Impact of four-stream radiative transfer algorithm on aerosol direct radiative effect and forcing.

Large uncertainties remain in the estimation of aerosol direct radiative effect (DRE) and forcing (DRF). In this work, using an aerosol-climate model with two- and four-stream radiation schemes, we show that the radiative transfer algorithms contribute to the uncertainties. Aerosol shortwave DREs and heating rate are underestimated significantly by the two-stream algorithm. For present-day conditions, the four-stream algorithms are found to enhance global annualmean aerosol shortwave DREs by more than 8% (14%) at the top of the atmosphere (TOA), 15% (18%) in the atmosphere, and 12% (15%) at the surface for all-sky (clear-sky) case. The regional-averaged relative differences in aerosol shortwave DREs between the two- and four-stream algorithms increase as latitude increases, exceeding 25% at the TOA and 30% at the surface in the high latitudes of the Southern Hemisphere. The DRE differences due to the four-stream algorithms are negative, except for the Arctic, Tibetan Plateau, Arabia, and Sahara, at the TOA, are positive in the atmosphere, and are negative at the surface, with the maximum exceeding 4.0Wm^-2. Increases in aerosol shortwave heating rates due to the four-stream algorithms are generally more than 10% and may even exceed 100%. Our results also show that the two-stream algorithm underestimates the DRFs due to anthropogenic aerosols. Significant underestimation appears in the middle latitudes of the Northern Hemisphere, with the maximum being close to the quantity of 0.6Wm^-2 for clear-sky case. This study indicates that a multi-stream radiative transfer algorithm is necessary to reduce the uncertainties of aerosol DREs and DRFs estimated by global climate models. [ABSTRACT FROM AUTHOR]