LMDZ5B: the atmospheric component of the IPSL climate model with revisited parameterizations for clouds and convection

Climate Dynamics, Apr 2012

Based on a decade of research on cloud processes, a new version of the LMDZ atmospheric general circulation model has been developed that corresponds to a complete recasting of the parameterization of turbulence, convection and clouds. This LMDZ5B version includes a mass-flux representation of the thermal plumes or rolls of the convective boundary layer, coupled to a bi-Gaussian statistical cloud scheme, as well as a parameterization of the cold pools generated below cumulonimbus by re-evaporation of convective precipitation. The triggering and closure of deep convection are now controlled by lifting processes in the sub-cloud layer. An available lifting energy and lifting power are provided both by the thermal plumes and by the spread of cold pools. The individual parameterizations were carefully validated against the results of explicit high resolution simulations. Here we present the work done to go from those new concepts and developments to a full 3D atmospheric model, used in particular for climate change projections with the IPSL-CM5B coupled model. Based on a series of sensitivity experiments, we document the differences with the previous LMDZ5A version distinguishing the role of parameterization changes from that of model tuning. Improvements found previously in single-column simulations of case studies are confirmed in the 3D model: (1) the convective boundary layer and cumulus clouds are better represented and (2) the diurnal cycle of convective rainfall over continents is delayed by several hours, solving a longstanding problem in climate modeling. The variability of tropical rainfall is also larger in LMDZ5B at intraseasonal time-scales. Significant biases of the LMDZ5A model however remain, or are even sometimes amplified. The paper emphasizes the importance of parameterization improvements and model tuning in the frame of climate change studies as well as the new paradigm that represents the improvement of 3D climate models under the control of single-column case studies simulations.

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LMDZ5B: the atmospheric component of the IPSL climate model with revisited parameterizations for clouds and convection

Frederic Hourdin 0 1 2 3 Jean-Yves Grandpeix 0 1 2 3 Catherine Rio 0 1 2 3 Sandrine Bony 0 1 2 3 Arnaud Jam 0 1 2 3 Frederique Cheruy 0 1 2 3 Nicolas Rochetin 0 1 2 3 Laurent Fairhead 0 1 2 3 Abderrahmane Idelkadi 0 1 2 3 Ionela Musat 0 1 2 3 Jean-Louis Dufresne 0 1 2 3 Alain Lahellec 0 1 2 3 Marie-Pierre Lefebvre 0 1 2 3 Romain Roehrig 0 1 2 3 0 M.-P. Lefebvre CNRM-Game, Meteo-France and CNRS, Toulouse, France 1 J.-Y. Grandpeix C. Rio S. Bony A. Jam F. Cheruy N. Rochetin L. Fairhead A. Idelkadi I. Musat J.-L. Dufresne A. Lahellec M.-P. Lefebvre LMD, Paris, France 2 F. Hourdin (&) Laboratoire de Meteorologie Dynamique , IPSL, UPMC, Tr 45-55, 3e et, B99, Jussieu, 75005 Paris, France 3 R. Roehrig CNRM-GAME, Toulouse, France Based on a decade of research on cloud processes, a new version of the LMDZ atmospheric general circulation model has been developed that corresponds to a complete recasting of the parameterization of turbulence, convection and clouds. This LMDZ5B version includes a mass-flux representation of the thermal plumes or rolls of the convective boundary layer, coupled to a bi-Gaussian statistical cloud scheme, as well as a parameterization of the cold pools generated below cumulonimbus by reevaporation of convective precipitation. The triggering and closure of deep convection are now controlled by lifting processes in the sub-cloud layer. An available lifting energy and lifting power are provided both by the thermal plumes and by the spread of cold pools. The individual parameterizations were carefully validated against the results of explicit high resolution simulations. Here we present the work done to go from those new concepts and developments to a full 3D atmospheric model, used in particular for climate change projections with the IPSLCM5B coupled model. Based on a series of sensitivity experiments, we document the differences with the previous LMDZ5A version distinguishing the role of parameterization changes from that of model tuning. Improvements found previously in single-column simulations of case studies are confirmed in the 3D model: (1) the convective boundary layer and cumulus clouds are better represented and (2) the diurnal cycle of convective rainfall over continents is delayed by several hours, solving a longstanding problem in climate modeling. The variability of tropical rainfall is also larger in LMDZ5B at intraseasonal timescales. Significant biases of the LMDZ5A model however remain, or are even sometimes amplified. The paper emphasizes the importance of parameterization improvements and model tuning in the frame of climate change studies as well as the new paradigm that represents the improvement of 3D climate models under the control of single-column case studies simulations. 1 Introduction The representation of turbulent, convective and cloud processes is critical for climate modeling for a series of reasons. Clouds affect the latitudinal gradients of diabatic heating in the atmosphere, thereby forcing the general circulation. Their representation is key for the simulation of prominent climate features such as the Inter Tropical Convergence Zone (ITCZ) organization (Lindzen and Hou 1988; Hou and Lindzen 1992) or Madden-Julian Oscillation (Zhang 2005). Cloud feedbacks also constitute a major source of dispersion in global warming projections (Bony and Dufresne 2005; Webb et al. 2006). A good representation of boundary layer and convective processes is also a key issue for the coupling with the other components of the climate system: surface energy fluxes (which depend on turbulence and clouds) and rainfall for coupling with the ocean and continental surfaces, vertical transport of gaseous molecules or lifting and scavenging of aerosols. It is also essential for so-called impact studies which generally rely on statistics on the near surface meteorological variables and fluxes which determine the climates in the geographers sense. In the last two decades, significant progress was made in the understanding of cloud and convective processes and paths towards new parameterizations were proposed. These works were coordinated at an international level in the framework of the GCSS1 or Eurocs2 projects. They benefited from the progress in observationsboth satellite and in-situ on the occasions of coordinated field campaign experimentsand from the development of limited area non-hydrostatic models. Explicit simulations, with socalled cloud resolving models (CRM, with horizontal resolution of 12 km) are indeed able to represent reasonably well some important aspects of deep convection (Guichard et al. 2004; Redelsperger et al. 2000). Large Eddy simulations (LES), with a resolution of 20100 m, are able to accurately simulate boundary layer dynamics (Moeng and Wyngaard 1988; Couvreux et al. 2005), cumulus clouds (Siebert and Frank 2003; Brown et al. 2002) or the transition from shallow to deep convection (Petch et al. 2002). A series of such explicit simulations, concerning var (...truncated)


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Frédéric Hourdin, Jean-Yves Grandpeix, Catherine Rio, Sandrine Bony, Arnaud Jam, Frédérique Cheruy, Nicolas Rochetin, Laurent Fairhead, Abderrahmane Idelkadi, Ionela Musat, Jean-Louis Dufresne, Alain Lahellec, Marie-Pierre Lefebvre, Romain Roehrig. LMDZ5B: the atmospheric component of the IPSL climate model with revisited parameterizations for clouds and convection, Climate Dynamics, 2012, pp. 2193-2222, Volume 40, Issue 9-10, DOI: 10.1007/s00382-012-1343-y