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Discussion papers
https://doi.org/10.5194/gmd-2018-307
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/gmd-2018-307
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Model evaluation paper 06 Mar 2019

Model evaluation paper | 06 Mar 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Geoscientific Model Development (GMD).

The global aerosol-climate model ECHAM6.3-HAM2.3 – Part 2: Cloud evaluation, aerosol radiative forcing and climate sensitivity

David Neubauer1, Sylvaine Ferrachat1, Colombe Siegenthaler-Le Drian1, Philip Stier2, Daniel G. Partridge3, Ina Tegen4, Isabelle Bey1,a, Tanja Stanelle1, Harri Kokkola5, and Ulrike Lohmann1 David Neubauer et al.
  • 1Institute of Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland
  • 2Department of Physics, University of Oxford, Oxford, UK
  • 3College of Engineering, Mathematics, and Physical Sciences, University of Exeter, Exeter, UK
  • 4Leibniz Institute for Tropospheric Research, Leipzig, Germany
  • 5Finnish Meteorological Institute, Kuopio, Finland
  • anow at: MeteoSwiss, Geneva, Switzerland

Abstract. The global aerosol-climate model ECHAM6.3-HAM2.3 (E63H23) and the previous model versions ECHAM5.5-HAM2.0 (E55H20) and ECHAM6.1-HAM2.2 (E61H22) are evaluated using global observational datasets for clouds and precipitation. In E63H23 low cloud amount, liquid and ice water path and cloud radiative effects are more realistic than in previous model versions. E63H23 has a more physically based aerosol activation scheme, improvements in the cloud cover scheme, changes in detrainment of convective clouds, changes in the sticking efficiency for accretion of ice crystals by snow, consistent ice crystal shapes throughout the model, changes in mixed phase freezing and an inconsistency in ice crystal number concentration (ICNC) in cirrus clouds was removed. Biases that were identified in E63H23 (and in previous model versions) are a too low cloud amount in stratocumulus regions, deep convective clouds in the Atlantic and Pacific oceans form too close to the continents and there are indications that ICNCs are overestimated.

Since clouds are important for effective radiative forcing due to aerosol-radiation and aerosol-cloud interactions (ERFari+aci) and equilibrium climate sensitivity (ECS), also differences in ERFari+aci and ECS between the model versions were analyzed. ERFari+aci is weaker in E63H23 (−1.0 W m−2) than in E61H22 (−1.2 W m−2) (or E55H20; −1.1 W m−2). This is caused by the weaker shortwave ERFari+aci (new aerosol activation scheme and sea salt emission parameterization in E63H23, more realistic simulation of cloud water) overcompensating the weaker longwave ERFari+aci (removal of an inconsistency in ICNC in cirrus clouds in E61H22).

The decrease in ECS in E63H23 (2.5 K) compared to E61H22 (2.8 K) is due to changes in the entrainment rate for shallow convection (affecting the cloud amount feedback) and a stronger cloud phase feedback.

David Neubauer et al.
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David Neubauer et al.
Data sets

Data for the publication "The global aerosol-climate model ECHAM6.3-HAM2.3 – Part 2: Cloud evaluation, aerosol radiative forcing and climate sensitivity" D. Neubauer, S. Ferrachat, C. Siegenthaler-Le Drian, P. Stier, D. G. Partridge, I. Tegen, I. Bey, T. Stanelle, H. Kokkola, and U. Lohmann https://doi.org/10.5281/zenodo.2541937

Model code and software

Scripts for the publication "The global aerosol-climate model ECHAM6.3-HAM2.3 – Part 2: Cloud evaluation, aerosol radiative forcing and climate sensitivity" D. Neubauer, S. Ferrachat, C. Siegenthaler-Le Drian, P. Stier, D. G. Partridge, I. Tegen, I. Bey, T. Stanelle, H. Kokkola, and U. Lohmann https://doi.org/10.5281/zenodo.2553892

David Neubauer et al.
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Short summary
The global aerosol-climate model ECHAM6.3-HAM2.3 and the previous model versions ECHAM5.5-HAM2.0 and ECHAM6.1-HAM2.2 are evaluated. The simulation of clouds has improved in ECHAM6.3-HAM2.3. This has an impact on effective radiative forcing due to aerosol-radiation and aerosol-cloud interactions and equilibrium climate sensitivity which both are weaker in ECHAM6.3-HAM2.3 than in the previous model versions.
The global aerosol-climate model ECHAM6.3-HAM2.3 and the previous model versions ECHAM5.5-HAM2.0...
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