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Offline Atmospheric Transport on a Global Mesh of Hexagons
No full textInternational audienceWe present a new version of the offline transport model from the LMDz atmospheric general circulation model. It paves the globe with hexagons and 12 pentagons of similar surface areas rather than with regular longitude-latitude rectangles. It is available in a complete, nonlinear version and in a linearized configuration for use in variational atmospheric inversions. It runs on graphics processing units like the previous version. The previous advection approach and physical parameterizations have been kept while the code has been restructured for better numerical efficiency. The change of mesh was made necessary by the evolution of the parent LMDz model, but the technical and scientific evaluation of the new version with a roughly constant number of cells shows some interesting advantages. This evaluation is based on an 11-year simulation of sulfur hexafluoride and on a 10-year atmospheric inversion assimilating column-averaged dry air mole fractions of carbon dioxide (CO 2 ) retrieved from measurements of NASA's second Orbiting Carbon Observatory. As it is used in variational inversion and at 90-km resolution with 79 layers in the vertical, we find that the new offline model is twice as fast. Further, it shows improved interhemispheric transport, some small improvements in terms of inferred (posterior) atmospheric concentrations and small differences in terms of inferred surface fluxes compared to the previous version. These assets allowed it to be commissioned in its 90-km configuration for the operational CO 2 inversions of the European Copernicus Atmosphere Monitoring Service.</div
Testing a low-complexity spatially distributed model to simulate the intra-annual dynamics of soil erosion and sediment delivery
No full textInternational audienc
The voices of glaciers: stories of grief and hope amidst shrinking glaciers in the tropics
No full textIRD’s paper ISBN : 978-2-7099-3073-4; IRD’s PDF ISBN : 978-2-7099-3074-1; IRD’s epub ISBN : 978-2-7099-3075-8International audienc
Evenness peaks in fire-resilient vegetation preceded ecosystem shifts in East Africa
No full textInternational audienceFire is often assumed to be a key driver in shaping tropical vegetation structure and composition in grass-dominated ecosystems, while forests experience infrequent but impactful fires that influence ecosystem resilience. Although short-term interactions between fire and vegetation are well-documented, long-term dynamics remain underexplored. This study examines fire regimes, vegetation dynamics, and its biodiversity over the past 17 000 years in southwest Tanzania, using sedimentary charcoal and pollen. Major ecological transformations of vegetation and fire regimes were recorded, with vegetation changes consistently preceded shifts in fire regimes. Increased grass pollen correlated with more frequent or larger fires, while high tree cover in Miombo woodland reduced fire activity. Interestingly, pollen evenness was a precursor to major ecological transformations as peaks preceded changes in ecosystem states. Changes in precipitation and moisture seems the major top–down drivers of these changes in vegetation and fire. Fire regimes were indirectly controlled by water availability, and vegetation exhibited resilience to fire at centennial timescales before reaching ecosystem shifts at 12 400 and 1700 cal BP. Our results emphasize the critical role of tree and grass cover in shaping fire regimes and highlight the interplay between climate, vegetation structure, and fire in East African ecosystems
Unveiling the link between phytoplankton molecular physiology and biogeochemical cycling via genome-scale modeling
No full textInternational audienceEarth system models (ESMs) highly simplify their representation of biological processes, leading to major uncertainty in the impacts of climate change. Despite a growing understanding of molecular networks from genomic data, describing how changing phytoplankton physiology affects biogeochemical processes remains elusive. Here, we embed genome-scale models within a state-of-the-art ESM to deliver an integrated understanding of how gradients of nutrients modulate the molecular physiology of various plankton. In particular, when applied to Prochlorococcus , we find that glycogen and lipid management can be interpreted in terms of acclimation to different environments. Generalized to other phytoplankton such as the diatom Thalassiosira , we estimate the production of 39 metabolites that constitute hot spots of dissolved organic carbon described by their amount of carbon produced and their diversity of associated metabolites in ESMs. This modeling approach shows how genome scale–enabled ESMs have the potential to advance our understanding of microbial ecosystem functioning in ocean biogeochemical processes
CHEX-MATE: The Impact of Triaxiality and Orientation on Planck SZ Cluster Selection and Weak Lensing Mass Measurements
No full textInternational audienceGalaxy cluster abundance measurements are a valuable tool for constraining cosmological parameters like the mass density () and density fluctuation amplitude (). Wide area surveys detect clusters based on observables, such as the total integrated Sunyaev-Zel'dovich effect signal () in the case of Planck. Quantifying the survey selection function is necessary for a cosmological analysis, with completeness representing the probability of detecting a cluster as a function of its intrinsic properties. Employing a Monte-Carlo method, we inject triaxial cluster profiles into random positions within the Planck all-sky maps, and subsequently determine the completeness of the Planck-selected CHEXMATE sample as a function of both geometry and SZ brightness. This is then used to generate 1000 mock CHEX-MATE cluster catalogs, and the distribution of shapes and orientations of the detected clusters, along with any associated bias in weak lensing-derived mass () due to this orientation-dependent selection, denoted as , is obtained. We show that cluster orientation impacts completeness, with a higher probability of detecting clusters elongated along the line of sight (LOS). This leads to values of for CHEXMATE clusters relative to a random population. The largest increase in is observed in the lowest mass objects, which are most impacted by orientation-related selection bias. This bias is relevant for upcoming SZ surveys like CMB-S4, and should be considered for surveys utilizing other probes for cluster detection, such as Euclid
A two-dimensional depth-integrated model for immiscible two-phase flow in open rough fractures
No full textInternational audienceImmiscible two-phase flows in geological fractures are relevant to various industrial applications, including subsurface fluid storage and hydrocarbon exploitation. Direct numerical simulations (DNS) of first-principle equations, which resolve three-dimensional (3-D) fluid-fluid interfaces, can address all types of flow regimes but are computationally intensive. To retain most of their advantages while reducing the computational cost, we propose a novel two-dimensional (2-D) model based on integrating the 3-D first-principle equations over the local fracture aperture, assuming the lubrication approximation and a parabolic out-of-plane velocity profile, and relying on the volume-of-fluid method for fluid-fluid interface capturing. Such existing models have, so far, been restricted to single-phase permanent flow in rough fractures and two-phase flow in 2-D porous media. Wall friction and out-of-plane capillary pressure are incorporated as additional terms in the 2-D momentum equation. The model then relies on a geometric description reduced to the fracture's aperture field and mean topography field. Implemented in OpenFOAM, it is validated against 3-D DNS results for viscous fingering in a Hele-Shaw cell, and applied to a realistic synthetic rough fracture geometry over a wide range of capillary numbers ( ). We then analyse to which extent, under which conditions and why this depth-integrated 2-D model, with a tenfold reduction in computational cost, provides convincing results compared with 3-D DNS predictions. We find that it performs surprisingly well over nearly the entire range of for which 3-D DNS models are relevant, in particular because it properly accounts for the out-of-plane capillary forces and wall friction
Radiometric Cross-Calibration of an Aerial Sensor with Satellite Top-of-Atmosphere Reflectance
No full textInternational audienceSurface reflectance (SR) is essential for many remote sensing applications, but retrieving it from aerial images is challenging due to the lack of in-flight radiometric calibration and varying acquisition conditions. We propose a novel method for radiometric cross-calibration of aerial imagery using satellite Top-of-Atmosphere (TOA) reflectance. The method involves estimating at-sensor reflectance at the airborne altitude from satellite TOA reflectance, followed by spectral band adjustment and spatial alignment between satellite and airborne imagery. A linear radiometric model is derived to relate the Digital Number (DN) to the at-sensor reflectance from a selected subset of robust aerial-satellite pixel correspondences. The radiometric calibration parameter was retrieved using linear regression. The method is particularly suitable for airborne campaigns that lack onboard or in-situ radiometric calibration equipment. An ablation study is presented to analyze the selection of reliable reference pixels
Carbon accretion and desorption by interstellar polycyclic aromatic hydrocarbons
No full textInternational audienceTwo key questions of the chemistry of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM) are addressed: (i) the way carbon is returned from PAHs to the interstellar gas after the very efficient accretion of C+ ions onto PAHs, and (ii) the PAH contribution to the high abundance of small carbon molecules observed in UV-irradiated regions. They are addressed based on the structure and stability of the various isomers of the complexes formed by PAHs and their cations with atomic carbon. Carbon complexes with coronene are studied by B3LYP/6-311+G** calculations, in order to determine the behaviour of C+ and C complexes with larger pericondensed interstellar PAHs, which are thought to be dominant in the ISM. The most stable forms of the [C-coronene]+ cation include 7C and 4C rings, C+ insertion into a CH bond, and a 5C ring with a short exocyclic cumulene chain, and similarly for neutral [C-coronene]. The subsequent evolution of similar complexes with pericondensed PAHs, in diffuse clouds, is discussed under the action of interstellar UV photons and H atoms as a function of the PAH size. Despite the complexity of this processing, it seems probable that, for small PAHs, these complexes efficiently lose a C2H2 molecule from repeated photodissociations. However, this conclusion needs to be confirmed by the identification of reaction paths and the computation of activation energies. The case of the evolution of larger [C-PAH] complexes is less clear. The processing may explain the observed balance between C+ and PAHs, at least in the diffuse ISM. The formation of C2H2 from PAH catalysis is a key input for the chemistry of small carbon molecules in diffuse clouds. C+ accretion might frequently form stable PAHs that contain a peripheral pentagonal ring and form a significant fraction of interstellar PAHs
The ESO SupJup Survey: VII. Clouds and line asymmetries in CRIRES<sup>+</sup> J-band spectra of the Luhman 16 binary
No full textInternational audienceContext. Brown dwarfs at the L–T transition likely experience an inhomogeneous clearing of the clouds in their atmospheres. The resulting surface of thin and thick cloudy patches has been put forward to explain the observed variability, J-band brightening, and re-emergence of FeH absorption. Aims. We studied the closest binary brown dwarfs, Luhman 16A and B, in an effort to constrain their chemical and cloud compositions. As this binary consists of an L7.5 and a T0.5 component, we gain insight into the atmospheric properties at the L–T transition. Methods. As part of the ESO SupJup Survey, we observed Luhman 16AB at high spectral resolution in the J band (1.1–1.4 μm) using CRIRES+. To analyse the spectra, we employed an atmospheric retrieval framework, coupling the radiative transfer code petitRADTRANS with the MultiNest sampling algorithm. Results. For both objects, we report detections of H2O, K, Na, FeH, and, for the first time in the J band, hydrogen fluoride (HF). The K doublet at 1250 nm shows asymmetric absorption in the blue line wings, which are reproduced via pressure- and temperature-dependent shifts in the line cores. We find evidence of clouds in both spectra and place constraints on an FeH depletion in the Luhman 16A photosphere. The inferred over-abundance of FeH for Luhman 16B is in contradiction with its predicted rainout into iron clouds. A two-column model, which emulates the patchy surface expected at the L–T transition, is weakly preferred (~1.8σ) for component B but disfavoured for A (~5.5σ). Conclusions. The results suggest a uniform surface on Luhman 16A, which is in good agreement with the reduced variability observed for this L-type component. While the presented evidence is not sufficient to allow us to draw conclusions about any inhomogeneity on Luhman 16B, future observations covering a broader wavelength range could help us test the cloud-clearing hypothesis