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Volume 14, issue 21 | Copyright
Atmos. Chem. Phys., 14, 11997-12022, 2014
© Author(s) 2014. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 14 Nov 2014

Research article | 14 Nov 2014

Impact of the representation of marine stratocumulus clouds on the anthropogenic aerosol effect

D. Neubauer1, U. Lohmann1, C. Hoose2, and M. G. Frontoso1,3,* D. Neubauer et al.
  • 1ETH Zurich, Institute for Atmospheric and Climate Science, Zurich, Switzerland
  • 2Karlsruhe Institute of Technology, Institute for Meteorology and Climate Research, Karlsruhe, Germany
  • 3ETH Zurich, Center for Climate System Modeling, Zurich, Switzerland
  • *now at: RMS, Zurich, Switzerland

Abstract. Stratocumulus clouds are important for climate as they reflect large amounts of solar radiation back into space. However they are difficult to simulate in global climate models because they form under a sharp inversion and are thin. A comparison of model simulations with the ECHAM6-HAM2 global aerosol climate model to observations, reanalysis and literature data revealed too strong turbulent mixing at the top of stratocumulus clouds and a lack of vertical resolution. Further reasons for cloud biases in stratocumulus regions are the too "active" shallow convection scheme, the cloud cover scheme and possibly too low subsidence rates.

To address some of these issues and improve the representation of stratocumulus clouds, we made three distinct changes to ECHAM6-HAM2. With a "sharp" stability function in the turbulent mixing scheme we have observed, similar to previous studies, increases in stratocumulus cloud cover and liquid water path. With an increased vertical resolution in the lower troposphere in ECHAM6-HAM2 the stratocumulus clouds form higher up in the atmosphere and their vertical extent agrees better with reanalysis data. The recently implemented in-cloud aerosol processing in stratiform clouds is used to improve the aerosol representation in the model.

Including the improvements also affects the anthropogenic aerosol effect. In-cloud aerosol processing in ECHAM6-HAM2 leads to a decrease in the anthropogenic aerosol effect in the global annual mean from −1.19 Wm−2 in the reference simulation to −1.08 Wm−2, while using a "sharp" stability function leads to an increase to −1.34 Wm−2. The results from the simulations with increased vertical resolution are diverse but increase the anthropogenic aerosol effect to −2.08 Wm−2 at 47 levels and −2.30 Wm−2 at 95 levels.

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Short summary
Several biases in the representation of clouds in the stratocumulus regime in the ECHAM6-HAM2 global climate model were found by evaluating the model in the stratocumulus cloud regime. Simulations with changes in model resolution and physics to better represent clouds and aerosol in the stratocumulus regime show that the human influence on clouds and thus climate by emission of aerosol particles is sensitive to the representation of (stratocumulus) clouds.
Several biases in the representation of clouds in the stratocumulus regime in the ECHAM6-HAM2...