Radiative forcing by aerosols as derived from the AeroCom present-day and pre-industrial simulations

[摘要] Nine different global models with detailed aerosol modules haveindependently produced instantaneous direct radiative forcing due toanthropogenic aerosols. The anthropogenic impact is derived from thedifference of two model simulations with prescribed aerosolemissions, one for present-day and one for pre-industrial conditions. Thedifference in the solar energy budget at the top of the atmosphere (ToA) yields anew harmonized estimate for the aerosol direct radiative forcing (RF) underall-sky conditions. On a global annual basis RF is −0.22 Wm⁻², rangingfrom +0.04 to −0.41 Wm⁻², with astandard deviation of ±0.16 Wm⁻². Anthropogenic nitrate and dust arenot included in this estimate. No model shows a significant positive all-skyRF. The corresponding clear-sky RF is −0.68 Wm⁻². The cloud-sky RF wasderived based on all-sky and clear-sky RF and modelled cloud cover. It wassignificantly different from zero and ranged between −0.16 and +0.34 Wm⁻².A sensitivity analysis shows that the total aerosol RF isinfluenced by considerable diversity in simulated residence times, massextinction coefficients and most importantly forcing efficiencies (forcingper unit optical depth). The clear-sky forcing efficiency(forcing per unit optical depth) has diversity comparable to that for the all-sky/clear-sky forcing ratio. While the diversity in clear-sky forcing efficiency isimpacted by factors such as aerosol absorption, size, and surface albedo,we can show that the all-sky/clear-sky forcing ratio is importantbecause all-sky forcing estimates requireproper representation of cloud fields and the correct relative altitudeplacement between absorbing aerosol and clouds. The analysis of the sulphateRF shows that long sulphate residence times are compensated bylow mass extinction coefficients and vice versa. This is explained by more sulphateparticle humidity growth and thus higher extinction in those models whereshort-lived sulphate is present at lower altitude and vice versa. Solaratmospheric forcing within the atmospheric column is estimated at +0.82±0.17 Wm⁻².The local annual average maxima of atmospheric forcing exceed +5 Wm⁻²confirming the regional character of aerosol impacts on climate. The annualaverage surface forcing is −1.02±0.23 Wm⁻². With the currentuncertainties in the modelling of the radiative forcingdue to thedirect aerosol effect we show here that an estimate from onemodel is not sufficient but a combination of several model estimatesis necessary to provide a mean and to explore the uncertainty.

[学科分类] 大气科学