Impact of the LMDZ atmospheric grid configuration on the climate and sensitivity of the IPSL-CM5A coupled model

Climate Dynamics, Mar 2013

The IPSL-CM5A climate model was used to perform a large number of control, historical and climate change simulations in the frame of CMIP5. The refined horizontal and vertical grid of the atmospheric component, LMDZ, constitutes a major difference compared to the previous IPSL-CM4 version used for CMIP3. From imposed-SST (Sea Surface Temperature) and coupled numerical experiments, we systematically analyze the impact of the horizontal and vertical grid resolution on the simulated climate. The refinement of the horizontal grid results in a systematic reduction of major biases in the mean tropospheric structures and SST. The mid-latitude jets, located too close to the equator with the coarsest grids, move poleward. This robust feature, is accompanied by a drying at mid-latitudes and a reduction of cold biases in mid-latitudes relative to the equator. The model was also extended to the stratosphere by increasing the number of layers on the vertical from 19 to 39 (15 in the stratosphere) and adding relevant parameterizations. The 39-layer version captures the dominant modes of the stratospheric variability and exhibits stratospheric sudden warmings. Changing either the vertical or horizontal resolution modifies the global energy balance in imposed-SST simulations by typically several W/m2 which translates in the coupled atmosphere-ocean simulations into a different global-mean SST. The sensitivity is of about 1.2 K per 1 W/m2 when varying the horizontal grid. A re-tuning of model parameters was thus required to restore this energy balance in the imposed-SST simulations and reduce the biases in the simulated mean surface temperature and, to some extent, latitudinal SST variations in the coupled experiments for the modern climate. The tuning hardly compensates, however, for robust biases of the coupled model. Despite the wide range of grid configurations explored and their significant impact on the present-day climate, the climate sensitivity remains essentially unchanged.

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Impact of the LMDZ atmospheric grid configuration on the climate and sensitivity of the IPSL-CM5A coupled model

Frederic Hourdin 0 1 2 3 Marie-Alice Foujols 0 1 2 3 Francis Codron 0 1 2 3 Virginie Guemas 0 1 2 3 Jean-Louis Dufresne 0 1 2 3 Sandrine Bony 0 1 2 3 Sebastien Denvil 0 1 2 3 Lionel Guez 0 1 2 3 Francois Lott 0 1 2 3 Josefine Ghattas 0 1 2 3 Pascale Braconnot 0 1 2 3 Olivier Marti 0 1 2 3 Yann Meurdesoif 0 1 2 3 Laurent Bopp 0 1 2 3 0 F. Codron V. Guemas J.-L. Dufresne S. Bony L. Guez F. Lott LMD, Paris, France 1 M.-A. Foujols S. Denvil J. Ghattas Institut Pierre-Simon Laplace (IPSL) , CNRS-UPMC, UVSQ, CEA, Paris, France 2 F. Hourdin (&) Laboratoire de Meteorologie Dynamique , (LMD/IPSL), CNRS-UPMC-ENS-EP, Tr 45-55, 3e et, B99 Jussieu, Paris 75005, France 3 P. Braconnot O. Marti Y. Meurdesoif L. Bopp Laboratoire des Sciences du Climate et de l'Environnement (LSCE/IPSL) , CNRS-CEA-UVSQ, Saclay, France The IPSL-CM5A climate model was used to perform a large number of control, historical and climate change simulations in the frame of CMIP5. The refined horizontal and vertical grid of the atmospheric component, LMDZ, constitutes a major difference compared to the previous IPSL-CM4 version used for CMIP3. From imposedSST (Sea Surface Temperature) and coupled numerical experiments, we systematically analyze the impact of the horizontal and vertical grid resolution on the simulated climate. The refinement of the horizontal grid results in a systematic reduction of major biases in the mean tropospheric structures and SST. The mid-latitude jets, located too close to the equator with the coarsest grids, move poleward. This robust feature, is accompanied by a drying at mid-latitudes and a reduction of cold biases in mid-latitudes relative to the equator. The model was also extended to the stratosphere by increasing the number of layers on the vertical from 19 to 39 (15 in the stratosphere) and adding relevant parameterizations. The 39-layer version captures the dominant modes of the stratospheric variability and exhibits stratospheric sudden warmings. Changing either the vertical or horizontal resolution modifies the global energy balance in imposedSST simulations by typically several W/m2 which translates in the coupled atmosphere-ocean simulations into a different global-mean SST. The sensitivity is of about 1.2 K per 1 W/m2 when varying the horizontal grid. A re-tuning of model parameters was thus required to restore this energy balance in the imposed-SST simulations and reduce the biases in the simulated mean surface temperature and, to some extent, latitudinal SST variations in the coupled experiments for the modern climate. The tuning hardly compensates, however, for robust biases of the coupled model. Despite the wide range of grid configurations explored and their significant impact on the present-day climate, the climate sensitivity remains essentially unchanged. 1 Introduction Numerical simulations with general circulation models are at the heart of climate change studies. They are used to quantify the impact of greenhouse gas increase on the evolution of the global climate, to unravel the physical mechanisms that control climate sensitivity, and to verify theoretical hypotheses or mechanisms while taking into account the complexity of the climate system. Those numerical models however still provide only an approximate representation of the real climate system, which constitutes a major source of uncertainty for assessing future climate changes. Improving the models should therefore be one of the main drivers of climate research. Among the limitations often emphasized is the rather coarse spatial resolution of the models used for long-term climate change simulations, such as those coordinated by the Coupled Model Intercomparison Project (CMIP, Meehl et al. 2007; Taylor et al. 2012). It is partly because of this coarse resolution that key processes such as convection or clouds have to be parameterized. Systematic centennial global simulations with meshes of the order of 50 m, which would be required to explicitly represent boundary layer clouds, will not be reachable before at least a couple of decades. It is however expected that significant improvements can already be achieved by increasing the spatial resolution of current climate models from a few hundreds to a few tens of kilometers, both because it allows a better resolution of the dominant atmospheric large scale dynamics and because it offers a finer description of surface conditions (orography, land/sea distribution). Among the expected improvements are a reduction of systematic biases in temperature, precipitation and winds (Pope and Stratton 2002; Roeckner et al. 2006; Hack et al. 2006), a better representation of the regional-scale climate (Williamson et al. 1995; Kobayashi and Sugi 2004; Navarra 2008; Byrkjedal et al. 2008), and a better representation of rainfall distributions (Kiehl and Williamson 1991; Deque et al. 1994). An important question in the frame of climate change simulations is to know whether the model limitations, and in part (...truncated)


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Frédéric Hourdin, Marie-Alice Foujols, Francis Codron, Virginie Guemas, Jean-Louis Dufresne, Sandrine Bony, Sébastien Denvil, Lionel Guez, François Lott, Josefine Ghattas, Pascale Braconnot, Olivier Marti, Yann Meurdesoif, Laurent Bopp. Impact of the LMDZ atmospheric grid configuration on the climate and sensitivity of the IPSL-CM5A coupled model, Climate Dynamics, 2013, pp. 2167-2192, Volume 40, Issue 9-10, DOI: 10.1007/s00382-012-1411-3