Direct climate forcing by biomass-burning aerosols: Impact of correlations between controlling variables

Iacobellis, SF, Frouin R, Somerville RCJ.  1999.  Direct climate forcing by biomass-burning aerosols: Impact of correlations between controlling variables. Journal of Geophysical Research-Atmospheres. 104:12031-12045.

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anthropogenic aerosols, approximation, atmospheric aerosols, clouds, emissions, greenhouse gases, model, radiation, sulfate


Estimates of the direct climate forcing by condensed organic species resulting from biomass burning have been made using bulk radiative transfer models of various complexity and the SUNRAY radiation code of the European Centre for Medium-Range Weather Forecasts general circulation model. Aerosols arising from the burning of tropical forests and savannas as well as those from biomass fires outside the tropics are considered. The bulk models give values ranging from -1.0 to -0.6 W m(-2), which compare with -0.7 W m(-2) using the SUNRAY code. There appears to be significant uncertainty in these values due to uncertainties in the model input parameters. The difference is only 13% between the forcing obtained by taking into account the spatial and temporal distribution of the controlling variables and the forcing obtained using global averages fur all the variables. This indicates that the effects of variations in the controlling variables tend to compensate. Yet the forcing varies by up to 34% depending on which variables are set to global averages. The SUNRAY results show that the efficiency at which the biomass-burning aerosols backscatter sunlight in cloudy conditions is 0.53, a value significantly higher than that reported for sulfate aerosols. Most of the difference is due to the relatively low latitude (hence low sun zenith angle) of the biomass-burning aerosol sources relative to the sulfate aerosol sources. The implication is that clouds should not be assumed to have a reflectivity of unity in bulk models. Comparison of SUNRAY and bulk model results points to other potential problems with bulk models. First, the use in bulk models of mean aerosol optical properties across the entire solar spectrum has significant impact on the calculated forcing and may account for 23% of the difference between SUNRAY and bulk model estimates in clear-sky conditions. Second, neglecting multiple scattering in bulk models introduces significant differences in the clear-sky forcing at high sun zenith angles.