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

Estimates of the direct climate forcing by condensed organic species result ing from biomass burning have been made using bulk radiative transfer model s of various complexity and the SUNRAY radiation code of the European Centr e for Medium-Range Weather Forecasts general circulation model. Aerosols ar ising from the burning of tropical forests and savannas as well as those fr om biomass fires outside the tropics are considered. The bulk models give v alues ranging from -1.0 to -0.6 W m(-2), which compare with -0.7 W m(-2) us ing the SUNRAY code. There appears to be significant uncertainty in these v alues due to uncertainties in the model input parameters. The difference is only 13% between the forcing obtained by taking into account the spatial a nd temporal distribution of the controlling variables and the forcing obtai ned using global averages fur all the variables. This indicates that the ef fects of variations in the controlling variables tend to compensate. Yet th e forcing varies by up to 34% depending on which variables are set to globa l averages. The SUNRAY results show that the efficiency at which the biomas s-burning aerosols backscatter sunlight in cloudy conditions is 0.53, a val ue significantly higher than that reported for sulfate aerosols. Most of th e difference is due to the relatively low latitude (hence low sun zenith an gle) of the biomass-burning aerosol sources relative to the sulfate aerosol sources. The implication is that clouds should not be assumed to have a re flectivity of unity in bulk models. Comparison of SUNRAY and bulk model res ults points to other potential problems with bulk models. First, the use in bulk models of mean aerosol optical properties across the entire solar spe ctrum 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 introduce s significant differences in the clear-sky forcing at high sun zenith angle s.