Observational evidence of a change in radiative forcing due to the indirect aerosol effect

Abstract

Anthropogenic aerosols enhance cloud reflectivity by increasing the number concentration of cloud droplets, leading to a cooling effect on climate known as the indirect aerosol effect. Observational support for this effect is based mainly on evidence that aerosol number concentrations are connected with droplet concentrations, but it has been difficult to determine the impact of these indirect effects on radiative forcing1,2,3. Here we provide observational evidence for a substantial alteration of radiative fluxes due to the indirect aerosol effect. We examine the effect of aerosols on cloud optical properties using measurements of aerosol and cloud properties at two North American sites that span polluted and clean conditions—a continental site in Oklahoma with high aerosol concentrations, and an Arctic site in Alaska with low aerosol concentrations. We determine the cloud optical depth required to fit the observed shortwave downward surface radiation. We then use a cloud parcel model to simulate the cloud optical depth from observed aerosol properties due to the indirect aerosol effect. From the good agreement between the simulated indirect aerosol effect and observed surface radiation, we conclude that the indirect aerosol effect has a significant influence on radiative fluxes.

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Figure 1: Cloud optical depth, as determined from the observed shortwave flux, plotted against LWP.
Figure 2: Cloud optical depth, as determined from the parcel model, plotted against LWP.
Figure 3: Modelled outgoing shortwave radiation at the SGP and NSA sites using the average albedo and zenith angle from the SGP site and the cloud properties from the adiabatic model.

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Acknowledgements

We thank P. Quinn for providing the composition data at the ARM SGP and NSA sites. During this study, X.D. was also supported by the NASA CERES project. This work was supported by the DOE ARM programme.

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Correspondence to Joyce E. Penner.

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Penner, J., Dong, X. & Chen, Y. Observational evidence of a change in radiative forcing due to the indirect aerosol effect. Nature 427, 231–234 (2004). https://doi.org/10.1038/nature02234

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