The transport model is an essential component of the CCDAS that relates the surface fluxes to the atmospheric CO
2 concentrations.
It follows the configuration of the General Circulation Model LMDz model (Hourdin et al., 2006) as implemented in the coupled climate simulations for the IPCC AR4 report
(IPCC, 2007). The dynamical part of the GCM is based on a finite-difference formulation of the primitive equations solved on a 3D Eulerian grid with a typical horizontal resolution of 3.75° (longitudes) x 2.5° (latitudes) and 19 sigma-pressure layers up to
3hPa. This corresponds to a vertical resolution of about 300-500 m in the planetary boundary layer (first level at 70 m height) and to a resolution of about 2 km at the tropopause (with 7-9 levels located in the stratosphere). The calculated winds (u and v) are relaxed to ECMWF analyzed meteorology with a relaxation time of 2.5h (nudging) in order to realistically account for large scale advection (Hourdin et al., 2000). The advection of tracers is calculated based on the finite-volume, second-order scheme as described by Hourdin and Armengaud (1999). Deep convection is parameterized according to the scheme of Tiedtke (1989) and the turbulent mixing in the planetary boundary layer is based on a local second-order closure formalism.
Representation of vertical and horizontal grids of LMDZ
Colors represent simulated surface and air temperatures, arrows represent wind’s direction.
© L. Fairhead (IPSL/LMD)
The model has been widely used to model climate (IPCC, 2001, 2007) as well as transport and chemistry of gas and particles (Peylin et al., 2005, Bousquet et al. 2005,
2006, Rivier et al., 2006, Hauglustaine et al., 2004, Folberth et al., 2005). LMDz-INCA is running operationally since 2006 and calculates every day a 5-day forecast of the main

atmospheric trace gases ( http://www-lsceinca.cea.fr
). For greenhouse gases, we have been using LMDz since 2003 at LSCE in an offline mode named LMDZt (with precalculated transport fields) to save computational time compared to the full GCM.
Few main changes have occurred since the first use of LMDz in the CCDAS. The model used for the first CARBONES re-analysis is based on LMDz version 3. Recent and ongoing developments at the LMD laboratory have led to an improved version, LMDz version 4, with the following changes:
 A more sophisticated parameterization of the planetary boundary layer has been implemented by Hourdin et al. (2002) based on the representation of a dry thermal in terms of mass fluxes, as in deep convection schemes. In this new version we introduced the nudging of air temperature T with the same relaxation time as for the wind (u and v).
 Several small inconsistencies have been cleared for the convection and advection schemes.
The updated version has led to significant improvement in terms of simulated atmospheric CO
2
concentrations. An internal report that can be requested from Philippe
Bousquet at LSCE ( bousquet@lsce.ipsl.fr
) compares the two LMDz versions together with two other state of the art global transport models (TM3 and TM5) to atmospheric
CO
2
data using a standard set of fluxes. Model skills for the simulation of i) large scale atmospheric features (TRANSCOM-I experiment), ii) upper air CO
2
data, iii) vertical profile within the planetary boundary layer, and iv) continuous surface CO
2
records are on averaged improved with the new LMDz version. Within the continental PBL, the standard LMDZ performances were limited by a too large diffusion that smoothes gradients of concentrations too efficiently. The new version 4 with an improved PBL scheme, presents equivalent capabilities to represent the synoptic variations of CO2 concentrations as other state-of-the-art models such as TM5. Such improvement is thus crucial to properly assimilate all surface CO
2 concentration data within the CCDAS.
The model is freely available under a free-software license: CECILL. It can be access through: http://lmdz.lmd.jussieu.fr/
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