Intercomparison of shortwave radiative transfer schemes in global aerosol modeling: Results from the AeroCom Radiative Transfer Experiment

In this study we examine the performance of 31 global model radiative transfer schemes in cloudfree
conditions with prescribed gaseous absorbers and no aerosols (Rayleigh atmosphere), with prescribed scatteringonly
aerosols, and with more absorbing aerosols. Results are compared to benchmark results from high-resolution, multiangular
line-by-line radiation models. For purely scattering aerosols, model bias relative to the line-by-line models in
the top-of-the atmosphere aerosol radiative forcing ranges from roughly −10 to 20 %, with over- and underestimates
of radiative cooling at lower and higher solar zenith angle, respectively. Inter-model diversity (relative standard deviation)
increases from 10 to 15% as solar zenith angle decreases. Inter-model diversity in atmospheric and surface
forcing decreases with increased aerosol absorption, indicating that the treatment of multiple-scattering is more variable
than aerosol absorption in the models considered. Aerosol radiative forcing results from multi-stream models are generally
in better agreement with the line-by-line results than the simpler two-stream schemes. Considering radiative fluxes,
model performance is generally the same or slightly better than results from previous radiation scheme intercomparisons.
However, the inter-model diversity in aerosol radiative forcing remains large, primarily as a result of the treatment of
multiple-scattering. Results indicate that global models that estimate aerosol radiative forcing with two-stream radiation
schemes may be subject to persistent biases introduced by these schemes, particularly for regional aerosol forcing.