ESTIMATION OF SOLAR RADIATIVE IMPACT DUE TO BIOMASS BURNING OVER THE AFRICAN CONTINENT

1. Unperturbed surface albedo seasonal variation driven by precipitation. Dry season Wet season Dry season Surface albedo cycle perturbed by man-made fire activities. ESTIMATION OF SOLAR RADIATIVE IMPACT DUE TO BIOMASS BURNING OVER THE AFRICAN CONTINENT • Y. Govaerts(1), G. Myhre(2), J. M. Haywood(3), T. K. Berntsen(4) and A. Lattanzio(1) • (1) EUMETSAT, Germany, +49(0)6151807362, govaerts@eumetsat.de • (2) University of Oslo, Norway, gunnar.myhre@geofysikk.uio.no • (3) Met Office, UK, jmhaywood@metoffice.com • (4) Center for International Climate and Environmental Research-Oslo, Norway, t.k.berntsen@geofysikk.uio.no ABSTRACT This study investigates the solar radiative impact of surface albedo decrease and aerosol emission resulting from biomass burning. Albedo decrease has a positive impact due to the increase in solar radiation absorbed by the surface. Aerosol emission is responsible for an increase in solar radiation reflected back to space and has therefore a negative impact. When these two effects are combined, surface albedo decrease tends to reduce the negative radiative impact of biomass burning aerosol. OBJECTIVE BACKGROUND The African continent is subject to intense biomass burning, almost all of it resulting from human activities. These fires are responsible for ecological, chemical and meteorological changes, among which aerosol emission is one of the most recognised effects. Biomass burning is also responsible for regional surface albedo decrease as large as 15% with respect to non-burnt areas. The radiative forcing due to these biomass burning perturbations is still very uncertain. This study investigates the direct radiative impact of biomass burning on the net solar radiative flux at the top of the atmosphere accounting for both aerosol emission and surface albedo decrease. Radiative impact is defined as the difference in the net solar radiative flux at the top of the atmosphere between the simulation with a perturbation (aerosol emission and/or surface albedo decrease) and the control experiment (simulation without aerosol or surface albedo perturbation). This is similar to the radiative forcing concept, except that both anthropogenic and natural abundance of the aerosols are included. METHOD Biomass burning emission and transport models A chemistry-transport model (OSLO-CTM2) based on ECMWF analysed data fields is used to simulate the distribution of the biomass burning aerosols [1]. The carbonaceous aerosol modelling (i.e., the hydrophobic fraction in the emissions, the transfer rate from hydrophobic to hydrophilic aerosols (aging), and the dry deposition velocities) are taken from Cooke et al. [2]. The size distribution and refractive index of the particles in the biomass burning plume are adopted from Haywood et al. [3] to model the optical properties (specific extinction coefficient, single scattering albedo, and asymmetry factor) using Mie theory. Estimation of albedo decrease due to biomass burning Surface albedo is derived from Meteosat observations [4]. A temporal analysis is applied to identify pixels whose albedo seasonal variations are perturbed by biomass burning [5]. The relative surface albedo decrease is estimated as the difference between the albedo of the unperturbed and perturbed pixels within each grid box. Probability of man-made fire-induced albedo decrease over North Africa in December derived from albedo temporal profile analysis. Albedo relative decrease due to biomass burning estimated for December. Missing data are shown in black. Example of monthly mean modelled biomass burning aerosol optical thickness in December RESULTS Impact of surface albedo decrease Biomass burning is responsible for an albedo decrease that translates into an increase in the energy absorbed by the surface and therefore a decrease in the outgoing solar radiation at the top of the atmosphere. It has thus a positive radiative impact. Over the studied area (1/7 of the Earth), the annual mean impact is about 0.04Wm-2 with local maximum values as large as +3Wm-2 . Impact of aerosol emission Biomass burning aerosol emission increases of the solar radiation reflected back to space. It has thus a negative radiative impact. Over the studied area (1/7 of the Earth), the annual mean impact is about -1.3Wm-2 with local minimum values as low as -7Wm-2 . Combined impact of surface albedo decrease an aerosol emission The presence of aerosol increases the radiation reflected back to space, decreasing the radiation available at the surface and thereby the impact of surface albedo decrease. Over fire affected areas such as the Sahelian region, surface albedo decrease reduces by about 1 Wm-1 the radiative impact of biomass burning aerosol. Mean annual solar radiative impact due to biomass burning aerosol emission without surface albedo decrease. Mean annual solar radiative impact due to biomass burning surface albedo decrease in the absence of aerosol emission. Mean annual solar radiative impact due to the combined effect of surface albedo decrease and biomass burning aerosol emission. CONCLUSION Aerosols weaken the effects of the surface albedo change as a result of a decrease in the radiation available at the surface. Further, the reduced surface albedo strengthen the radiative effect of the aerosols as scattering dominate over absorption for biomass burning aerosols. Consequently, surface albedo decrease due to fire activities has not a major impact, but globally reduces the radiative effect of biomass burning aerosol by about 3% over the African continent. References [1] Myhre, G., et al. (2003) Modeling the solar radiative impact of aerosols from biomass burning during the Southern African Regional Science Initiative (SAFARI-2000) experiment, JGR, 108, doi:10.1029/2002JD002313. [2] Cooke, W.F., et al. (1999) Construction of a 10 X 10 fossil-fuel emission dataset for carbonaceous aerosols and implementation and radiative impact in the ECHAM-4 model, JGR, 104, 22,137– 22,162. [3] Haywood, J., et al. (2003) The mean physical and optical properties of biomass burning aerosol measured by the C-130 aircraft during SAFARI-2000, JGR, 108, 8473, doi:10.1029/2002JD002226. [4] Pinty, B., et al. (2000). Surface Albedo Retrieval from METEOSAT. Part 2: Applications, JGR, 105, 18113-18134. [5] Govaerts, Y.M ., et al. (2002) Impact of Fires on Surface Albedo Dynamics over the African Continent, JGR, 107, DOI:10.1029/2002JD002388.