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L'exploitation des données anciennes
24/11/2026 14:00
Le thème « Système Solaire » de l’IPSL vous convie à une demi-journée consacrée à l’exploitation des données anciennes le mardi 24 novembre 2026.
IPSL Masterclass: “(Paleo)Climate Modelling for experimental scientists”
14/10/2026 11:54
The masterclass is aimed at PhD students and postdocs coming from an experimental background who want to understand how a climate model works and how they might use climate model output in their own research. All career stages are welcome to attend, though priority will be given to PhD students/postdocs if capacity becomes limiting.
Retrouvez tous les séminaires.
What can AI contribute to weather forecasting ? State-of-the-art and current research projects at Météo-France
02/12/2026 14:00
Séminaire du LMD à l’ENS.
Wildfires in Fontainebleau – What difference make fires of summer 2026?
17/11/2026 11:00
Séminaire du cycle Géosciences et société du LGENS.
Retrouvez toutes les soutenances de thèses et de HDR.
New approach for the detection and quantification of carbon monoxide plumes from hyperspectral measurements from space or under HAPS
23/11/2026 09:30
Carbon monoxide (CO) is a product of incomplete combustion. It regulates the oxidising capacity of the troposphere, and it is one of the best tracers of wildfire and pollution plumes. As fires become more frequent and next-generation infrared sounders such as IASI-NG and MTG-IRS multiply the volume of data to process, the conventional processing chain becomes a bottleneck. In this chain, every interferogram is converted into a calibrated spectrum before an expensive full-physics retrieval is run. This thesis answers whether CO can instead be retrieved directly in the interferogram domain, and from how small a part of the interferogram.
The line-by-line radiative transfer and retrieval code LARA was extended to the interferogram domain. The extension includes the Fourier transform of modelled and observed spectra, an optical path difference (OPD) sampling adapted to the CO micro-window (2143-2181.5 cm-1), and a noise model derived from the instrument NEΔT. Since raw IASI interferograms are not distributed, we reconstruct them from Level-1C spectra, and the assumptions this process involves are stated explicitly. Tests on 14 941 scenes over the Atlantic Ocean show that neither the OPD sampling nor the assumed noise can be chosen freely. A factor four undersampling biases the CO column by +2.6% and removes 20% of the degrees of freedom for signal.
The first question is where the CO information lies in the interferogram, and whether it can be used without going back to a spectrum. An observing system simulation experiment (OSSE) was built with LARA on the TIGR archive, including 2311 atmospheric situations, crossed with eight CO profiles and seven surface temperatures, so that the true state is known exactly. A Jacobian analysis shows that the CO information is concentrated around the first harmonic of the ~4 cm-1 CO line spacing, near 0.25 cm OPD, and its multiples. Three algorithms of increasing complexity were then trained to map interferogram samples directly onto CO, H2O and surface temperature. The best of them, a CNN-LSTM neural network using about thirty OPD samples, retrieves these quantities to within 15%, 20% and 0.3% respectively. It saves an estimated five hours of computation per hundred-pixel scene compared with a full-physics retrieval. However, it underestimates background CO levels by about 25%, exactly where a fire plume must be told apart from the background, and this low bias still prevents operational use.
The second question is whether the result holds on real observations with a physically based retrieval. The optimal-estimation retrieval of LARA was applied to IASI on Metop-B in three configurations that share the same forward model, a priori and noise, and differ only in the measurement vector, using spectral radiances, the full interferogram (0-2 cm OPD), and a partial interferogram (0.2-0.3 cm OPD). Over more than 11 500 soundings of a North Atlantic biomass-burning plume, the full interferogram and spectral retrievals agree to within 0.34% (r = 0.9998). Both agree with the operational FORLI product (biases of -0.4% and -0.7%, r = 0.975). Over nine years (2017-2025) at two TCCON stations, Park Falls and East Trout Lake, both agree with the ground-based reference within its stated uncertainty once the difference in vertical sensitivity is accounted for. To our knowledge, this is the first multi-year validation of an interferogram-domain product against a ground-based reference. The partial interferogram holds only 8 samples, a reduction of a factor 19 of the measurement vector (compared to the full interferogram), and loses about 27% of the DOFs. Yet it gives the smallest residual bias against TCCON (+0.73% and -0.56%).
Finally, an innovative instrument concept is introduced for a potential CO mission. Based on a static Fabry-Pérot array, it can only ever measure a partial interferogram. Hence, a preliminary thermal-infrared design adapted from the NanoCarb concept is proposed for CO and optimised with a Cramér-Rao lower bound analysis. The expected CO precision is about 10% for a single pixel and about 2% after along-track co-addition. This design opens the way to a compact CO sensor for persistent surveillance of wildfire plumes from a High-Altitude Platform System (HAPS).
Glacio-hydrogeological dynamic of catchments fed by temperate glaciers through the case of South-East Iceland
25/11/2026 16:00
Since the beginning of the 21st century, glaciers worldwide have been rapidly losing mass due to anthropogenic climate change. This mass loss generates natural hazards such as floods and landslides; and jeopardizes freshwater resources and energy supply in mountainous regions.
The role of groundwater in the water cycle of catchments fed by temperate glaciers is poorly characterized, especially the water fluxes occurring between the subglacial drainage system, the aquifers, and the surface flow network; as well as the glacial meltwater repartition in between these compartments. This is explained by the diversity of research fields and methods involved, the complexity of the hydrological system, the impossibility of directly observing the subglacial drainage system, and the fact that groundwater in glacial catchments has been studied very little to date, resulting in a lack of data on these aquifers.
In this thesis, I address these issues by collecting contemporary data in all compartments of the hydrosystem, so that co-occurring processes can be distinguished. I study four outlet glaciers of Vatnajökull ice cap, South-East of Iceland.
Two main aquifer formations are present in the region, separated by an aquitard of approximately 1 meter thickness, which may be discontinuous. The tills and glacio-fluvial deposits aquifer is unconfined, has a thickness of 2 to 54 m, and a hydraulic conductivity of 1e-6 to 1e-5 m/s. The basalt aquifer is unconfined when outcropping and confined when covered by the aquitard and the tills and glacio-fluvial deposits formation. It has a thickness of 200 to 400 meters, and a hydraulic conductivity of 1e-10 to 1e-5 m/s.
The rivers are braided or torrential and isolated, so it is not possible to measure surface water flow using classic gauging stations. I use passive seismic acquisition and develop two methods to calibrate the seismic records with a few point discharge measurements. Both methods provide accurate time series of absolute water flow rate, with a relative uncertainty of 5 to 15 %.
I observe subglacial channels under the ablation zone of the glaciers through dye-tracing experiments; with cross-sectional areas of 1 to 2 m2, and subglacial water velocities of 0.16 to 0.5 m/s. In 2023, the system was channelized from June to September. I observe a contemporary inefficient drainage system in early June and a solely inefficient drainage system in late September, with a subglacial flow rate of 0.19 m/s. These results will help understand and numerically model the water fluxes occurring between the aquifers and the subglacial drainage system.
I use glacier runoff data obtained by Liebsch et al., in prep through ablation modeling; and estimate forefield runoff through soil water balance computation. I compare the total runoff to the discharge time series acquired by passive seismic monitoring. The water balance computation at catchment scale indicates that 70 % of the water flowing beneath the glacial ablation zone infiltrates into the underlying aquifers during the melting season, and a smaller proportion beneath the glacier accumulation zone. In winter, exfiltration towards rivers or the subglacial drainage system occurs on a daily time scale, but the net balance in winter is still 60 % infiltration. Over a hydrological year, 50 % of the water available below the glacier infiltrates in the ground. The hydraulic head and groundwater temperature timeseries collected in a network of boreholes shows that upward vertical flow from the basaltic aquifer towards the tills and glacio-fluvial deposits aquifer is occurring in winter.
All these results help to improve the conceptual model of water flows in catchments fed by temperate glaciers, and the complementary data collected can contribute to the calibration and validation of a global numerical model of the area.
Apport de la modélisation des transferts d’eau et de température en milieu karstique pour l’étude de l’impact du changement climatique sur le devenir des grottes ornées : application à la grotte de Villars (Dordogne, France)
19/11/2026 14:00
Le changement climatique modifie les conditions de conservation des grottes ornées, par le réchauffement de l’atmosphère et par l’augmentation attendue de l’intensité et de la fréquence de certains épisodes pluvieux extrêmes. Cette thèse évalue comment ces changements peuvent affecter la grotte ornée de Villars, en Dordogne. La réponse à cette problématique est abordée selon quatre axes complémentaires :
(1) mieux caractériser le karst du site de Villars par des méthodes géophysiques et des mesures pétrophysiques en laboratoire,
(2) modéliser le transfert d’eau dans l’épikarst,
(3) simuler le transfert thermique entre l’air et le sol situé au-dessus de la grotte et
(4) modéliser le transfert thermique dans la grotte ainsi que les processus de diagenèse associés. Les modélisations sont calibrées dans les conditions actuelles à partir de mesures de terrain. Les modèles calibrés sont ensuite utilisés pour étudier la réponse du système à différents scénarios de changement climatique.
Cette thèse a d’abord permis, sur le site de Villars, de mettre en évidence la présence de deux lithologies aux propriétés pétrophysiques distinctes, caractérisées par des contrastes de perméabilité et de porosité de fissures. La modélisation hydrologique a ensuite montré que les principales dynamiques du goutte-à-goutte d’une stalactite située dans la grotte peuvent être reproduites en combinant un modèle conceptuel de réservoir épikarstique avec un modèle distribué à double perméabilité.
Les projections climatiques indiquent une augmentation des événements extrêmes de goutte-à-goutte sous la stalactite plus marquée que celle des précipitations extrêmes qui en sont la cause.
La modélisation thermique, quant à elle, a montré que l’estimation de la température du sol au-dessus de la grotte à partir de la température de l’air dépend fortement de l’approche utilisée : un modèle d’apprentissage profond (deep learning) est plus performant à court terme, tandis que les modèles à base physique semblent plus fiables pour les projections à long terme.
Enfin, la modélisation 3D de différents phénomènes physiques (conduction, convection et rayonnement) a permis de retrouver la température mesurée en différents points de la grotte.
Les projections thermiques indiquent que les zones proches de la surface ou des entrées/sorties de la grotte auront des conditions plus favorables à l’évaporation et à la précipitation de calcite, d’où un risque d’apparition d’un voile de calcite sur les parois ornées.
Les conditions dans les zones plus profondes seront au contraire plus favorables à la condensation et au retrait de calcite, ce qui pourrait représenter une menace pour la stabilité des pigments et la conservation des peintures rupestres.
