Modelling the radiative impact of aerosols from biomass burning during SAFARI-2000   Gunnar Myhre, Terje K. Berntsen, James M. Haywood, Jostein K. Sundet, Brent N. Holben, Mona Johnsrud, and Frode Stordal

Biomass Burning (OC & BC) -0.2Wm-2

Key Issues Can we model the aerosols from BB? NILU Key Issues Can we model the aerosols from BB? Distribution of aerosols (spatial and temporal) Radiative effects of the aerosols Compare the model results with a wide range of measurements; satellite data, ground based observations, and aircraft measurements What is the climatic effect of reducing BC from biomass burning? NOSA NOSA

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. The size distribution and refractive index of the particles in the biomass burning plume are adopted from the Met Office C-130 aircraft to model the optical properties (specific extinction coefficient, single scattering albedo, and asymmetry factor) using Mie theory. A BC/OC ratio of 0.12 and a OM/OC ratio of 2.6 from measurements are used in the calculation of the optical properties. We reproduce the single scattering albedo at 0.55 µm of 0.90, which was in accordance with the measurements. Further, the decrease with wavelength in specific extinction and single scattering, which is important for the radiative transfer calculations, is also well reproduced.

Fig1: Aerosol optical depth

Aerosol optical depth (AOD) Monthly mean AOD for September 2000 A maximum AOD of nearly 1.0 Transport pattern to the north west and south east Fig 2: Monthly mean modelled AOD for September 2000 in the upper panel and AOD for September 2000 from MODIS in the lower panel.

Fig 3: Comparison with AERONET data

Fig 4: Vertical profile over land and ocean Temperature Scattering Land Ocean

Comparison of vertical profile during three aircraft flights Fig 5

Comparison of measured and modelled radiative effect of biomass burning aerosols Aircraft measurments of AOD is 0.31 and radiative effect of –25 Wm-2, which gives a normalized radiative effect of –82 Wm-2/AOD. The modelled normalized radiative effect is –68 Wm-2/AOD. The Met Office C-130

Fig 6: Radiative effect of the aerosols

Radiative effect of biomass burning aerosols Total Clear sky Fig 7: 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.

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. Reduction of BC from biomass burning lead to warming 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