École thématique "Autour du 2°C" - Édition 2021

19/09/2021 00:00

Journée scientifique et technique au SIRTA

17/09/2021 00:00

Are cities on track to net-zero emissions? The next generation of greenhouse gas information systems

06/09/2021 00:00

À l'origine des fleuves dans les Andes semi-arides : le rôle hydrologique des glaciers rocheux et des tourbières vu par les méthodes hydrogéophysiques et isotopiques

07/05/2021 12:00

Dans les Andes semi-aride du Chili, les glaciers rocheux sont d’une importance majeure, car ils délivrent de l’eau aux saisons les plus critiques. Ils sont de plus prépondérants par rapport aux glaciers blancs dans cette région des Andes.

Drivers of low‑level cloud seasonality over sea ice and Greenland: Insights from 13 years of spaceborne lidar observations,

20/10/2025 09:30

Arctic clouds play a critical role on the sea-ice and Greenland Ice-sheet by modulating the energy received at the surface. The surface cloud radiative effect (CRE) results from two competitive effects: a shortwave cooling effect, the umbrella effect, and a longwave warming effect, the blanket effect. When considering both the Greenland ice-sheet and the sea-ice, the CRE is dominated by the longwave component and lead to a net warming effect at the surface, except for a short period in summer. As a result, they may limit sea-ice growth during winter, trigger melt events in spring or delay the freeze-up of both sea-ice and the Greenland Ice-sheet during fall. This thesis investigates the drivers of low-level clouds seasonality over the sea-ice and the GrIS, which remain poorly understood, and the associated effect on the surface radiative budget, using 13 years of space-based active cloud remote sensing by CALIPSO.

We distill three key results that clarify the seasonal controls on Arctic low-level clouds and their surface radiative impact:

(i) The steep spatially homogeneous increase of low-level optically thick clouds over the sea-ice in spring was attributed to the increase of lower troposphere temperature. While moisture transport from mid-latitudes is already sufficient to trigger the transition earlier in spring, the steep transition of temperature, from ∼−20°C in March to ∼−13oC in May, favors the formation of more liquid-containing clouds (optically thick) against ice clouds (optically thin).

(ii) From winter to spring, we reveal a weak but statistically significant control of surface pressure on the radiative effect of clouds over the Arctic sea-ice. This weak control is likely to explain the regional patterns of surface cloud radiative warming for a given period: minimum in April over the Beaufort Sea and maximum in winter-early spring over the Barents Sea.

(iii) Using a machine learning approach, we demonstrate that the surface cloud radiative warming increase of +10W/m² in September compared to July over the Greenland west coast is attributed to frequent « polar low » circulation in the Baffin Sea, advecting clouds over the coast. This maximum occurs still in a period considered as the melt season, over a region representing 22% of the total mass loss of the ice-sheet over the last two decades.

These results suggest that space-based active cloud observations over the sea-ice and the GrIS provide strong observation constraints, helping to overcome past limitations due to the scarcity of Arctic cloud measurements. Future studies should exploit new missions such as EarthCARE to better understand the drivers of Arctic cloud seasonality and radiative effects in summer for instance.

From coherent structures to convective storms: statistical, observational and modelling approaches to turbulent flows

15/10/2025 10:00

The understanding of climate change and its impacts has significantly advanced in recent decades, revealing the formidable challenges it poses to humanity. In particular, it is now well established that the continued anthropogenic emissions of greenhouse gases are not only driving an increase in global mean temperature but also amplifying the frequency and intensity of extreme weather events such as heatwaves and heavy precipitation. However, some aspects remain poorly understood, such as the influence of climate change on complex weather phenomena like convective storms, which can produce significant hazards, including floods, hail, lightning, and severe winds. This knowledge gap arises mainly from the multiscale, multivariate, and nonlinear nature of such phenomena, which involve intricate interactions among atmospheric variables. These processes are governed by turbulence, characterised by highly disordered fluid motions that present major challenges for both theoretical understanding and numerical modeling. In particular, turbulent flows require a very large number of degrees of freedom to be comprehensively accounted for in computer simulations, which is far beyond the capabilities of current computational resources in realistic environmental or industrial settings. As a result, numerical models use subgrid-scale modelling strategies with many adjustable parameters to represent unresolved processes, which introduces uncertainties.

This thesis addresses these challenges by focusing in particular on the analysis and modelling of coherent structures—organized, recurrent flow patterns such as eddies that emerge from the apparent disorder and provide a more tractable view of turbulence. Special emphasis is placed on convective storms in the context of climate change. First, we show how high-dimensional atmospheric data can be simplified through statistical approaches that extract recurring patterns. Specifically, a generative probabilistic model originally developed for topic modeling in text analysis is adapted to decompose maps of atmospheric variables into spatial patterns that can be identified as atmospheric coherent structures. Second, we examine the challenges in studying convective storms and their evolution with climate change, outlining the limitations of available observations and numerical models. In particular, an initial climatology of derechos, a severe type of convective storm, is constructed for France using multiple observational datasets. We also analyse historical changes in atmospheric environments associated with such events with a methodology that accounts for the conditioning role of the large-scale atmospheric configuration. Finally, a modelling approach for turbulence based on the dynamics of coherent structures is proposed. This approach accounts for the intermittent nature of energy dissipation and leverages the sparsity of coherent structures to limit computational costs. Such strategy could provide more efficient and accurate alternatives to current subgrid-scale models of turbulence, with potential application for atmospheric convection and climate models.

Modélisation numérique de la pollution et du climat régional

09/10/2025 10:00

Je présenterai certains des travaux que j’ai menés autour de la modélisation régionale du climat et de la composition atmosphérique au Laboratoire de Météorologie Dynamique depuis 2012.

Tout d’abord, sur la région Méditerranéenne, les programmes internationaux HyMEx et ChArMEx, m’ont permis d’étudier les précipitations dans le bassin méditerranéen et leur lien avec le couplage atmosphère-océan ainsi qu’avec les effets directs et indirects des aérosols dans le cadre des thèses de doctorat de Ségolène Berthou et de Nicolas Da Silva. Je présenterai également des travaux plus proches des aspects numériques de la modélisation, portant en particulier sur les méthodes de résolution numérique de l’équation de l’advection dans le cadre des modèles de chimie-transport.

Ces travaux ont porté sur toute la chaîne de production qui mène des cas académiques jusqu’à des études réalistes sur des panaches volcaniques ou radioactifs. Je présenterai ensuite quelques contributions que j’ai pu faire au modèle CHIMERE. Enfin, je proposerai quelques perspectives pour mon travail des prochaines années, en particulier autour de l’évolution des modèles de chimie-transport et de leur maillage ainsi que sur l’étude des panaches atmosphériques. Enfin, j’évoquerai le développement du réseau de recherche international ChimSur avec des collègues sud-américains, qui est aussi une perspective importante de mon travail.