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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
01 Mar 2018
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).
Aerosol and physical atmosphere model parameters are both important sources of uncertainty in aerosol ERF
Leighton Regayre1, Jill Johnson1, Masaru Yoshioka1, Kirsty Pringle1, David Sexton2, Ben Booth2, Lindsay Lee1, Nicolas Bellouin3, and Kenneth Carslaw1 1Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
2UK Hadley Centre Met Office, Exeter, Fitzroy Road, Exeter, Devon, EX1 3PB, UK
3Department of Meteorology, School of Mathematical & Physical Sciences, Faculty of Science, University of Reading, Reading, RG6 6BB, UK
Abstract. Changes in aerosols cause a change in net top-of-the-atmosphere (ToA) short-wave and long-wave radiative fluxes, rapid adjustments in clouds, water vapour and temperature, and cause an effective radiative forcing (ERF) of the planetary energy budget. The diverse sources of model uncertainty and the computational cost of running climate models make it difficult to isolate the main causes of aerosol ERF uncertainty and to understand how observations can be used to constrain it. We explore the aerosol ERF uncertainty by using fast model emulators to generate a very large set of aerosol-climate model variants that span the model uncertainty due to twenty-seven parameters related to atmospheric and aerosol processes. Sensitivity analyses shows that the uncertainty in the ToA flux is dominated (around 80 %) by uncertainties in the physical atmosphere model, particularly parameters that affect cloud reflectivity. However, uncertainty in the change in ToA flux caused by aerosol emissions over the industrial period (the aerosol ERF) is controlled by a combination of uncertainties in aerosol (around 60 %) and physical atmosphere (around 40 %) parameters. Four of the atmospheric and aerosol parameters that cause uncertainty in short-wave ToA flux (mostly parameters that directly scale cloud reflectivity, cloud water content or cloud droplet concentrations) also account for around 60% of the aerosol ERF uncertainty. The common causes of uncertainty mean that constraining the modelled planetary brightness to tightly match satellite observations changes the lower 95 % credible aerosol ERF value from −2.65 Wm−2 to −2.37 Wm−2. This suggests the strongest forcings (below around −2.4 Wm−2) are inconsistent with observations. These results show that, regardless of the fact that the ToA flux is an order of magnitude larger than the aerosol ERF, the observed flux can constrain the uncertainty in ERF because their values are connected by constrainable process parameters. The key to reducing the aerosol ERF uncertainty further will be to identify observations that can additionally constrain individual parameter ranges and/or combined parameter effects, which can be achieved through sensitivity analysis of perturbed parameter ensembles.
Citation: Regayre, L., Johnson, J., Yoshioka, M., Pringle, K., Sexton, D., Booth, B., Lee, L., Bellouin, N., and Carslaw, K.: Aerosol and physical atmosphere model parameters are both important sources of uncertainty in aerosol ERF, Atmos. Chem. Phys. Discuss.,, in review, 2018.
Leighton Regayre et al.
Leighton Regayre et al.
Leighton Regayre et al.


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