Radiative forcing estimates in coupled climate-chemistry models with emphasis on the role of the temporal variability
1Laboratoire des Sciences du Climat et de l'Environnement, UMR8212, IPSL, CEA-CNRS-UVSQ, Cedex, France
2Laboratoire de Météorologie Dynamique, LMD/IPSL, CNRS-UPMC, Paris, France
Abstract. This paper describes the impact on the sulphate aerosol radiative effects of coupling the radiative code of a global circulation model with a chemistry-aerosol module. With this coupling, temporal variations of sulphate aerosol concentrations influence the estimate of aerosol radiative impacts. Effects of this coupling have been assessed on net fluxes, radiative forcing and temperature for direct and first indirect effects of sulphate.
The direct effect responds almost linearly to rapid changes in concentrations whereas, the first indirect effect shows a strong non-linearity. In particular, sulphate temporal variability causes a large modification of the short wave net fluxes at the top of the atmosphere (+0.24 and +0.22 W m−2 for respectively, the present and preindustrial periods that are about 30 % of the total radiative forcing of sulfate). The effect is particularly important in regions with low-level clouds and intermediate sulphate aerosol concentrations (from 0.1 to 0.8 μg (SO4) m−3 in our model).
If computation of the aerosol direct radiative forcing is quite straightforward and has few effects; quantifying the first indirect radiative forcing requires first to tackle technical issues. We show that preindustrial sulphate concentrations have to be calculated with the same meteorological trajectory than that used for computing present ones. If this condition is not satisfied, the error on the estimation of the first indirect radiative forcing is of 60 %. Solutions are proposed to assess radiative forcing properly. In the reference method, the coupling between chemistry and climate results in a global average increase of 8 % in the first indirect radiative forcing. This change reaches 50 % in the most sensitive regions. However, the reference method is not suited to run long climate simulations. We present other methods that are simpler to implement in a coupled chemistry/climate model and that offer the possibility to assess radiative forcing.