became interested in proxies of GPP, knowing that GPP is not directly measurable starting from the ecosystem scale.

I first studied solar-induced fluorescence (SIF), then the uptake of carbonyl sulfide (COS) by continental vegetation. My aim for the future is to better characterize the response of GPP to abiotic stress events (droughts and high temperatures) which will intensify in the context of climate change, endangering the functioning of ecosystems.

 

The defense will be in French, with slides in English.