LE TREUT Herve

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Self-organizing map (SOM) is used to simulate summer daily precipitation over the Yangtze-Huaihe river basin in Eastern China, including future projections. SOM shows good behaviors in terms of probability distribution of daily rainfall and spatial distribution of rainfall indices, as well as consistency of multimodel simulations. Under RCP4.5 Scenario, daily rainfall at most sites (63%) is projected to shift towards larger values. For the early 21 st century (2016-2035), precipitation in the central basin increases, yet decreases occur over the middle reaches of the Yangtze River as well as a part of its southeast area. For the late 21 st century (2081-2100), the mean precipitation and extreme indices experience an overall increase except for a few southeast stations. The total precipitation in the lower reaches of the Yangtze River and in its south area is projected to increase from 7% at 1.5°C global warming to 11% at 2°C, while the intensity enhancement is more significant in southern and western sites of the domain. A clustering allows to regroup all SOM nodes into four distinct regimes. Such regional synoptic regimes show remarkable stability for future climate. The overall intensification of precipitation in future climate is linked to the occurrence-frequency rise of a wet regime which brings longitudinally closer the South Asia High (eastward extended) and the Western Pacific Subtropical High (westward extended), as well as the reduction of a dry pattern which makes the two atmospheric centers of action move away from each other.

Not allowing the planet to warm to a level more than 1.5 times that of the pre-industrial period is imperative in terms of the risks to the most vulnerable territories. But the possibility of achieving this is considerably more limited than it was 30 years ago, just before the Rio Earth Summit in 1992. At that time, CO emissions were 5 to 6 billion tons of carbon per year - they have doubled since then. In fact, we have already made a significant commitment to the future, as evidenced by phenomena such as the melting of glaciers and ice floes, the deep warming of the oceans, or the evolution of ecosystems. The situation is even more serious if we look to the near future: the warming of the next few decades is already strongly constrained by past emissions, and the rapid mixing of greenhouse gases by atmospheric circulation makes it a growing global problem that is binding on all. We have less and less possibility to significantly modify the local climate evolution through our own actions. It is therefore necessary to adapt progressively to developments that are both the already irremediable part of future changes and the delays or failures of international actions. In this context, the territories have a privileged role to play. It is, in fact, on their scale that the major impacts of climate change develop and that a large part of greenhouse gas emissions are produced. It is therefore at this level that strategies for co-benefits between mitigation and adaptation to climate change should be developed. And it is also where trade-offs between different land uses, different biodiversity protection strategies, or different management of vulnerable areas, such as cities, mountains or coastlines, will have to be defined. This also requires an in-depth and multidisciplinary knowledge of these territories, which are all different from one another. The example of New Aquitaine, where the Acclimaterra project has implemented a very large series of regional visits (http://www.acclimaterra.fr) shows that it is necessary to take the measure of the evolutions in progress, both by listening to the expression of the various social imperatives and by facilitating the decision making of all regional actors by "putting into narrative" the scientific diagnosis. The Acclimaterra project was also materialized by the production of two reports that brought together 400 scientists and were published in the form of carefully edited books to reach the general public. This work has shown that the regions, by taking advantage of the link that unites them with their population, can thus constitute an important link of innovation and reflection and bring important elements of solution to the climate problem.

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The results of the sixth phase of the Coupled Model Intercomparison Project (CMIP) are currently being analysed and will form the basis of the IPCC Sixth Assessment Report. Since its creation in the mid-1990s, CMIP has had an increasing influence on climate research. While the principle behind it has always remained the same-comparing different climate models under similar conditions-its design and motivations have evolved significantly over the phases of the project. This evolution is closely linked to the one of the IPCC, since, historically as well as today, the results of CMIP have played a major role in the panel reports. This role increased the visibility of CMIP-over time, more and more people started to be interested in CMIP and to analyze its results. Despite this success, the way CMIP is used today raises methodological issues. In fact, CMIP has promoted a particular way of doing climate research, centred on a single tool, Global Coupled Models (GCMs), and creating a gap between model developers and model users. Due to the debates regarding the interpretation of multi-model ensembles and the validation of GCMs, whether the emphasis on this particular way of studying climate is serving the progress of climate science is questionable.

Nesting an RCM (regional climate model) into a GCM (global climate model) is generally realized through a relaxation applied at boundaries of the RCM. This procedure of using GCM as a driver to nudge RCM may produce climate deteriorations and biases. The present work elaborates an idealized protocol to assess the uncertainty and performance of the nesting procedure for both long-term mean climate (illustrated here for seasonal-mean T2m drifts) and concomitance of synoptic sequences between the RCM and GCM.

Quantile-Quantile Adjustment (QQadj), Equidistant CDF Matching (EDCDF) and Transform CDF (CDF-t), are applied to five daily precipitation datasets over China produced by LMDZ4-regional that was nested into five global climate models (GCMs), BCC-CSM1-1m, CNRM-CM5, FGOALS-g2, IPSL-CM5A-MR and MPI-ESM-MR, respectively. A unified mathematical framework can be used to define the four bias correction methods, which helps understanding their natures and essences for identifying the most reliable probability distributions of projected climate. CDF-t is shown to be the best bias correction method based on a comprehensive evaluation of different precipitation indices. Future precipitation projections corresponding to the global warming levels of 1.5°C and 2°C under RCP8.5 were obtained using the bias correction methods. The multi-method and multi-model ensemble characteristics allow to explore the spreading of projections, considered as a surrogate of climate projection uncertainty, and to attribute such uncertainties to different sources. It is found that the spread among bias correction methods is smaller than that among dynamical downscaling simulations. The four bias correction methods, with CDF-t at the top, all reduce the spread among the downscaled results. Future projection using CDF-t is thus considered having higher credibility.

What will be the impact of climate change on the scale of Aquitaine? How would a global warming of several degrees affect its landscapes and resources? What would be the prospects for adaptation of the environment and humans? These are the questions that the work directed by Hervé Le Treut, climatologist, member of the French Academy of Sciences and expert to the IPCC (Intergovernmental Panel on Climate Change) attempts to answer, relying on the collaboration of more than 150 researchers from all disciplines. The authors reflect on the future of Aquitaine and its vulnerability to climate change, considering the likely consequences on the economy (agriculture, viticulture, forests, etc.), the landscape (coastline, mountains, estuaries, forests, etc.) and the population. Through the Aquitaine region, this book describes the main issues that need to be addressed now. Richly illustrated and accessible to all, this book gives an overall vision, but anchored locally, of the impacts of climate change and recommends solutions, ways to be favored so that Aquitaine societies can adapt to these changes.

Nationally Determined Contributions (NDCs), submitted by Parties to the United Nations Framework Convention on Climate Change (UNFCCC) before and after the 21st Conference of Parties (COP21), summarize domestic objectives for greenhouse gases (GHG) emissions reductions for the 2025-2030 time horizon. In the absence, for now, of detailed guidelines for NDCs format, ancillary data are needed to interpret some NDCs and project GHG emissions in 2030. Here, we provide an analysis of uncertainty sources and their impacts on 2030 global GHG emissions based on the sole and full achievement of the NDCs. We estimate that NDCs project into 56.8 to 66.5 Gt CO2eq yr-1 emissions in 2030 (90% confidence interval), which is higher than previous estimates, and with a larger uncertainty range. Despite these uncertainties, NDCs robustly shift GHG emissions towards emerging and developing countries and reduce international inequalities in per capita GHG emissions. Finally, we stress that current NDCs imply larger emissions reduction rates after 2030 than during the 2010-2030 period if long-term temperature goals are to be fulfilled. Our results highlight four requirements for the forthcoming "climate regime": a clearer framework regarding future NDCs' design, an increasing participation of emerging and developing countries in the global mitigation effort, an ambitious update mechanism in order to avoid hardly feasible decarbonization rates after 2030 and an anticipation of steep decreases in global emissions after 2030.

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This thesis manuscript reports some methodological studies on climate regionalization by the dynamic approach. The geographical domain covers a large area from the mid-North Atlantic to Eastern Europe, and from the Sahel to the Arctic. The quest for regional climate improvement at all costs is not the focus of the manuscript. However, the focus is on three key points, generally encountered by all attempts to regionalize climate. The first point concerns the nesting scheme: one-way nesting from the GCM to the RCM (OWN) or two-way nesting between the GCM and the RCM (TWN). The second point examines the technical realization of nesting, which is usually a Newtonian relaxation operation added to the prognostic equations of the model. The third point is on the effect of mesh refinement in the RCM. The general spirit of the manuscript is to conceptualize and perform numerical simulations to treat these three points with tricks to isolate and quantify them. The general circulation model LMDZ is used for all the experiments. It plays both the role of the GCM and the RCM. In both cases, it strictly keeps its physical parameterizations and its dynamic configuration, as well as all external forcings or parameters. The experimentation strategy, qualified as Master versus Slave, consists in carrying out simulations under two linked protocols: "DS-300-to-300" refers to downscaling from the GCM at 300 km horizontal resolution to the RCM which is identical to the GCM, also at 300 km spatial resolution. "DS-300-to-100" refers to downscaling from 300 km (GCM) to 100 km (RCM). It is clear that "DS-300-to-300" is an idealized framework, particularly appropriate for evaluating the effect of the relaxation operation. The "DS-300-to-100" protocol, subtracted from "DS-300-to-300", allows for a very accurate evaluation of the effect of the increased RCM resolution. In each protocol, two communication schemes between the RCM and the GCM have been implemented, one (OWN) is the classical one-way methodology of driving the RCM by the GCM outputs, the other (TWN) is to establish a mutual exchange between the two models. Regional climate is sensitive to the choice of communication schemes between the RCM and GCM, especially at mid-latitudes. TWN brings a clear improvement on the representation of boundary information. At the level of regional atmospheric circulation modes, expressed in EOF structures, OWN and TWN are both able to reproduce them, but with slight distortions in space. Newtonian relaxation, widely used in climate regionalization, allows the RCM to track the GCM synoptic path well. However, the temporal concomitance and spatial similarity are dependent on the variables considered, the seasons, the weather regimes, and the spatio-temporal scales of atmospheric circulations. Cases of de-correlation are remarkable when the dominant circulation of the region is of small scales. Mesh refinement increases the freedom of the RCM to develop its internal dynamics, especially at small scales, but also at the whole spectrum of the circulation through the interaction of scales. Thus the RCM becomes more independent and deviates more from the GCM. This thesis, around the methodological aspects of climate regionalization, helps to have a better understanding on the practice. It also sends a cautionary message to the RCM community and invites them to check their regionalization methodology carefully.

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Presentation of the laureates' thesis work by a "senior" researcher, then by the laureate. The topics covered are diverse: the snows of the Antarctic plateau (climate physics), the mechanics of the cell (biology), the fight against infectious diseases (genetics), the brain in the man-machine interface (computer science) and randomness (mathematics).

In this paper we analyze research gaps and identify new directions of research in relation to a number of facets of the Paris Agreement, including the new 1.5 °C objective, the articulation between near-term and long-term mitigation pathways, negative emissions, verification, climate finance, non-Parties stakeholders, and adaptation.

The back cover states: "Global warming is a reality that has been identified as a major threat to the population of the Earth. There are two imperative reasons to change our way of life: to limit the warming of the atmosphere and to adapt to the inevitable climate changes. To do this, we need to understand the causes of this climate change, which are complex: from the intensity of solar radiation, linked to long-term astronomical variations, to gaseous emissions into the atmosphere, of natural (volcanoes) or anthropogenic (industry, transport) origin. It is then necessary to understand how these factors modify the climate, to predict the evolutions, to anticipate the changes in order to try to avoid the main damages. The gases sent into the atmosphere create, through a complex chemistry, aerosols that modify the intensity of solar radiation, or, like carbon dioxide, inhibit the evacuation of energy from the Earth through the greenhouse effect. Through complex chemical mechanisms, warming affects life on Earth: plant or animal, land or sea, in short everything that makes up the human environment and must be understood and anticipated. To adapt to these inescapable vital changes, chemistry has mobilized its skills. Since "renewable energies" are intermittent, chemistry is providing the means to store them (new batteries). Since transportation must be oil-free, it is proposing other fuels (hydrogen, biofuels). it is developing new materials for lighter, more fuel-efficient vehicles. it is proposing to use carbon dioxide as a raw material for a new organic chemistry or to grow microalgae, which could themselves provide interesting biofuels. This book presents the research, understanding and innovations of high-level professionals, active in these fields in industry or public research. Its reading facilitates the understanding of these considerable changes that we are living".

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Considering that limiting global warming to below 2°C implies a CO2 budget not to be exceeded and near-zero emissions by 21OO (IPCC), we can assess global 2030 greenhouse gas emissions implied by INDCs in comparison to long-term trajectories. Ahead of the COP21, we estimate that submitted INDCs would bring global greenhouse gas emissions in the range of 55 to 64 GtC02eq in 2030. Under this assumption,global emissions in 2030 are thus higher than the level of 51GtC0 2eq for the year 2012. However, this is not in contradiction with a peaking of global emissions that can only be expected after 2020, given in particular the projected dynamics of emissions in China and other developing countries. The published INDCs represent a significant step towards trajectories compatible with the 2°C goal,but remain insufficient to join trajectories presenting a reasonable probability of success. ln order to increase the chance of meeting the 2°C objective, the ambition of the short-term contributions needs to be strengthened in future negotiations. ln order to sustain a high pace in emissions reductions after 2030,structural measures are also needed, which, in order to have a rapi impact, should be prepared as early as possible. Continued efforts are needed to accelerate the development of low carbon solutions on the one hand,and demonstrate the feasibility of negative emissions on the other hand.

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Hervé Le Treut, French climatologist and member of the International Panel on Climate Change (IPCC), gave a lecture on the theme of climate modeling. Since January 2009, he has been the director of the Institut Pierre Simon Laplace (IPSL), a federation that coordinates the work of nine research laboratories in the Paris region.From the outset of his talk, Hervé Le Treut explained how the issues associated with climate change are evolving: because of the passing of certain key dates and the crossing of critical thresholds, the deadlines - once distant and unclear - are becoming clearer and closer. After illustrating this state of affairs on diagrams that specify the associated time scales, the speaker presents the various elements necessary for climate modeling, distinguishing between what we can say for sure and what is a matter of uncertainty, and then he discusses future risks.This lecture, given on June 18, 2015 on the occasion of the Inria scientific days, before an audience mainly composed of researchers, ends with an invitation addressed to all citizens. Hervé Le Treut suggests that everyone should take an interest in climate change on a territorial scale, to allow for more concrete observation and greater awareness.

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This essay focuses on the "technological practices" of collecting, producing, storing, disseminating and using satellite data applied to the Earth sciences in France. Through the description of a process of "conciliation-normalization", we study the efforts made by the promoters of space technologies (at CNES, CNRS and universities) to enlist a large scientific community in the execution of space experiments, and in particular to attract researchers from various fields of Earth sciences. On the one hand, space technologies must adapt to the practices and representations of these disciplines. On the other hand, the appropriation of satellite data requires researchers to learn, because the types of data are foreign to their expertise. It is during this process, taking place between 1980 and 2000, that research objects, tools and methods, as well as scientific communities were forged, communities sharing the belief that satellite data (in many forms) are relevant tools for scientific investigation. Our case study focuses on the processing of POLDER radiometer data. The socio-technical configuration at stake, borrowed from NASA, operates a number of framings. It gives a dominant place to data and processes with an explicit geophysical meaning, compared to other types of variables. It puts forward a specific epistemic community. It privileges certain technological practices, as well as particular forms of data production, storage and delivery. It also influences the architecture and planning of missions. Finally, this dynamic guides the ways in which data are used and makes them difficult to use by other researchers. We have thus analyzed the specific case of certain climate modelers whose work requires data produced according to other representations, sometimes opposed to the "norm".

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Recent studies have shown that global warming and associated sea-surface temperature (SST) changes may trigger an important rainfall increase in southeastern South America (SESA) during the austral summer (December-January-February, DJF). The goal of this paper is to provide some insight into processes which may link global and SESA changes. For this purpose, a "two-way nesting" system coupling interactively the regional and global versions of the LMDZ4 atmospheric model is used to study the response to prescribed SST changes. The regional model is a variable-grid version of the global model, with a zoom focused over South America. An ensemble of simulations forced by distinct patterns of DJF SST changes has been carried out using a decomposition of full SST changes into their longitudinal and latitudinal components. The full SST changes are based on projections for the end of the twenty-first century from a multi-model ensemble of WCRP/CMIP3. Results confirm the presence of a major rainfall dipole structure, characterized by an increase in SESA and a decrease in the South Atlantic Convergence Zone region. Rainfall changes found in the WCRP/CMIP3 models are largely explained as a response of this dipole structure to the zonally-asymmetric (or longitudinal) component of SST changes. The rainfall response to the zonal-mean (or latitudinal) SST changes (including the global warming signal itself) shows an opposite contribution. The processes explaining the role of zonally-asymmetric SST changes involve remote effects of SST warming over the equatorial Indian and Pacific oceans inducing an atmospheric wave-train extended across the South Pacific into South America. © 2013 Springer-Verlag Berlin Heidelberg.

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The winter time weather variability over the Mediterranean is studied in relation to the prevailing weather regimes (WRs) over the region. Using daily geopotential heights at 700 hPa from the ECMWF ERA40 Reanalysis Project and Cluster Analysis, four WRs are identified, in increasing order of frequency of occurrence, as cyclonic (22.0 %), zonal (24.8 %), meridional (25.2 %) and anticyclonic (28.0 %). The surface climate, cloud distribution and radiation patterns associated with these winter WRs are deduced from satellite (ISCCP) and other observational (E-OBS, ERA40) datasets. The LMDz atmosphere-ocean regional climate model is able to simulate successfully the same four Mediterranean weather regimes and reproduce the associated surface and atmospheric conditions for the present climate (1961-1990). Both observational- and LMDz-based computations show that the four Mediterranean weather regimes control the region's weather and climate conditions during winter, exhibiting significant differences between them as for temperature, precipitation, cloudiness and radiation distributions within the region. Projections (2021-2050) of the winter Mediterranean weather and climate are obtained using the LMDz model and analysed in relation to the simulated changes in the four WRs. According to the SRES A1B emission scenario, a significant warming (between 2 and 4 °C) is projected to occur in the region, along with a precipitation decrease by 10-20 % in southern Europe, Mediterranean Sea and North Africa, against a 10 % precipitation increase in northern European areas. The projected changes in temperature and precipitation in the Mediterranean are explained by the model-predicted changes in the frequency of occurrence as well as in the intra-seasonal variability of the regional weather regimes. The anticyclonic configuration is projected to become more recurrent, contributing to the decreased precipitation over most of the basin, while the cyclonic and zonal ones become more sporadic, resulting in more days with below normal precipitation over most of the basin, and on the eastern part of the region, respectively. The changes in frequency and intra-seasonal variability highlights the usefulness of dynamics versus statistical downscaling techniques for climate change studies. © 2013 Springer-Verlag Berlin Heidelberg.

The back cover states: "The impacts of climate change in Aquitaine What will be the impact of climate change on the scale of Aquitaine? How would a global warming of several degrees affect its landscapes and resources? What would be the prospects for adaptation of the environment and humans? These are the questions that the work directed by Hervé Le Treut, climatologist, member of the French Academy of Sciences and expert to the IPCC (Intergovernmental Panel on Climate Change) attempts to answer, relying on the collaboration of more than 150 researchers from all disciplines. The authors reflect on the future of Aquitaine and its vulnerability to climate change, considering the likely consequences on the economy (agriculture, viticulture, forests, etc.), the landscape (coastline, mountains, estuaries, forests, etc.) and the population. Through the Aquitaine region, this book describes the main issues that need to be addressed now. Richly illustrated and accessible to all, this book gives an overall view, but anchored locally, of the impacts related to climate change and recommends solutions, tracks to be favored so that Aquitaine societies can adapt to these changes.

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This chapter addresses the issue of climate change from the point of view of its consequences and the social, economic and political transformations that it could generate. As the vulnerabilities of societies to climate change and the adaptation capacities to be developed cannot be analysed exhaustively, a selection of issues that concern Aquitaine in particular are addressed: cities and demography, forests, coastline, water resources, tourism.

State of the art concerning the major natural resources of the Aquitaine region.

Southeast South America (SESA) is one of the few subtropical regions where WCRP/CMIP3 climate models project a significant increase in austral summer precipitation for the late 21st century under a global warming scenario. This signal is associated with an increase in the frequency of summers identified as positive phases of the dominant mode of precipitation variability in the region, this mode being defined by above (below) normal precipitation in the SESA (South Atlantic Convergence Area). This trend is associated with an increase in sea surface temperature (SST) in the equatorial Pacific. This is confirmed by numerical simulation experiments with the LMDZ4 interactive two-way nesting system, which also show that the projected increase in precipitation in the SESA is associated with the zonally asymmetric signal of SST warming.

Variations in the large-scale tropical circulation during climate change may have important consequences in terms of impact on human activities. First, we study the tropical meridional circulation in different coupled climate models. During global warming, we find that the Hadley meridional circulations and subtropical jets are significantly shifted towards the poles, and that the intensity of these circulations decreases slightly during climate change. In a second part, simulations using the GCM LMDZ4 in different configurations, show that the intensity and displacement of Hadley cells are related to the uniform change in sea temperature but also to the change in meridional temperature gradients. Finally, we show that changes in Hadley circulation can then have a significant impact on water vapor feedback.

Using a new automatic scheme, mid-latitude North Pacific synoptic low and high trajectories were constructed from daily sea level pressure (SLP) data over the period 1950-2001. Over the eastern North Pacific, high pressure strength decreases and their frequency becomes more variable, while lows intensify and exhibit more southerly trajectories from the mid-1970s. Thus, winter low-pressure and high-pressure activity are significantly anti-correlated. These changes in transient activity result in weaker PNMs at the Aleutian Low, consistent with the positive phase of the North Pacific Oscillation, inducing positive ocean surface temperature (OST) anomalies along the west coast of North America and negative OST anomalies over the central basin. These conditions are associated with warmer temperatures and more precipitation from Alaska to Mexico and over the southwestern United States. According to the simulations of the Coupled Global Model of the National Center for Meteorological Research version 3, the winter frequency of lows could be significantly reduced in a context of strong increase of greenhouse gases. This modification could be favorable to anticyclonic conditions over the East, inducing a weighting of anthropogenic warming along the North American west coast and a reduction/increase in precipitation over the regions located south and north of 45°N respectively.

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A simplified surface ocean-atmosphere coupled model already existing at the lmd has been adapted here to make a tool strictly comparable to the ipsl global coupled model. A comparative analysis of climate simulations conducted with these two parallel ocean-atmosphere coupled models, using the same atmosphere model but ocean models of different complexity, has allowed us to highlight common systematic biases, which are also present in other climate models (kiehl, 1998). These are a cooling of the western tropical Pacific basin and a drying of the central Pacific accompanied by the penetration of a tongue of cold water from the east of the basin. This bias is also marked by a cooling of the tropical upper troposphere above 30°C. In order to reveal the role of the atmosphere in the tropical thermal drift of the simulated climate, we have performed sensitivity experiments which we believe significantly affect the energetics of the upper troposphere. These sensitivity experiments concern: the high ice clouds, the representation of convection and the ozone content of the upper atmospheric layers. The results of these simulations allow us to suggest that the tropical thermal drift at the ocean surface is controlled by an energetic imbalance at the top of the atmosphere. The transmission of these biases to the ocean surface is then made by the subsidence branches of the hadley-walker cells. This analysis of atmospheric feedbacks is completed by a study of atmospheric energy transport (tae). We validate the tae simulated by the lmd mcga against the transports derived from the ecmwf reanalyses and then focus on the variation of these transports in a changing climate. For the different energy types, the response of atmospheric energy transport to a prescribed change in ocean surface temperatures corresponding to a 2co 2 is directly related to the equator-tropic thermal gradient. However, the variations in the different energy terms partially offset each other in terms of the wet static energy transport. The analysis in terms of divergence of the atmospheric energy transport allows to quantify the importance of the dynamic feedbacks compared to the radiative feedbacks. The wet static energy transport varies by 10-20 wm 2 . These variations are of the same order of magnitude as the variations in radiative fluxes induced at the tropopause in a 2co 2 experiment. On the other hand, the variations in the different types of energy are of an order of magnitude larger. Finally, we show that the hydrological component of the tae is affected by the parameterization of the precipitation process.

Cloud cover is one of the major parameters in the study of the climate system. The lack of knowledge about cloud cover represents one of the main uncertainties in climate modeling and climate change prediction. In particular, the vertical distribution of clouds in the atmosphere is a determining factor of the earth's radiation balance. In order to study this vertical structure, we use two sets of data, surface observations (warren et al. data) and satellite observations (isccp data) that we start by comparing. We show the complementarity of these two data sets. If a good coherence is found for the detection of the total cloud cover, significant differences appear when we focus on a particular type of cloud (low clouds for example). The validation of the cloud cover simulated by a general circulation model (GCM) then becomes delicate. Considering the great heterogeneity of the cloud cover, it is essential to compare not only the simulated and observed mean radiation balance but also the cloud characteristics (top pressure, optical thickness) determined at a smaller spatial scale than the model and to compare them with the same parameters derived from the observation. A method of comparing the cloud cover simulated by a mcg with isccp satellite data has been developed. We use this method to validate physical parameterizations, wet convection and boundary layer scheme, in a mcg. However, this type of comparison does not allow a complete validation of the vertical distribution of the cloud cover. Indeed, the isccp climatology provides a description in terms of radiatively equivalent monolayer clouds at the top of the atmosphere to the actual cloud distribution. This representation, in the case of multilayered clouds or clouds with strong vertical development, is not representative of the true vertical structure of the cloud layers. It is therefore important to improve in the future the measurement techniques and to consider other instruments to be embarked in space to solve in particular the problem of detection of cloud systems organized in multilayers. It is in this perspective that we have tried to show the interest of embarking a backscattering lidar in space by coupling it to a passive radiometry instrument and to measure its limits. From the cloud cover simulated by the mcg, we have performed simulations of the restitution of this cloud cover from lidar and radiometric measurements. We have tested different hypotheses on some parameters such as the signal to noise ratio and sampling in order to determine the optimal conditions for the use of lidar. The lidar will allow a better study of the vertical distribution of clouds in the atmosphere. It will allow to improve the restitution of the altitude of the top of semi-transparent clouds such as cirrus and to highlight the multi-layer systems including cirrus as well as the systems of split cloud cover.

The aim of this thesis is the exploitation of a coupled model of atmosphere, surface ocean, sea ice, for the study of the interactions between the surface ocean and the atmosphere. The coupled model, which has never been tested before, is described in a first part (chapter 2). A particular interest is given to the term representing the heat transport by the oceans: its impact on the thermodynamics of the ocean and on the general representation of is analyzed (chapter 3). Chapter 4 is devoted to the analysis of the model results at the seasonal scale. The behavior of the model is satisfactory and stable in its representation of the annual cycle. But there are systematic anomalies. Thanks to various sensitivity studies, the origin of these anomalies is better identified. But the interest of these experiments is wider since they allow to reveal some preponderant factors of the coupling (ocean-radiation interaction, surface wind friction) and to better understand the interactions between the surface ocean (or the sea ice) and the atmosphere. Finally, the last chapter (chapter 5) analyzes the climate drift experienced by our model. We obtain original results, in the sense that we highlight a slow atmospheric component that destabilizes the climate system during the first five years of simulation. In particular, we address the problem of the representation of the radiative properties of high clouds (cirrus) in the equatorial region: these cloud-radiation interactions are crucial phenomena but nevertheless little known, for the sensitivity of climate models to disturbances.

The estimation of climate sensitivity to natural or anthropogenic forcing is linked to our understanding of the mechanisms that regulate radiative exchange between the earth and space. Using satellite observations erbe (earth radiation budget experiment) and ssm/i (special sensor microwave imager) and numerical climate simulation models (the lmd model and the arpege model developed by meteo-france), seasonal variations of the terrestrial radiation budget are analyzed and the respective roles of temperature, water vapor and clouds in these variations are examined. The ability of general circulation models to simulate the seasonal cycle of water vapor, clear-sky radiative fluxes and clouds radiative forcing is evaluated, and the models are used in turn to interpret some observed characteristics. This allows to identify more precisely the factors that modulate the greenhouse effect on seasonal and interannual scales and under climate change scenarios. This study shows the essential role of the variability of the vertical atmospheric structure (in particular the relative humidity profile and the vertical temperature gradient) in the observable variations of the climate as well as in its global sensitivity, and shows the need to differentiate these effects in the analysis of the climate sensitivity at various time scales. Finally, this study highlights the importance of validating seasonal and interannual variations in the vertical atmospheric structure for the application of climate simulation models to future scenarios.

In order to establish the role of feedbacks related to a decrease in snow cover in the context of global warming due to greenhouse gases, a new parameterization of albedo has been developed. This, using a global climate model, considers the most significant parameters of the observed large variability, such as vegetation, snow age, snow cover renewal rate and fraction of the surface covered by snow. Sensitivity studies to this parameterization are performed in seasonal and perpetual mode. We study aspects of the model's response concerning both surface albedo, ground temperature, snow fraction and some hydrological variables. For the validation of the results of the lmd model, a percentage snow cover climatology has been established using satellite data provided by noaa. The climatic impact of boreal deforestation was also analyzed over the northern hemisphere.

With the intensification of human activities, greenhouse gases are increasing in the atmosphere. This can lead to a warming of the earth system and a change in the global climate. The sensitivity of the atmospheric system in response to external forcing obviously depends on many physical and dynamic processes. Perhaps the most uncertain, in our present state of knowledge, is the cloud-radiation interaction. The present study investigates the modification of the sensitivity of the climate system by this interaction, using two numerical models: the atmospheric general circulation model of the laboratory of dynamic meteorology (lmd) and a one-dimensional vertical model developed in this study.