Modelling the radiative impact of aerosols from biomass burning during SAFARI-2000

1
Modelling the radiative impact of aerosols from
biomass burning during SAFARI-2000 Gunnar
Myhre1,2 Terje K. Berntsen3,1 James M. Haywood4
Jostein K. Sundet1 Brent N. Holben5 Mona
Johnsrud2 Frode Stordal2,1 1Department of
Geophysics, University of Oslo, Oslo,
Norway2Norwegian Institute for Air Research
(NILU), Kjeller, Norway3Center for International
Climate and Environmental Research - Oslo
(CICERO), Oslo, Norway4Met Office, Bracknell,
UK5Biospheric Sciences Branch, NASA Goddard
Space Flight Center, Greenbelt, Maryland

• 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 (1.87x1.87).
• 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.

• 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

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.

• 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.

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 1 Monthly mean modelled AOD for September
2000 in the upper panel and AOD for September
2000 from MODIS in the lower panel.

• 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.

Comparison with AERONET data
The paper can be found at http//folk.uio.no/gunn
army/manuscript/revised/safari/safari_ctm.pdf A
similar paper from the SHADE campaign can be
found at http//folk.uio.no/gunnarmy/manuscript/r
evised/shade/shade.pdf
Fig 2 Comparison of AOD from the modelled with
AERONET for 10 stations