- Simulated Antarctic precipitation and surface mass balance at
the end of the 20th and 21st centuries.
Krinner, G., O. Magand, I. Simmonds, C. Genthon, and J.-L. Dufresne.
Clim. Dyn. , DOI: 10.1007/s00382-006-0177-x, 2006.
Abstract : The aim of this work is to assess potential future Antarctic surface mass balance changes, the underlying mechanisms, and the impact of these changes on global sea level. To this aim, this paper presents simulations of the Antarctic climate for the end of the 20th and 21st centuries. The simulations were carried out with a stretched-grid atmospheric general circulation model, allowing for high horizontal resolution (60 km) over Antarctica. It is found that the simulated present-day surface mass balance is good on continental scales. Errors on regional scales are moderate when observed sea surface conditions are used; more significant regional biases appear when sea surface conditions from a coupled model run are prescribed. The simulated Antarctic surface mass balance increases by 32 mm water equivalent per year in the next century, corresponding to a sea level decrease of 1.2 mm yr-1 by the end of the 21st century. This surface mass balance increase is largely due to precipitation changes, while changes in snow melt and turbulent latent surface fluxes are weak. The temperature increase leads to an increased moisture transport towards the interior of the continent because of the higher moisture holding capacity of warmer air, but changes in atmospheric dynamics, in particular off the Antarctic coast, regionally modulate this signal.
Texte in pdf .- The LMDZ4 general circulation model: climate performance and sensitivity to parametrized physics with emphasis on tropical convection
Hourdin, F., I. Musat, S. Bony, P. Braconnot, F. Codron, J.-L. Dufresne, L. Fairhead, M.-A. Filiberti, P. Friedlingstein, J.-Y. Grandpeix, G. Krinner, P. LeVan, Z.-X. Li and F. Lott.
Climate Dynamics , Vol. 19, No. 15, pages 3445-3482, August 2006
Abstract : The LMDZ4 general circulation model is the atmospheric component of the IPSL-CM4 coupled model which has been used to perform climate change simulations for the 4th IPCC assessment report. The main aspects of the model climatology (forced by observed sea surface temperature) are documented here, as well as the major improvements with respect to the previous versions, which mainly come form the parametrization of tropical convection. A methodology is proposed to help analyse the sensitivity of the tropical Hadley- Walker circulation to the parametrization of cumulus convection and clouds. The tropical circulation is characterized using scalar potentials associated with the horizontal wind and horizontal transport of geopotential (the Laplacian of which is proportional to the total vertical momentum in the atmospheric column). The effect of parametrized physics is analysed in a regime sorted framework using the vertical velocity at 500 hPa as a proxy for large scale vertical motion. Compared to Tiedtke s convection scheme, used in previous versions, the Emanuel s scheme improves the representation of the Hadley-Walker circulation, with a relatively stronger and deeper large scale vertical ascent over tropical continents, and suppresses the marked patterns of concentrated rainfall over oceans. Thanks to the regime sorted analyses, this differences are attributed to intrinsic differences in the vertical distribution of convective heating, and to the lack of self-inhibition by precipitating downdraughts in Tiedtke s parametrization. Both the convection and cloud schemes are shown to control the relative importance of large scale convection over land and ocean, an important point for the behaviour of the coupled model.
Texte in pdf .- Radiative forcing by well-mixed greenhouse gases: Estimates from climate models in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4),
W. D. Collins, V. Ramaswamy, M. D. Schwarzkopf, Y. Sun, R. W. Portmann, Q. Fu, S. E. B. Casanova, J.-L. Dufresne, D. W. Fillmore, P. M. D. Forster, V. Y. Galin, L. K. Gohar, W. J. Ingram, D. P. Kratz, M.-P. Lefebvre, J. Li, P. Marquet, V. Oinas, Y. Tsushima, T. Uchiyama, and W. Y. Zhong
J. Geophys. Res. - Atmospheres , Vol. 111, D14317, doi:10.1029/2005JD006713, August 2006
Abstract : The radiative effects from increased concentrations of well-mixed greenhouse gases (WMGHGs) represent the most significant and best understood anthropogenic forcing of the climate system. The most comprehensive tools for simulating past and future climates influenced by WMGHGs are fully coupled atmosphere-ocean general circulation models (AOGCMs). Because of the importance of WMGHGs as forcing agents it is essential that AOGCMs compute the radiative forcing by these gases as accurately as possible. We present the results of a radiative transfer model intercomparison between the forcings computed by the radiative parameterizations of AOGCMs and by benchmark line-by-line (LBL) codes. The comparison is focused on forcing by CO2, CH4, N2O, CFC-11, CFC-12, and the increased H2O expected in warmer climates. The models included in the intercomparison include several LBL codes and most of the global models submitted to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). In general, the LBL models are in excellent agreement with each other. However, in many cases, there are substantial discrepancies among the AOGCMs and between the AOGCMs and LBL codes. In some cases this is because the AOGCMs neglect particular absorbers, in particular the near-infrared effects of CH4 and N2O, while in others it is due to the methods for modeling the radiative processes. The biases in the AOGCM forcings are generally largest at the surface level. We quantify these differences and discuss the implications for interpreting variations in forcing and response across the multimodel ensemble of AOGCM simulations assembled for the IPCC AR4.
Texte in pdf . "© American Geophysical Union. Further reproduction or electronic distribution is not permitted."- Jean-Baptiste Joseph Fourier et la découverte de l'effet de serre
(Jean-Baptiste Joseph Fourier and the discovery of the greenhouse effect.)
J.-L. Dufresne La météorologie , N. 53, pp. 42-46, mai 2006.
Abstract : Joseph Fourier is well known for his mathematical studies and for his work on heat diffusion. Fourier series, Fourier coefficients, Fourier integrals, Fourier analysis, Fourier equation are common expressions. Fourier most popular work is probably the "analytical theory of heat". Here we present the important contribution of J. Fourier to the Earth and space sciences by analysing his "memoir on the temperature of the Earth and planetary spaces".
Résumé Joseph Fourier est bien connu pour ses travaux mathématiques et pour ceux sur la diffusion de la chaleur. On parle de séries de Fourier, de coefficients de Fourier, d'intégrales de Fourier, d'analyse de Fourier, d'équation de Fourier. Son oeuvre probablement la plus connue est la «Théorie analytique de la chaleur». Ici nous présentons l'importante contribution de J. Fourier aux sciences de l'univers à travers l'analyse de son «mémoire sur les températures du globe terrestre et des espaces planétaires.»
Texte en pdf .- How Well do we Understand and Evaluate Climate Change Feedback Processes?
S. Bony , R. Colman, V.M. Kattsov, R.P. Allan, C.S. Bretherton, J.-L. Dufresne, A. Hall, S. Hallegatte, M.M. Holland, W. Ingram, D.A. Randall, B.J. Soden, G. Tselioudis, M.J. Webb
J. of Clim., to be published.
Abstract : Processes in the climate system that can either amplify or damp the climate response to an external perturbation are referred to as climate feedbacks. Climate sensitivity estimates depend critically on radiative feedbacks associated with water vapor, lapse-rate, clouds, snow and sea-ice, and global estimates of these feedbacks differ among general circulation models. By reviewing recent observational, numerical and theoretical studies, this paper shows that there has been progress since the Third Assessment Report of the Intergovernmental Panel of Climate Change in (i) the understanding of the physical mechanisms involved in these feedbacks, (ii) the interpretation of inter-model differences in global estimates of these feedbacks, and (iii) the development of methodologies of evaluation of these feedbacks (or of some components) using observations. This suggests that continuing developments in climate feedback research will progressively get us to the point where it will be possible to constrain the GCMs range of climate feedbacks and climate sensitivity through an ensemble of diagnostics based on physical understanding and observations.
Abstract or texte in pdf . "© American Meteorological Society." - The LMDZ4 general circulation model: climate performance and sensitivity to parametrized physics with emphasis on tropical convection
-
Marine boundary layer clouds at the heart of cloud
feedback uncertainties in climate models.
Bony, S. , J-L Dufresne;
Geophys. Res. Lett., Vol. 32, No. 20, L20806, doi: 10.1029/2005GL023851, Oct. 2005.
Abstract : The radiative response of tropical clouds to global warming exhibits a large spread among climate models, and this constitutes a major source of uncertainty for climate sensitivity estimates. To better interpret the origin of that uncertainty, we analyze the sensitivity of the tropical cloud radiative forcing to a change in sea surface temperature that is simulated by 15 coupled models simulating climate change and current interannual variability. We show that it is in regimes of large-scale subsidence that the model results (1) differ the most in climate change and (2) disagree the most with observations in the current climate (most models underestimate the interannual sensitivity of clouds albedo to a change in temperature). This suggests that the simulation of the sensitivity of marine boundary layer clouds to changing environmental conditions constitutes, currently, the main source of uncertainty in tropical cloud feedbacks simulated by general circulation models.
Texte in pdf . "© American Geophysical Union. Further reproduction or electronic distribution is not permitted."- Contrasts in the effects on climate of anthropogenic sulfate aerosols between the 20th and the 21st century
Jean-Louis Dufresne, Johannes Quaas, Olivier Boucher, Sébastien Denvil, Laurent Fairhead
Geophys. Res. Lett., Vol. 32, No. 21, L21703, doi: 10.1029/2005GL023619, Nov. 2005.
Abstract : In this study, we examine the time evolution of the relative contribution of sulfate aerosols and greenhouse gases to anthropogenic climate change. We use the new IPSL-CM4 coupled climate model for which the first indirect effect of sulfate aerosols has been calibrated using POLDER satellite data. For the recent historical period the sulfate aerosols play a key role on the temperature increase with a cooling effect of 0.5K, to be compared to the 1.4K warming due to greenhouse gas increase. In contrast, the projected temperature change for the 21st century is remarkably independent of the effects of anthropogenic sulfate aerosols for the SRES-A2 scenario. Those results are interpreted comparing the different radiative forcings, and can be extended to other scenarios. We also highlight that the first indirect effect of aerosol strongly depends on the land surface model by changing the cloud cover.
Texte (pdf) in with color figures or black and white olny "© American Geophysical Union. Further reproduction or electronic distribution is not permitted."- Net exchange reformulation of radiative transfer in the CO2 15um band on Mars
Jean-Louis Dufresne, Richard Fournier, Christophe Hourdin, Frédéric Hourdin
to be published in Journal of the Atmospheric Sciences,, 2005.
Abstract : The Net Exchange Formulation (NEF) is an alternative to the usual radiative transfer formulation. It was proposed by two authors in 1967, but until now, this formulation has been used only in a very few cases for atmospheric studies. The aim of this paper is to present the NEF and its main advantages, and to illustrate them in the case of planet Mars.
In the NEF, the radiative fluxes are no more considered. The basic variables are the net exchange rates between each pair of atmospheric layers i,j. NEF offers a meaningful matrix representation of radiative exchanges, allows to quantify the dominant contributions to the local heating rates and provides a general framework to develop approximations satisfying reciprocity of radiative transfer as well as first and second principle of thermodynamic. This may be very useful to develop fast radiative codes for GCMs.
We present a radiative code developed along those lines for a GCM of Mars. We show that computing the most important optical exchange factors at each time step and the others exchange factors only a few times a day strongly reduces the CPU time without any significant precision lost. With this solution, the CPU time increases proportionally to the number N of the vertical layers and no more proportionally to its square N^2. We also investigate some specific points such as numerical instabilities that may appear in the high atmosphere and errors that may be introduced if inappropriate treatments are performed when reflection at the surface occurs.- Simulation of Absorbing Aerosol Indices for African Dust
Masaru Yoshioka, Natalie Mahowald, Jean-Louis Dufresne, and Chao Luo
J. Geophys. Res. - Atmospheres , 110, D18S17, doi:10.1029/2004JD005276, June 2005.
Abstract : It has been speculated that the vegetation change and human land use have modulated the dust sources in North Africa and contributed to the observed increase of dust since 1960s. However, the roles of such surface disturbances on dust generation are not well constrained due to limitations in the available data and models. This study addresses this issue by simulating the Total Ozone Mapping Spectrometer (TOMS) Absorbing Aerosol Indices (AAIs) for model predicted dust and comparing them with the observations. Model simulations are conducted for natural topographic depression sources with and without adding sources due to vegetation change and cultivation over North Africa. The simulated AAIs capture the previously reported properties of TOMS AAI as well as observed magnitude and spatial distribution reasonably well, although there are some important disagreements with observations. Statistical analyses of spatial and temporal patterns of simulated AAI suggest that simulations using only the natural topographic source capture the observed patterns better than those using 50% of surface disturbance sources. The AAI gradients between Sahara (north) and Sahel (south) suggests the best mixture of surface disturbance sources are 20-25%, while spatial and temporal correlations suggests the optimum mixture of 0-15% with the upper bound of 25-40%. However, sensitivity studies show that uncertainties associated with meteorology and source parameterization are large and may undermine the findings derived from the simulations. Such uncertainties in the model simulations need to be reduced in order to constrain the roles of different types of dust sources better using AAI simulation.
Texte in (pdf) . "© American Geophysical Union. Further reproduction or electronic distribution is not permitted."- Estimates of global multicomponent aerosol optical depth and direct radiative perturbation in the Laboratoire de Météorologie Dynamique general circulation model
M. S. Reddy, O. Boucher, N. Bellouin, M. Schulz, Y. Balkanski, J-L Dufresne, and M. Pham
Journal of GeophysicalResearch - Atmospheres , 110, D10S16, doi:10.1029/2004JD004757, April 2005.
Abstract : The global cycle of multicomponent aerosols including sulfate, black carbon (BC), organic matter (OM), mineral dust, and sea salt is simulated in the Laboratoire de Me´te´orologie Dynamique general circulation model (LMDZT GCM). The seasonal open biomass burning emissions for simulation years 2000-2001 are scaled from climatological emissions in proportion to satellite detected fire counts. The emissions of dust and sea salt are parameterized online in the model. The comparison of model-predicted monthly mean aerosol optical depth (AOD) at 500 nm with Aerosol Robotic Network (AERONET) shows good agreement with a correlation coefficient of 0.57(N = 1324) and 76% of data points falling within a factor of 2 deviation. The correlation coefficient for daily mean values drops to 0.49 (N = 23,680). The absorption AOD (tau at 670 nm) estimated in the model is poorly correlated with measurements (r = 0.27, N = 349). It is biased low by 24% as compared to AERONET. The model reproduces the prominent features in the monthly mean AOD retrievals from Moderate Resolution Imaging Spectroradiometer (MODIS). The agreement between the model and MODIS is better over source and outflow regions (i.e., within a factor of 2). There is an underestimation of the model by up to a factor of 3 to 5 over some remote oceans. The largest contribution to global annual average AOD (0.12 at 550 nm) is from sulfate (0.043 or 35%), followed by sea salt (0.027 or 23%), dust (0.026 or 22%), OM (0.021 or 17%), and BC (0.004 or 3%). The atmospheric aerosol absorption is predominantly contributed by BC and is about 3% of the total AOD. The globally and annually averaged shortwave (SW) direct aerosol radiative perturbation (DARP) in clear-sky conditions is -2.17 W^2 and is about a factor of 2 larger than in all-sky conditions ( 1.04 W^2 ). The net DARP (SW + LW) by all aerosols is -1.46 and -0.59 W^2 in clear- and all-sky conditions, respectively. Use of realistic, less absorbing in SW, optical properties for dust results in negative forcing over the dust-dominated regions.
Texte in pdf . "© American Geophysical Union. Further reproduction or electronic distribution is not permitted."- SIRTA, a ground-based atmospheric observatory for cloud and aerosol research
M. Haeffelin, L. Barthes, O. Bock, C. Boitel, S. Bony, D. Bouniol, H. Chepfer, M. Chiriaco, J. Cuesta, J. Delanoe, P. Drobinski, J.-L. Dufresne, C. Flamant, M. Grall, A. Hodzic, F. Hourdin, F. Lapouge, Y. Lemaitre, A. Mathieu, Y. Morille, C. Naud, V. Noel, W. O Hirok, J. Pelon, C. Pietras, A. Protat, B. Romand, G. Scialom, and R. Vautard.
to be published in Annales Geophysicae, 2005.
Abstract : Ground-based remote sensing observatories have a crucial role to play in providing data to improve our understanding of atmospheric processes, to test the performance of atmospheric models, and to develop new methods for future space-borne observations. Institut Pierre Simon Laplace, a French research institute in environmental sciences, created the Site Instrumental de Recherche par T&eactue;l&eactue;d&eactue;tection Atmosph&eactue;rique (SIRTA), an atmospheric observatory with these goals in mind. Today SIRTA, located 20 km south of Paris, operates a suite a state-of-the-art active and passive remote sensing instruments dedicated to routine monitoring of cloud and aerosol properties, and key atmospheric parameters. Detailed description of the state of the atmospheric column is progressively archived and made accessible to the scientific community. This paper describes the SIRTA infrastructure and database, and provides an overview of the scientific research associated with the observatory. Researchers using SIRTA data conduct research on atmospheric processes involving complex interactions between clouds, aerosols and radiative and dynamic processes in the atmospheric column. Atmospheric modellers working with SIRTA observations develop new methods to test their models and innovative analyses to improve parametric representations of sub-grid processes that must be accounted for in the model. SIRTA provides the means to develop data interpretation tools for future active remote sensing missions in space (e.g. CloudSat and CALIPSO). SIRTA observation and research activities take place in networks of atmospheric observatories that allow scientists to access consistent data sets from diverse regions on the globe.
Texte in (pdf)- A boundary-Based Net Exchange Monte-Carlo Method for absorbing an scattering thick media
V. Eymet, R. Fournier, S. Blanco, J.L. Dufresne
Journal of Quantitative Spectroscopy and Radiative Transfer Volume 91, Issue 1 , 15 February 2005, Pages 27-46
Abstract : A boundary-based net-exchange Monte Carlo method was introduced (JQSRT 74 (2001) 563) that allows to bypass the difficulties encountered by standard Monte Carlo algorithms in the limit of optically thick absorption (and/or for quasi-isothermal configurations). With the present paper, this method is extended to scattering media. Developments are fully 3D, but illustrations are presented for plane parallel configuration. Compared to standard Monte Carlo algorithms, convergence qualities have been improved over a wide range of absorption and scattering optical thicknesses. The proposed algorithm still encounters a convergence difficulty in the case of optically thick, highly scattering media.
Texte in (pdf) "© Elsevier Science." - Contrasts in the effects on climate of anthropogenic sulfate aerosols between the 20th and the 21st century
- Impacts of greenhouse gases and aerosol direct and indirect
effects on clouds and radiation in atmospheric GCM simulations of the
1930-1989 period.
Johannes Quaas, Olivier Boucher, Jean-Louis Dufresne, Hervé Le Treut
Climate Dynamics, doi:10.1007/s00382-004-0475-0, Volume 23, Numbers 7-8, pp. 779-789, December 2004.
Abstract : Among anthropogenic perturbations of the Earth's atmosphere, greenhouse gases and aerosols are considered to have a major impact on the energy budget through their impact on radiative fluxes. We use three ensembles of simulations with the LMDZ general circulation model to investigate the radiative impacts of five species of greenhouse gases (CO2, CH4, N2O, CFC-11 and CFC-12) and sulfate aerosols for the period 1930-1989. Since our focus is on the atmospheric changes in clouds and radiation from greenhouse gases and aerosols, we prescribed sea surface temperatures in these simulations. Besides the direct impact on radiation through the greenhouse effect and scattering of sunlight by aerosols, strong radiative impacts of both perturbations through changes in cloudiness are analysed. The increase in greenhouse gas concentration leads to a reduction of clouds at all atmospheric levels, thus decreasing the total greenhouse effect in the longwave spectrum and increasing absorption of solar radiation by reduction of cloud albedo. Increasing anthropogenic aerosol burden results in a decrease in high-level cloud cover through a cooling of the atmosphere, and an increase in the low-level cloud cover through the second aerosol indirect effect. The trend in low-level cloud lifetime due to aerosols is quantified to 0.5 min day-1 decade-1 for the simulation period. The different changes in high (decrease) and low-level (increase) cloudiness due to the response of cloud processes to aerosols impact shortwave radiation in a contrariwise manner, and the net effect is slightly positive. The total aerosol effect including also the aerosol direct and first indirect effects remains strongly negative. Texte in (pdf)
"© ;Springer-Verlag. Further reproduction or electronic distribution is not permitted. The original publication is available at springerlink.com."
- Longwave radiative analysis of cloudy scattering atmospheres using a
Net Exchange Formulation
V. Eymet, J.L. Dufresne, P. Ricchiazzi, R. Fournier, and S. Blanco
Atmospheric Research, Volume 72, Issues 1-4, Pages 239-261 2004, November-December 2004.
Abstract : The Net Exchange Formulation (NEF) is an alternative to the usual radiative transfer equation. It was proposed in 1967 by Green for atmospheric sciences and by Hottel for engineering sciences. Until now, the NEF has been used only in a very few cases for atmospheric studies. Recently we have developped a longwave radiative code based on this formulation for a GCM of the Mars planet. Here, we will present results for the Earth atmosphere, obtained with a Monte Carlo Method based on the NEF. In this method, fluxes are not addressed any more. The basic variables are the net exchange rates (NER) between each pair of atmospheric layer (i,j), i.e. the radiative power emitted by i and absorbed by j minus the radiative power emitted by j and absorbed by i. The graphical representation of the NER matrix highlights the radiative exchanges that dominate the radiative budget of the atmosphere and allows one to have a very good insight of the radiative exchanges. Results will be presented for clear sky atmospheres with Mid-Latitude Summer and Sub-Arctic Winter temperature profiles, and for the same atmospheres with three different types of clouds. The effect of scattering on longwave radiative exchanges will also be analysed.
Texte in (pdf) "© Elsevier Science."- Will marine dimethylsulfide emissions amplify or alleviate global warming? - A model study.
L. Bopp, O. Boucher, O. Aumont, S. Belviso, J-L Dufresne, P. Montfray, M. Pham
Canadian Journal of Fisheries and Aquatic Sciences,, Vol. 61, No. 5, pp. 826-835, May 2004
Abstract : Dimethylsulfide (DMS) is the most abundant volatile sulfur compound at the sea surface and has a strong marine phytoplanktonic origin. Once outgased in the atmosphere, it is oxidized and contributes to form sulfate aerosol particles that affect the radiative budget as precursors of cloud condensation nuclei (CCN). More than 15 years ago, Charlson et al. (1987) have postulated that climate may be partly modulated by variations in DMS production. We test this hypothesis in the context of anthropogenic climate change and present here a first modeled estimate of the radiative impact due to changes in DMS air-sea fluxes caused by global warming. Following previous work on modeling the marine DMS response to climate change (Bopp et al. 2003), we use an atmospheric model of the global sulfur cycle to simulate the responses of DMS sea-to-air flux and its associated radiative impact to increased greenhouse gas concentration (a 1 % increase per year in atmospheric CO2 until the present-day concentration has doubled). At 2xCO, our model estimates a small increase (3 %) in the global DMS flux to the atmosphere but with large spatial heterogeneities (from -15 % to 30 % in the zonal mean). The radiative perturbation due to the DMS-induced change in cloud albedo is estimated to be -0.05 W.m-2, which represents only a small negative climate feedback on global warming. However there are large regional changes, such as a perturbation of up to -1.5 W.m-2 in summer between 40°S and 50°S, which can significantly impact the regional climate. In the Southern Ocean, the radiative impact due to changes in the DMS cycle may partly alleviate the radiative forcing due to anthropogenic CO2 (which is estimated between +2.5 and +3 W.m-2 at 2xCO2 compared to 1xCO2 conditions).
Keywords: dimethylsulfide, global warming, radiative impact- On dynamic and thermodynamic components of cloud changes.
S. Bony, J-L Dufresne, H. Le Treut, J-J Morcrette, and C. Senior,
Climate Dynamics, , DOI: 10.1007/s00382-003-0369-6, Vol. 22, No. 2-3, pp.71-86, March 2004
Abstract : Clouds are sensitive to changes in both the large-scale circulation and the thermodynamic structure of the atmosphere. In the Tropics, temperature changes that occur on seasonal to decadal timescales are often associated with circulation changes. Therefore, it is difficult to determine the part of cloud variations that results from a change in the dynamics from the part that may result from the temperature change itself. This study proposes a simple framework to unravel the dynamic and non-dynamic (referred to as thermodynamic) components of the cloud response to climate variations.
It is used to analyze the contrasted response, to a prescribed ocean warming, of the tropically-averaged cloud radiative forcing (CRF) simulated by the ECMWF, LMD and UKMO climate models. In each model, the dynamic component largely dominates the CRF response at the regional scale, but this is the thermodynamic component that explains most of the average CRF response to the imposed perturbation. It is shown that this component strongly depends on the behaviour of the low-level clouds that occur in regions of moderate subsidence (e.g. in the trade wind regions). These clouds exhibit a moderate sensitivity to temperature changes, but this is mostly their huge statistical weight that explains their large influence on the tropical radiation budget.
Several propositions are made for assessing the sensitivity of clouds to changes in temperature and in large-scale motions using satellite observations and meteorological analyses on the one hand, and mesoscale models on the other hand.
Texte in (pdf) "© ;Springer-Verlag. Further reproduction or electronic distribution is not permitted. The original publication is available at springerlink.com."- Sensitivity of TOMS aerosol index to boundary layer height: Implications for detection of mineral aerosol sources,
N. M. Mahowald, J-L Dufresne,
Geophys. Res. Lett., Vol. 31, No. 3, L03103, DOI: 10.1029/2003GL018865, 04 February 2004.
Abstract : The TOMS aerosol index (AI) is a powerful tool in determining the sources of mineral aerosols. The sensitivity of the AI to the height of the aerosol layer has been noted previously, but the implications of this sensitivity for deducing sources has not been explicitly considered. Here, we present a methodology and a sensitivity test to show the importance of spatial and temporal variations of the planetary boundary layer height to deducing sources using the AI. These results suggest that while dry topographic low sources may be large sources of desert dust, conclusions eliminating other sources may be premature, especially when these sources occur on the edges of deserts, where boundary layer heights are lower, and human influences more important. The compounding problem of differentiating downwind transport and local sources suggests it may not currently be possible to use the AI to conclusively determine mineral aerosol source regions.
Texte in pdf file. "© American Geophysical Union. Further reproduction or electronic distribution is not permitted." - Will marine dimethylsulfide emissions amplify or alleviate global warming? - A model study.
- How positive is the feedback between climate change and the carbon cycle.
Friedlingstein, P., J.-L. Dufresne, P.M. Cox, and P. Rayner
Tellus, april 2003, 55B (2), pp. 692-700
Abstract : Future climate change induced by atmospheric emissions of greenhouse gases is believed to have a large impact on the global carbon cycle. Several offline studies focusing either on the marine or on the terrestrial carbon cycle highlighted such potential adverse effects. Two recent online studies, using ocean-atmosphere general circulation models coupled to land and ocean carbon cycle models, investigated in a consistent way, the feedback between the climate change and the carbon cycle. These two studies used observed anthropogenic CO2 emissions for the 1860-1995 period and IPCC scenarios for the 1995-2100 period to force the climate-carbon cycle models. The study from the Hadley Centre group, showed a very large positive feedback, atmospheric CO2 reaching 980 ppmv by 2100 if future climate impacts on the carbon cycle, but only about 700 ppmv if the carbon cycle is assumed to be insensitive to the climate change. The IPSL coupled climate-carbon cycle model, simulated a much smaller positive feedback: climate impact on carbon cycle leads by 2100 to an addition of less than 100 ppmv in the atmosphere. Here, we perform a detailed feedback analysis to show that such differences are due to two key processes that are still poorly constrained in these coupled models, first Southern Ocean circulation which primarily controls the geochemical uptake of CO2, and second vegetation and soil carbon response to global warming. Our analytical analysis reproduces remarkably the results obtained by the fully coupled models. Also it allows us to identify, that amongst the two processes mentioned above, the last one, the land response to global warming is the one that essentially explains the differences between the IPSL and the Hadley results.
Texte in (pdf) "© Further reproduction or electronic distribution is not permitted."
- Global response of the terrestrial biosphere to CO2 and
climate change using a coupled climate-carbon cycle model.
M. Berthelot, P. Friedlingstein, P. Ciais, P. Monfray ,J. L. Dufresne, H. Le Treut, and L. Fairhead
Global Biogeochem. Cycles, 16(4), 1084, doi:10.1029/2001GB001827, 2002.
Abstract : We study the response of the land biosphere to climate change by coupling a climate general circulation model to a global carbon cycle model. This coupled model was forced by observed CO2 emissions for the 1860 1990 period and by the IPCC SRES-A2 emission scenario for the 1991 2100 period. During the historical period, our simulated Net Primary Production (NPP) and net land uptake (NEP) are comparable to the observations in term of trend and variability. By the end of the 21st century, we show that the global NEP is reduced by 56% due to the climate change. In the tropics, increasing temperature, through an increase of evapotranspiration, acts to reduce the soil water content, which leads to a 80% reduction of net land CO2 uptake. As a consequence, tropical carbon storage saturates by the end of the simulation, some regions becoming sources of CO2. On the contrary, in northern high latitudes, increasing temperature stimulates the land biosphere by lengthening the growing season by about 18 days by 2100 which in turn leads to a NEP increase of 11%. Overall, the negative climate impact in the tropics is much larger than the positive impact simulated in the extratropics, therefore, climate change reduce the global land carbon uptake. This constitutes a positive feedback in the climate-carbon cycle system.
Texte in pdf file. "© American Geophysical Union. Further reproduction or electronic distribution is not permitted."- Monte Carlo methods and sensitivity estimations
A. de Lataillade, S. Blanco, Y. Clergent, J.L. Dufresne, M. El Hafi and R. Fournier
J.Quant. Spectrosc. Radiat. Transfer, Vol. 75, N.5, pp.529-538, Oct 2002
Abstract : It is shown that, starting from any existing Monte Carlo algorithm for estimation of a physical quantity A, it is possible to implement a simple additional procedure that simultaneously estimates the sensitivity of A to any problem parameter. The corresponding supplementary cost is very low as no additional random sampling is required. The principle is presented on a formal basis and simple radiative transfer examples are used for illustration.
Texte in ps or pdf file. " © 2002 Published by Elsevier Science Ltd. Further reproduction or electronic distribution is not permitted."- A Net Exchange Monte Carlo approach to radiation in optically thick systems
A. de Lataillade, J.L. Dufresne, M. El Hafi, V, Eymet, R. Fournier
J.Quant. Spectrosc. Radiat. Transfer, Vol. 74, N.5, pp.563-584, Jul 2002
Abstract : A Monte Carlo approach to radiative transfer in participating media is described and tested. It solves to a large extent the well known problem of Monte Carlo simulation of optically thick absorption configurations. The approach which is based on a net-exchange formulation and on adapted optical path sampling procedures is carefully designed to insure satisfactory convergence for all types of optical thicknesses. The need for such adapted algorithms is mainly related to the problem of gaseous line spectra representation in which extremely large ranges of optical thicknesses may be simultaneously encontered. The algorithm is tested against various band average computations for simple geometries using the Malkmus statistical narrow band model.
Texte in pdf file. " © 2002 Published by Elsevier Science Ltd.Further reproduction or electronic distribution is not permitted."- Longwave Scattering Effects of Mineral Aerosols
Jean-Louis Dufresne, Catherine Gautier, Paul Ricchiazzi, Yves Fouquart
J. Atmospheric Science, Vol. 59, N.12, pp.1959-1966, 15 June 2002
Abstract : Scattering in the longwave domain has been neglected in the first generation of radiative codes and is still neglected in most current GCMs. Scattering in the longwave domain does not play any significant role for clear sky conditions but recent works have shown that it is not negligible for cloudy conditions. In this paper we highlight the importance of scattering by mineral aerosols in the longwave domain for a wide range of conditions commonly encountered during dust events. We show that neglecting scattering may lead to an underestimate of longwave aerosol forcing. This underestimate may reach 50% of the longwave forcing at the top of atmosphere and 15% at the surface for aerosol effective radius greater than a few tenths of a micron. For an aerosol optical thickness of one and for typical atmospheric conditions, the longwave forcing at the TOA increases to 8 W.m-2 when scattering effects are included. In contrast, the heating rate inside the atmosphere is only slightly affected by aerosol scattering: neglecting it leads to an underestimate by no more than 10% of the cooling caused by aerosols.
Texte in pdf or in ps file. "© 2002 American Meteorological Society. Further reproduction or electronic distribution is not permitted."- On the magnitude of positive feedback between future \\ climate change and the carbon cycle.
Dufresne J-L, Friedlingstein P, Berthelot M, Bopp L, Ciais P, L. Fairhead, H. LeTreut, P. Monfray
Geophys. Res. Lett., 29(10), 10.1029/2001GL013777, 23 May 2002
Abstract : We use an ocean-atmosphere general circulation model coupled to land and ocean carbon models to simulate the evolution of climate and atmospheric CO2 from 1860 to 2100. Our model reproduces the observed global mean temperature changes and the growth rate of atmospheric CO2 for the period 1860-2000. For the future, we simulate that the climate change due to CO2 increase will reduce the land carbon uptake, leaving a larger fraction of anthropogenic CO2 in the atmosphere. By 2100, we estimate that atmospheric CO2 will be 18% higher due to the climate change impact on the carbon cycle. Such a positive feedback has also been found by [Cox et al.,2000]. However, the amplitude of our feedback is three times smaller than the one they simulated. We show that the partitioning between carbon stored in the living biomass or in the soil, and their respective sensitivity to increased CO2 and climate change largely explain this discrepancy.
Texte in pdf or in ps file. "© American Geophysical Union. Further reproduction or electronic distribution is not permitted." - Monte Carlo methods and sensitivity estimations
- Positive feedback between future climate change and the carbon cycle.
Friedlingstein P, Bopp L, Ciais P, Dufresne JL, Fairhead L, LeTreut H, Monfray P, Orr J,
Geophys. Res. Lett., Vol.28, No.8, pp. 1543-1546, Apr 2001
Abstract : Future climate change due to increased atmospheric CO2 may affect land and ocean efficiency to absorb atmospheric CO2. Here, using climate and carbon three-dimensional models forced by a 1% per year increase in atmospheric CO2, we show that there is a positive feedback between the climate system and the carbon cycle. Climate change reduces land and ocean uptake of CO2, respectively by 54% and 35% at 4 x CO2. This negative impact implies that for prescribed anthropogenic CO2 emissions, the atmospheric CO2 would be higher than the level reached if climate change does not affect the carbon cycle. We estimate the gain of this climate-carbon cycle feedback to be 10% at 2 x CO2 and 20% at 4 x CO2. This translates into a 15% higher mean temperature increase.
Texte in pdf or in ps file. "© 2001 American Geophysical Union. Further reproduction or electronic distribution is not permitted."
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Les rétroactions " climat-carbone " associées aux rejets anthropiques de CO2
à l'aide du modèle couplé de l'IPSL : un possible effet amplificateur?
J-L.Dufresne,P. Friedlingstein
Lettre n.11 du Programme International Géosphère Biosphère-Programme Mondial de Recherches sur le Climat (PIGB-PMRC)
Résumé : Depuis plus d'un siècle, la concentration en CO2 atmosphérique mesurée a augmenté de 25% passant de 280 ppmv en 1860 à 360 ppmv de nos jours. Cette augmentation serait environ deux fois plus forte si tout le CO2 émis par les activités humaines restaient dans l'atmosphère ; environ la moitié de ce CO2 émis est capté par la biosphère et par l'océan. Comment réagissent ces puits à un changement climatique?
- The influence of resolution in simulating inter-annual and
inter-decadal variability in a coupled ocean-atmosphere GCM,
with emphasis over the North Atlantic
C. Laurent, H Le Treut, Z.X.Li, L. Fairhead and J.L. Dufresne
Notes du pôle de modélisation de l'IPSL, N.8, 1998
Abstract : Multidecadal numerical experiments have been realized at different resolutions using the coupled ocean-atmosphere general circulation model developed at IPSL (Institut Pierre Simon Laplace) in Paris. The atmospheric LMD model and the oceanic OPA-LODYC model are coupled without any flux adjustment. The mean simulated climate is stable and differs from reality through a few well established systematic errors. The variability simulated by the model at these different resolutions is compared for various areas, and more specifically the North Atlantic. A statistical method, the Multi-channel Singular Spectrum Analysis (M-SSA), is used to detect sea surface temperature oscillations at different time scales. Oscillations over the North Atlantic display a well-known tripole feature, which seems to depend primarily on the ocean resolution. The period of the oscillation (from 8 to 12 years) seems more sensitive to the atmospheric resolution.- Simulations couplées globales des changements climatiques associés a une augmentation de la teneur atmosphérique en CO2
( Global coupled simulations of climate change due to increased atmospheric CO2 concentration )
P. Barthelet, S. Bony, P. Braconnot, A. Braun, D. Cariolle, E. Cohen-Solal, J.-L. Dufresne, P. Delecluse, M. Deque, L. Fairhead, M.-A. Filiberti, M. Forichon, J.-Y. Grandpeix, E. Guilyardi, M.-N. Houssais, M. Imbard, H. Le Treut, C. Levy, Z.X. Li, G. Madec, P. Marquet, O. Marti, S. Planton, L. Terray, O. Thual, S. Valcke
Compte Rendu Académie des Sciences Paris, Série II a, Vol.326, No.10, p.677-684, mai 1998
Abstract : Two transient CO2 experiments using two coupled general circulation models developped by the french GASTON group have been realized using the same methodology . No flux corrections at the air-sea interface were used in these experiments. The main features of the present climate are reasonably well captured by both coupled models in the control simulations, although the biases are not the same. The transient CO2 simulations show a global warming, ranging between 1.6 C and 2.0 C at the time of CO2 doubling(+70 years). These values, and the main geographical characteristics of climate change are in agreement with previous studies published by other research groups, using either flux corrected or non-flux corrected models.
Résumé : Deux expériences climatiques d'augmentation de la teneur en CO2 atmosphérique, utilisant deux modèles de circulation générale couplés océan/glace/atmosphère développés au sein du groupe GASTON, ont été réalisées selon la même méthodologie, sans correction de flux artificielle à l'interface air-mer. Les simulations de contrôle reproduisent convenablement les caractéristiques principales du climat actuel, les biais des deux modèles couplés étant cependant différents. Les simulations de scénario d'augmentation de la teneur en CO2 atmosphérique (1% par an) indiquent un réchauffement global de la température à la surface du globe compris entre 1,6°C et 2,0°C au moment du doublement de la teneur en CO2 (+70 ans). L'amplitude ainsi que la répartition géographique du réchauffement sont en accord avec les résultats d'autres groupes de recherche utilisant des modèles à flux corrigés ou non. - Simulations couplées globales des changements climatiques associés a une augmentation de la teneur atmosphérique en CO2
- Monte Carlo simulation of radiation in gases with a narrow-band
model and a Net-Exchange Formulation.
Cherkaoui M, Dufresne JL, Fournier R, Grandpeix JY, Lahellec A;
ASME Journal of Heat Transfer, Vol.118, No.2, pp. 401-407, May 1996
Abstract : The Monte Carlo method is used for simulation of radiative heat transfers in non-gray gases. The proposed procedure is based on a Net-Exchange Formulation (NEF). Such a formulation provides an efficient way of systematically fulfilling the reciprocity principle, which avoids some of the major problems usually associated with the Monte Carlo method~: numerical efficiency becomes independent of optical thickness, strongly non uniform grid sizes can be used with no increase in computation times and configurations with small temperature differences can be addressed with very good accuracy. The Exchange Monte Carlo Method (EMCM) is detailed for a one-dimensional slab with diffusely or specularly reflecting surfaces.- Raccordement des Modèles Thermodynamiques de Glace, d'Océan et d'Atmosphère
( Sea-ice/ocean/atmosphere thermodynamic coupling )
Dufresne J.L, Grandpeix J.Y.
Laboratoire de Météorologie Dynamique, Note Interne 205, juin 1996
Abstract : L'interface océan-glace de mer-atmosphère est extrêmement hétérogène. Le modèle de glace-océan du LODYC, utilisé par la comunauté "GASTON", reproduit en partie cette hétérogénéité (mailles mixtes contenant de l'océan libre et différents types de glace) alors que les modèles d'atmosphère (Arpège, LMD 5) ne la reproduisent pas ou peu (mailles homogènes ou mailles mixtes océan libre - un seul type de glace). De plus, aux hautes latitudes, une maille atmosphérique recouvre un nombre élevé de mailles océaniques.
Nous avons développé un modèle de raccordement qui répartit de façon différenciée les flux calculés par le modèle d'atmosphère sur les diff'erents types de surface (océan libre ou variétés de glace) en fonction de leur distribution statistique (fraction surfacique) et de leur caracteristiques individuelles (temp. de surface, albédo, etc...). Ce modèle de raccordement garantit la conservation des flux à l'interface.
Dans cette note, nous abordons tout d'abord le problème de la stabilité numérique du raccordement glace-atmosphère, puis présentons le modèle de raccordement développé.- Monte Carlo simulation of radiation in gases with a narrow-band model and a Net-Exchange Formulation : numerical results for one dimesionnal configurations;
Cherkaoui M., Dufresne J.L., Fournier R. , Grandpeix J.Y. , Lahellec A.
Laboratoire de Météorologie Dynamique, Internal Report 204, Jun 1996
Abstract : Additional numerical results to the paper of (Cherkaoui et al., 1996) - Raccordement des Modèles Thermodynamiques de Glace, d'Océan et d'Atmosphère
- Procédure d'identification inclusive d'un système
thermique. Etude de cas : caractérisation d'un capteur solaire à air
en régime dynamique;
( Inclusive identification procedure for a thermal system dynamical characterization. Case study : an air solar collector wall componenet. )
Dufresne, J.L., Lahellec, A., Chounet L.M.,
Revue de Physique Appliquée, Vol.25, No.11, p. 1139-1160, Nov. 1990
Abstract. - The dynamical characterisation of a solar component - taken as a case study - introduced us to the general problem of non linear multiparametric model identification (10 parameters in our case). The elaboration of such a model has three goals : first, it has to allow the designer to understand and improve his components ; then it can be integrated in a global'system model (the building and control devices) for real performances study ; and finally, it is a guidance for the monitoring process in the purpose of fulfilling the two previous objectives (global coherence). The method and its implementation are described, and the results obtained for our case study given ; we set a particular emphasis on the embedded coherence between the monitoring stage - performed in a "Technical Center" in the context of a normalisation procedure - and the physical model parameters identification. In opposition to classical measurement operation which allow different experiments to decouple each parameter, the described method determines at once the whole set of parameters versus the whole set of available experimental data (inclusive identification). We then display a few procedure intrinsic coherence tests as well as the accuracy obtained on the determined parameters. One of the critical point of the procedure is the measurement error analysis, because of its importance not only for the resulting model accuracy, but as an essential information for a proper use of the experimental data in the identification procedure itself.Résumé. - Une étude de cas concernant la caractérisation en régime dynamique d'un composant solaire de bâtiment nous a permis d'aborder le problème général que soulève l'identification d'un modèle physique, non linéaire, à nombre élevé de paramètres libres (ici dix). Le point de vue retenu est que le modèle à construire et à déterminer a trois objectifs couplés : apporter au concepteur une compréhension suffisante du fonctionnement du composant afin de guider ses tentatives d'améliorations, autoriser son intégration dans un système plus vaste (modèle de bâtiment et d'organes de commande) afin d'éprouver ses performances en situation réelle, et enfin guider le choix de séquences expérimentales pour répondre aux mieux aux deux objectifs précédents (cohérence globale). On décrit la méthode, sa mise en oeuvre, et on donne les résultats obtenus en mettant en évidence la cohérence explicite qui est établie entre la phase expérimentale - effectuée dans un contexte de normalisation par un centre technique - et la détermination proprement dite des grandeurs physiques du modèle. Contrairement aux méthodes classiques de mesure pour lesquelles on peut concevoir des expériences découplant chacun des paramètres, la procédure choisie identifie d'un coup l'ensemble des paramètres libres sur l'ensemble des données expérimentales (identification inclusive). Un accent particulier est mis sur l'analyse des erreurs de mesure dont l'importance est manifeste, non seulement pour déterminer la précision du modèle obtenu, mais également pour utiliser correctement l'ensemble de l'information expérimentale pour la phase d'identification proprement dite.
- Présentation d'une procédure expérimentale pour la caractérisation d'un composant en régime dynamique : application à un mur-capteur solaire à air
(Experimental procedure for dynamic characterisation of a thermal component. Example : wall-air solar collecter model identification)
Dufresne, J.L., Chounet L.M., Picard D., Gaillard P., Noppe J.M.,
Revue Générale de Thermique, Vol.29, No.339, pp. 135-151, mars 1990
Résumé. - Presentation d' une procedure experimentale d'identification des modeles permettant de caracteriser precisement en regime dynamique des composants physiquement complexes. Application a l'etude d' un mur-capteur solaire a air. Description de l'installation experimentale- Approche thermique, économique et architecturale de la réhabilitation des grands ensembles d'habitation ;
Chounet L.M., Dufresne, J.L., Franchisseur R., Lahellec, A., Rego-Texeira A.,
Revue Générale de Thermique, Vol.28, No.335-336, p. 700-711, nov-dec 1989
Résumé. - L'evaluation d'operations de rehabilitation requiert du point de vue energetique un diagnostic initial correct et un outil de prevision suffisamment fiable. Nous presentons cet ensemble methodologique sur le suivi d'une operation prototype de renovation d'une cite OPHLM de 593 logements de la region parisienne. Nous montrons qu'un code de calcul comme CALECO-DOE2, tres repandu est tout a fait adapte a ce type d'analyse. Une analyse de credibilite de l'ensemble de la demarche est resumee. Les conclusions portent sur les apports methodologiques de notre travail et leurs limites, ainsi que sur le developpement souhaitable des outils de modelisation
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- Présentation d'une procédure expérimentale pour la caractérisation d'un composant en régime dynamique : application à un mur-capteur solaire à air