1. Modelling the radiative impact of aerosols from biomass burning during SAFARI-2000 Gunnar Myhre 1,2 Terje K. Berntsen 3,1 James M. Haywood 4 Jostein K. Sundet 1 Brent N. Holben 5 Mona Johnsrud 2 Frode Stordal 2,1 1 Department of Geophysics, University of Oslo, Oslo, Norway 2 Norwegian Institute for Air Research (NILU), Kjeller, Norway 3 Center for International Climate and Environmental Research - Oslo (CICERO), Oslo, Norway 4 Met Office, Bracknell, UK 5 Biospheric Sciences Branch, NASA Goddard Space Flight Center, Greenbelt, Maryland Radiative forcing Clouds strongly influence the radiative forcing due to aerosol from biomass burning as can be seen from Fig 3. Clouds have a stronger impact on the radiative forcing due to biomass aerosol than for sulfate aerosols. Aerosol optical depth (AOD) The modelled September 2000 monthly mean AOD is shown in Figure 1. A maximum AOD of nearly 1.0 is estimated with transport pattern to the north west and south east. Introdution  Based on modelling linked to measurements we estimate the radiative impact of aerosols from biomass burning during the SAFARI-2000 campaign,  A chemistry-transport model (the Oslo CTM) with meteorological data for the actual period is adopted to simulate the distribution of the biomass aerosols.  A radiative transfer scheme is adopted in the calculations of the radiative impact of the biomass aerosols.  A thorough comparison between our model results and available observations are made with regard to aerosol optical depth (AOD), the vertical profile, and the radiative impact of the biomass aerosols. Observations include in situ data from the Met Office C-130 aircraft, ground based data, and satellite data Comparison with AERONET data Summary  Using the ECMWF meteorological data for the campaign period the model manages to reproduce some of the main patterns of AOD during period, found both in satellite retrievals and ground based AERONET measurements.  The modelled radiative impact of the biomass aerosols compares reasonably well to measurements (within 20%).  Local radiative cooling and warming up to 50 Wm -2 magnitude is modelled.  The clouds strongly influence the radiative impact of the aerosols.  Globally the aerosols from biomass burning in southern Africa in September 2000 result in a global mean radiative impact of -0.13 Wm -2. References Cooke, W.F., C. Liousse, H. Cachier, and J. Feichter, Construction of a 1x1 fossil-fuel emission dataset for carbonaceous aerosols and implementation and radiative impact in the ECHAM-4 model, J. Geophys. Res., 104, 22,137-22,162, 1999. Formenti, P., W. Elbert, W. Maenhaut, C. Jost, D. Sprung, M.O. Andreae, H. Barjat, J. Haywood, P. Francis, and S. Osborne, The C-130 airborne measurements of water soluble and carbonaceous aerosols during the SAFARI 2000 dry season intensive: chemical characteristics, relevance to the optical properties and emission inventories of African biomass burning aerosols, J. Geophys. Res., accepted 2002. Haywood, J., S. Osborne, P. Francis, P. Formenti, and M.O. Andreae, The mean physical and optical properties of biomass burning aerosol measured by the C-130 aircraft during SAFARI-2000, J. Geophys. Res., accepted 2002. Myhre, G., T. K. Berntsen, J. M. Haywood, J. K. Sundet, B. N. Holben, M. Johnsrud, and F. Stordal, Modelling the solar radiative impact of aerosols from biomass burning during SAFARI-2000, accepted J. Geophys. Res., 2002. Fig 1: Monthly mean modelled AOD for September 2000 in the upper panel and AOD for September 2000 from MODIS in the lower panel. Method A 3-dimentional off-line CTM with pre-calculated meteorological fields from ECMWF is adopted to calculate the distribution of aerosol from biomass burning. The horizontal resolution used in the simulations is T63 (1.87°x1.87°). The treatment of black carbon (BC) and organic carbon (OC) for biomass burning is adopted from Cooke et al. [1999]. Both BC and OC are separated in a hydrophobic fraction and a hydrophilic fraction (see further details [Myhre et al., 2002)]. The size distribution and refractive index of the particles in the biomass burning plume are adopted from the Met Office C-130 aircrfat [Haywood et al., 2002] to model the optical properties (specific extinction coefficient, single scattering albedo, and asymmetry factor) using Mie theory. A BC/OC ratio of 0.12 from [Haywood et al., 2002] and a OM/OC ratio of 2.6 from Formenti et al. [2002] is used in the calculation of the optical properties. We reproduce the single scattering albedo at 0.55 µm of 0.90, which was estimated by Haywood et al. [2002]. Further, the decrease with wavelength in specific extinction and single scattering, [Haywood et al., 2002], which is important for the radiative transfer calculations, is also well reproduced. The paper can be found at http://folk.uio.no/gunnarmy/manuscript/revised/safari/safari_ctm.pdf A similar paper from the SHADE campaign can be found at http://folk.uio.no/gunnarmy/manuscript/revised/shade/shade.pdf Fig 3: Monthly mean radiative forcing due to aerosols from biomass buring during September 2000. a) Clouds included in the radiative transfer calculations, b) clouds excluded in the radiative transfer calculations. Fig 2: Comparison of AOD from the modelled with AERONET for 10 stations