Climate change projections using the IPSL-CM5 Earth System Model: from CMIP3 to CMIP5

We present the global general circulation model IPSL-CM5 developed to study the long-term response of the climate system to natural and anthropogenic forcings as part of the 5th Phase of the Coupled Model Intercomparison Project (CMIP5). This model includes an interactive carbon cycle, a representation of tropospheric and stratospheric chemistry, and a comprehensive representation of aerosols. As it represents the principal dynamical, physical, and bio-geochemical processes relevant to the climate system, it may be referred to as an Earth System Model. However, the IPSL-CM5 model may be used in a multitude of configurations associated with different boundary conditions and with a range of complexities in terms of processes and interactions. This paper presents an overview of the different model components and explains how they were coupled and used to simulate historical climate changes over the past 150 years and different scenarios of future climate change. A single version of the IPSL-CM5 model (IPSL-CM5A-LR) was used to provide climate projections associated with different socio-economic scenarios, including the different Representative Concentration Pathways considered by CMIP5 and several scenarios from the Special Report on Emission Scenarios considered by CMIP3. Results suggest that the magnitude of global warming projections primarily depends on the socio-economic scenario considered, that there is potential for an aggressive mitigation policy to limit global warming to about two degrees, and that the behavior of some components of the climate system such as the Arctic sea ice and the Atlantic Meridional Overturning Circulation may change drastically by the end of the twenty-first century in the case of a no climate policy scenario. Although the magnitude of regional temperature and precipitation changes depends fairly linearly on the magnitude of the projected global warming (and thus on the scenario considered), the geographical pattern of these changes is strikingly similar for the different scenarios. The representation of atmospheric physical processes in the model is shown to strongly influence the simulated climate variability and both the magnitude and pattern of the projected climate changes

Use of A-train satellite observations (CALIPSO-PARASOL) to evaluate tropical cloud properties in the LMDZ5 GCM

International audienceThe evaluation of key cloud properties such as cloud cover, vertical profile and optical depth as well as the analysis of their intercorrelation lead to greater confidence in climate change projections. In addition, the comparison between observations and parameterizations of clouds in climate models is improved by using collocated and instantaneous data of cloud properties. Simultaneous and independent observations of the cloud cover and its three-dimensional structure at high spatial and temporal resolutions are made possible by the new space-borne multi-instruments observations collected with the A-train. The cloud cover and its vertical structure observed by CALIPSO and the visible directional reflectance (a surrogate for the cloud optical depth) observed by PARASOL, are used to evaluate the representation of cloudiness in two versions of the atmospheric component of the IPSL-CM5 climate model (LMDZ5). A model-to-satellite approach, applying the CFMIP Observation Simulation Package (COSP), is used to allow a quantitative comparison between model results and observations. The representation of clouds in the two model versions is first evaluated using monthly mean data. This classical approach reveals biases of different magnitudes in the two model versions. These biases consist of (1) an underestimation of cloud cover associated to an overestimation of cloud optical depth, (2) an underestimation of low- and mid-level tropical clouds and (3) an overestimation of high clouds. The difference in the magnitude of these biases between the two model versions clearly highlights the improvement of the amount of boundary layer clouds, the improvement of the properties of high-level clouds, and the improvement of the simulated mid-level clouds in the tropics in LMDZ5B compared to LMDZ5A, due to the new convective, boundary layer, and cloud parametrizations implemented in LMDZ5B. The correlation between instantaneous cloud properties allows for a process-oriented evaluation of tropical oceanic clouds. This process-oriented evaluation shows that the cloud population characterized by intermediate values of cloud cover and cloud reflectance can be split in two groups of clouds when using monthly mean values of cloud cover and cloud reflectance: one group with low to intermediate values of the cloud cover, and one group with cloud cover close to one. The precise determination of cloud height allows us to focus on specific types of clouds (i.e. boundary layer clouds, high clouds, low-level clouds with no clouds above). For low-level clouds over the tropical oceans, the relationship between instantaneous values of the cloud cover and of the cloud reflectance reveals a major bias in the simulated liquid water content for both model versions. The origin of this bias is identified and possible improvements, such as considering the sub-grid heterogeneity of cloud properties, are investigated using sensitivity experiments. In summary, the analysis of the relationship between different instantaneous and collocated variables allows for process-oriented evaluations. These evaluations may in turn help to improve model parameterizations, and may also help to bridge the gap between model evaluation and model development

Erratum: Erratum to: Use of A-train satellite observations (CALIPSO-PARASOL) to evaluate tropical cloud properties in the LMDZ5 GCM

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Influence combinée de la physique atmosphérique et de l'hydrologie sur la météorologie simulée au SIRTA

Le couplage surface/atmosphère est essentiel pour déterminer les conditions aux limites en eau et énergie du système atmosphérique sur les surfaces continentales. Il a été montré qu'il était à l'origine du biais chaud dont le modèle de climat de l'IPSL souffre en été aux moyennes latitudes. Les contributions du modèle d'hydrologie, et des paramétrisations de la couche limite ont été étudiées en détail en utilisant l'approche " zoomée-guidée " (Coindreau et al. 2007) et les observations recueillies et élaborées au SIRTA (Chéruy et al. 2011). Le modèle d'hydrologie à 11 couches (De Rosnay, 2000) qui assure une description physique des flux verticaux d'eau dans le sol couplé avec LMDZ a permis d'accroitre l'évaporation du sol en été, évitant de simuler de " fausses sécheresses " responsables du biais chaud. Cette amélioration est constatée avec les versions standard (ST) comme " nouvelle physique " (NP) du modèle atmosphérique LMDZ5 (Hourdin et al. 2011). Les nouvelles paramétrisations physiques proposées dans le modèle LMDZ5-NP (Hourdin et al. 2011), assurent une représentation plus réaliste des nuages de couche limite de type " petits cumulus " en été. Ces améliorations sont également observées dans les simulations de type climatique effectuées en mode forcé pour les SST. Nous sommes ainsi mieux armés pour représenter les évènements extrêmes pour lesquels le couplage sol/atmosphère est essentiel comme par exemples les canicules estivales

Combined influence of atmospheric physics and soil hydrology on the simulated meteorology at the SIRTA atmospheric observatory

International audienceThe identification of the land-atmosphere interactions as one of the key source of uncertainty in climate models calls for process-level assessment of the coupled atmosphere/land continental surface system in numerical climate models. To this end, we propose a novel approach and apply it to evaluate the standard and new parametrizations of boundary layer/convection/clouds in the Earth System Model (ESM) of Institut Pierre Simon Laplace (IPSL), which differentiate the IPSL-CM5A and IPSL-CM5B climate change simulations produced for the Coupled Model Inter-comparison Project phase 5 exercise. Two different land surface hydrology parametrizations are also considered to analyze different land-atmosphere interactions. Ten-year simulations of the coupled land surface/atmospheric ESM modules are confronted to observations collected at the SIRTA (Site Instrumental de Recherche par Télédection Atmosphérique), located near Paris (France). For sounder evaluation of the physical parametrizations, the grid of the model is stretched and refined in the vicinity of the SIRTA, and the large scale component of the modeled circulation is adjusted toward ERA-Interim reanalysis outside of the zoomed area. This allows us to detect situations where the parametrizations do not perform satisfactorily and can affect climate simulations at the regional/continental scale, including in full 3D coupled runs. In particular, we show how the biases in near surface state variables simulated by the ESM are explained by (1) the sensible/latent heat partitionning at the surface, (2) the low level cloudiness and its radiative impact at the surface, (3) the parametrization of turbulent transport in the surface layer, (4) the complex interplay between these processes. We also show how the new set of parametrizations can improve these biases

Understanding the West African Monsoon from the analysis of diabatic heating distributions as simulated by climate models

International audienceVertical and horizontal distributions of diabatic heating in the West African monsoon (WAM) region as simulated by four model families are analyzed in order to assess the physical processes that affect the WAM circulation. For each model family, atmosphere-only runs of their CMIP5 configurations are compared with more recent configurations which are on the development path toward CMIP6. The various configurations of these models exhibit significant differences in their heating/moistening profiles, related to the different representation of physical processes such as boundary layer mixing, convection, large-scale condensation and radiative heating/cooling. There are also significant differences in the models' simulation of WAM rainfall patterns and circulations. The weaker the radiative cooling in the Saharan region, the larger the ascent in the rainband and the more intense the monsoon flow, while the latitude of the rainband is related to heating in the Gulf of Guinea region and on the northern side of the Saharan heat low. Overall, this work illustrates the difficulty experienced by current climate models in representing the characteristics of monsoon systems, but also that we can still use them to understand the interactions between local subgrid physical processes and the WAM circulation. Moreover, our conclusions regarding the relationship between errors in the large-scale circulation of the WAM and the structure of the heating by small-scale processes will motivate future studies and model development

Control of deep convection by sub-cloud lifting processes: The ALP closure in the LMDZ5B general circulation model

International audienceRecently, a new conceptual framework for deep convection scheme triggering and closure has been developed and implemented in the LMDZ5B general circulation model, based on the idea that deep convection is controlled by sub-cloud lifting processes. Such processes include boundary-layer thermals and evaporatively-driven cold pools (wakes), which provide an available lifting energy that is compared to the convective inhibition to trigger deep convection, and an available lifting power (ALP) at cloud base, which is used to compute the convective mass flux assuming the updraft vertical velocity at the level of free convection. While the ALP closure was shown to delay the local hour of maximum precipitation over land in better agreement with observations, it results in an underestimation of the convection intensity over the tropical ocean both in the 1D and 3D configurations of the model. The specification of the updraft vertical velocity at the level of free convection appears to be a key aspect of the closure formulation, as it is weaker over tropical ocean than over land and weaker in moist mid-latitudes than semi-arid regions. We propose a formulation making this velocity increase with the level of free convection, so that the ALP closure is adapted to various environments. Cloud-resolving model simulations of observed oceanic and continental case studies are used to evaluate the representation of lifting processes and test the assumptions at the basis of the ALP closure formulation. Results favor closures based on the lifting power of sub-grid sub-cloud processes rather than those involving quasi-equilibrium with the large-scale environment. The new version of the model including boundary-layer thermals and cold pools coupled together with the deep convection scheme via the ALP closure significantly improves the representation of various observed case studies in 1D mode. It also substantially modifies precipitation patterns in the full 3D version of the model, including seasonal means, diurnal cycle and intraseasonal variability. © 2012 Springer-Verlag