Télescope intérieur, une œuvre spatiale d’Eduardo Kac

01/07/2022 12:30

Tout comprendre (ou presque) sur le climat

17/06/2022 12:30

Patrimoine Instrumental Spatial : Enjeux et méthodes

10/06/2022 12:30

Ce que la « diversité » fait au machine learning

05/06/2023 11:00

Séminaire du département de Géosciences de l’ENS.

Voyage en Terres Australes Françaises

02/06/2023 13:00

Romane est de retour de mission lointaine.

Revealing the statistics of extreme events hidden in short weather forecast data: A case study of Sudden Stratospheric Warmings

01/06/2023 11:00

Séminaire du LMD à l’ENS.

Modeling the Dynamics of Water and CO2 Ice on the Planet Mars

21/11/2024 14:00

Mars is surrounded by a thin atmosphere composed mostly of CO₂ with a small amount of water. Every night and every winter, CO₂ and water can locally condense as frost. In the geologically recent past, when the obliquity of Mars was larger, the Martian atmosphere was enriched in water and a perennial mantle of water ice formed from the poles to the mid-latitudes. When the obliquity decreased again, these ices sublimated and got buried under a lag deposit in the Martian subsurface, while much of the atmospheric CO₂ condensed at the poles to form massive glaciers. Today, we can observe the remains of these periods via the water ice buried in the subsurface at mid-latitudes and the perennial CO₂ cap at the South Pole. This thesis aims to improve our understanding of the stability and dynamics of these ices over time, from the diurnal cycle to the million-year scale. I used space observations and, even more, numerical models such as the global climate model « Mars Planetary Climate Model » (PCM) and the new « Planetary Evolution Model » (PEM) for which I actively invested to develop new capabilities.

In a first study, by comparing the different pressure data acquired on the surface of Mars by space probes over the last 50 years, I proved that the average mass of the Martian atmosphere had not varied at these scales and therefore that the perennial CO₂ cap had a neutral mass balance. I then showed that the diurnal cycle of CO₂ frost formation in dusty areas could prevent dust from aggregating, maintaining the presence of reservoirs of mobilizable dust on the surface.

At the same time, I developed a new numerical tool for modeling microclimates on Martian slopes within the PCM and then the PEM.This tool is able to predict the presence of Martian ice on cold slopes in agreement with the detections from orbit. In a dedicated study, using observations from the THEMIS visible and thermal camera and this new model, I showed that the water frost accumulating on these slopes cannot melt in the current climate. It may eventually form brines if this frost is in contact with a significant amount of salts.

I then studied the stability of water ice in the subsurface. In particular, I proved that buried water ice at tropical latitudes under poleward slopes was not stable. The presence of such a buried water ice had previously been presented as essential to explain the absence of seasonal CO₂ ice on these slopes, because of its high thermal inertia. However, my model showed that this absence of CO₂ ice could also be explained by atmospheric heat transport, previously neglected. By adapting a new comprehensive model of exchange between permafrost and the Martian atmosphere, I revisited the effect of the water cycle and near-surface atmospheric conditions on the stability of buried ice. In parallel, we reconstructed the history of buried water ice in the mid-latitude subsurface and suggested that it may be a remnant of the last ice ages of 630,000 years ago. Finally, as for the surface frost, I showed that this buried water ice could not melt, even under the most favorable conditions we could imagine for Mars: the case where a ground flow suddenly exposes the underground ice to solar heating.

In a final study, using the new « Planetary Evolution Model », I simulated the formation of CO₂ glaciers when the entire atmosphere condenses in the polar regions during periods of low obliquity. By decreasing the obliquity from the current value of 25.2° to 15°, I showed that the mean pressure drops to a value of 250 Pa. The humidity of the atmosphere is reduced to a few precipitable microns. Massive CO₂ glaciers form on poleward slopes at high latitudes. Despite the loss of more than half of the Martian atmosphere, CO₂ remains the major component. This work constitutes a new step in our understanding of Martian paleoclimates and in particular of the geological activity induced by recent climatic variations.

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.