AERIS fête ses 10 ans

27/01/2025 13:00

Transition écologique et devenir humain

18/12/2024 13:30

Festival Cinéscience 2024

12/12/2024 19:00

Climate scientists as analysts, advisors, advocates, or activists: what’s the difference and does it matter?

21/11/2024 11:00

Un séminaire de la branche Early-career du LSCE.

Sobriété : qu’en dit la sciences et ou en sommes-nous sur le plan politique

20/11/2024 16:00

Premier opus du cycle de séminaires organisé dans le cadre du cours « transition énergétique » du département de Géosciences de l’ENS-PSL.

Intégration du climat et de l'eau dans les études de transition énergétique aux États-Unis

20/11/2024 14:00

Les systèmes électriques dépendent de plus en plus des énergies renouvelables et la prise en compte de la non-stationnarité des conditions aux frontières extrarégionales s’avère fondamentale. Nous avons démontré que la variabilité interannuelle de l’eau seule pouvait entraîner une variation des coûts d’exploitation du système de +/-10 %…

Event-driven numerical modelling of early diagenesis in coastal ecosystems: application to flood deposits in Rhône River prodelta

13/10/2023 14:00

The main purpose of this work is to study the biogeochemical response of coastal seafloor subject to episodic massive sediment deposition from floods events. The Rhône River and its connected coastal margins serve as a case-study site for quantifying the impact of these extreme events on early diagenetic processes because it receives significant inputs of sediment (estimated to be up to 80%) during short and intense events. These extreme events are rare and unpredictable, thus the assessment of their impact on sediment biogeochemical processes is difficult.

In order to study the short and intermediate terms response of the sediment biogeochemistry under these abruptly changing conditions, an event-driven numerical model of early diagenesis was specifically developed during this thesis. Using published data of two contrasting floods in year 2008, the model showed reliable capability to simulate the changes induced by the sediment input on the porewater profiles for various solutes. The model suggests that these floods could produce differing biogeochemical response, the extent of which is determined by the underlying characteristics of the flood layer deposit. We found a two-fold increase in overall mineralization rates during the 2008 spring flood event from pre-flood conditions in the spring, which increased further in the fall when a very labile carbon enriched sediment was deposited. My research demonstrated that these differences were due to the nature of organic carbon delivered to  Rhone delta as well as the scale (thickness) of deposition.

These intrinsic characteristics might also be responsible for constraining the relaxation timescale of the various porewater solutes (e.g oxygen, dissolved inorganic carbon, sulfate) to a few months as observed in the field. Furthermore, this research also demonstrated that the strong internal cycling and the role of secondary redox processes such as pyrite precipitation (which were enhanced during these flood events) might be responsible for the maintenance of non-sulfidic condition observed in Rhône prodelta sediment. The thesis also briefly explores the concept of ”memory effect” of temporally connected flood depositions. Multiple occurrence of these events can also trigger temporal interaction between floods which has a substantial effect on the processes operating in the deep (such as methanogenesis and sulfate reduction) but negligible for superficial oxic and suboxic processes. This has significant ramification in future scenarios of increasing frequency of these extreme events.

More recent time series of porewater composition obtained during winter campaigns in 2021-22 investigates the temporal evolution of the porewater following an estimated 25 cm of sediment deposition. A remarkable modification of the dissolved inorganic carbon, sulfate and dissolved methane profiles were observed which was distinguishable from the pre-flood situation. Model simulations describes adequately the dataset and showed that these winter events can result to as much as 75% increase in total carbon mineralization, thus enhancing longer-term DIC production in the sediment. This winter flood also leads to a decoupling of the two pathways for sulfate reduction – organoclastic sulfate reduction and anaerobic oxidation of methane and is associated to vertical displacement of the sulfate-methane transition zone. This observation has important implications since further deepening of the AOM maximum zone due to flood deposition could enhance the effective trapping of methane (a ”green house” gas crucial in the context of climate change).

Overall, the numerical exploration in this thesis provides for the first time, a synthesis of the role of episodic event such as the massive flood deposition on spatio-temporal dynamics of the biogeochemical processes in the sediment.

Les précipitations au-dessus de la calotte Antarctique : une approche conjointe observations et modélisation

01/09/2023 14:00

Résumé
La calotte Antarctique, dont le bilan de masse de surface est principalement alimenté par les précipitations, stocke plus de 30 millions de km³ de glace (l’équivalent d’environ 60 mètres de niveau des mers). Toute perturbation de celle-ci a donc un impact direct sur le niveau global des mers. Malheureusement les processus à l’origine des précipitations sont extrêmement difficiles à observer du fait des conditions météorologiques complexes de l’environnement antarctique, et ainsi très peu connus.

En croisant les données issues de différents instruments – radar embarqué sur satellite, mesures in-situ et télédétection depuis la surface ainsi que les modèles météorologiques et de climat – l’objectif est d’obtenir la meilleure synthèse possible, de représenter le plus fidèlement et précisément les précipitations dans leur environnement (comprenant la dimension verticale) et la meilleure capacité à prévoir leur évolution. Une première étude climatologique globale ainsi que régionale des précipitations cumulées en surface a été réalisée.

Une évaluation des récentes avancées en modélisation (CMIP et ERA5) par rapport à l’unique jeu de données issues d’observations satellitales (CloudSat) pendant 4 années consécutives à l’échelle du continent a permis de mettre en avant la complexité et les nombreux désaccords dans la représentation des précipitations au pôle Sud.

Il y a moins de valeurs aberrantes dans les quantités de précipitations simulées mais aucune amélioration notable du biais positif des modèles (surestimation des taux moyens de précipitations aux échelles continentale et régionale, toute l’année, indépendamment de la saison). Puis une analyse événementielle locale des précipitations a été poursuivie dans le cadre de la campagne YOPP, permettant d’étudier à la fois la surface et la verticale pendant une saison d’été austral complète.

La divergence entre les modèles et réanalyses atmosphériques ainsi qu’avec les observations concernant les quantités de précipitations est notable. Un biais positif doit être pris en compte lors de l’utilisation des données de modélisation, tant sur l’intensité que la fréquence d’occurrence des événements de précipitation. L’analyse des données locales d’observation de Dumont d’Urville qui acquises pendant plusieurs années consécutives permettent de mettre la campagne YOPP dans un contexte pluriannuel.

D’autre part, l’étude des réanalyses ERA5 ainsi que des observations satellitales CloudSat sur la verticale complète et nous permet partiellement d’extrapoler l’analyse locale au continent Antarctique. En parallèle, l’exploitation de rapports météorologiques à différentes stations et des réanalyses ERA5 concernant la phase des précipitations a mené à la réalisation d’une analyse à l’échelle climatologique de l’occurrence des précipitations liquides en Antarctique. Ces événements pluvieux se manifestent généralement lors de l’apparition d’une intrusion maritime chaude et humide associée à un blocage anticyclonique.

Malgré les différences concernant les quantités de précipitations liquides prédites, les simulations réalisées avec de nombreux modèles (CMIP6) et dans plusieurs scénarios futurs suggèrent que le réchauffement global futur de l’Antarctique s’accompagnera d’épisodes pluvieux plus fréquents et plus intenses.

Abstract
The Antarctic ice sheet, whose surface mass balance is primarily driven by precipitation, stores over 30 million cubic kilometers of ice (equivalent to about 60 meters of global sea level rise). Any disturbance to this ice sheet has a direct impact on the global sea level. Unfortunately, the processes responsible for precipitation are extremely challenging to observe due to the complex weather conditions in the Antarctic environment, and thus, they are not well understood.

By combining data from various instruments – satellite-borne radar, in-situ measurements, surface remote sensing, as well as weather and climate models – the objective is to obtain the best possible synthesis, accurately representing precipitation in its environment (including the vertical dimension) and improving the ability to forecast its evolution. A preliminary global and regional climatological study of cumulative surface precipitation has been conducted. An evaluation of recent modeling advancements (CMIP and ERA5) compared to the single dataset derived from satellite observations (CloudSat) over four consecutive years at the continental scale has highlighted the complexity and significant discrepancies in representing precipitation at the South Pole.

There are fewer outliers in the simulated precipitation amounts, but there is no notable improvement in the positive bias of the models (overestimation of mean precipitation rates on both continental and regional scales, throughout the year, regardless of the season). Subsequently, a local event-based analysis of precipitation was pursued as part of the YOPP campaign, allowing for the study of both surface and vertical aspects during a complete austral summer season.

The divergence between atmospheric models, reanalyses, and observations concerning precipitation amounts is noteworthy. A positive bias needs to be considered when using modeling data, affecting both the intensity and frequency of precipitation events. The analysis of local observational data from Dumont d’Urville, acquired over multiple consecutive years, provides a multi-year context for the YOPP campaign. Additionally, studying ERA5 reanalyses and CloudSat satellite observations throughout the vertical dimension partially enables extrapolation of the local analysis to the Antarctic continent.

Concurrently, the utilization of meteorological reports from various stations and ERA5 reanalyses regarding the precipitation phase has led to a climatological-scale analysis of liquid precipitation occurrence in Antarctica. These rainy events typically occur during the arrival of a warm and moist maritime intrusion associated with anticyclonic blocking.

Despite differences in predicted amounts of liquid precipitation, simulations conducted with numerous models (CMIP6) and under multiple future scenarios suggest that future global warming in Antarctica will be accompanied by more frequent and intense rainy episodes.

Modélisation du recouvrement vertical des nuages et impacts sur le rayonnement

23/06/2023 14:00

Radiative transfer is a crucial process in atmospheric and climate modelling, as well as for cli-
mate change simulations. Computations of radiative fluxes at the top of the atmosphere and at the
surface allow us to estimate the radaitive budget of the planet, which is very important to represent
correctly when it comes to climate simulations. Many elements interact with the radiation in the
atmosphere : gases, aerosols, clouds, and different types of surfaces (vegetation, oceans, snow…).
These different components do not interact in the same way with solar radiation, that comes from
the sun, and with infrared radiation, that comes from the earth’s surface and the atmosphere itself.
In both situations, clouds, composed of liquid water droplets and/or solid water crystals, represent
an important modeling difficulty. Clouds are complex objects, because of their composition, their
geometry, and their multiple interactions with the radiation field. Cloud-radiation interaction has
been studied for many years, and it has been shown that it represents one of the most important
obstacles to the improvement of global climate models.

In this work, we focus on one of the key aspects in the representation of the effect of clouds on
radiation : vertical cloud overlap. This notion is indeed directly linked to the cloud cover, which is
a quantity of first order importance in the calculation of the albedo of a cloud scene. Within the
framework of the vertical cloud overlap, we develop a formalism allowing us to explore in depth
various hypotheses of cloud overlap, in particular exponential-random overlap. We show that this
overlap hypothesis can, under certain conditions, allow a very good representation of cloud proper-
ties, both geometric and radiative, even from a coarse resolution vertical cloud profile. We show that
the vertical subgrid variability of the cloud fraction, although not taken into account by large-scale
atmospheric models, can have a significant impact on the solar fluxes calculated at the top of the
atmosphere. The rigorous consideration of vertical resolutions by the overlap is also an important
factor.

We then focus on incorporating these overlap results into a Monte Carlo radiative transfer code
(RadForce). The use of this new algorithm, which also uses a line-by-line approach for the different
atmospheric gases, allows us to model the emission altitudes of each atmospheric component. These
new tools allow us to analyze in a new way the radiative forcings linked to greenhouse gases, as
well as the impact of taking into account the vertical overlap of clouds and their vertical subgrid
heterogeneity.