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Geoscientific Model Development An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/gmd-2019-148
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/gmd-2019-148
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: model experiment description paper 11 Jun 2019

Submitted as: model experiment description paper | 11 Jun 2019

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

Description of the resolution hierarchy of the global coupled HadGEM3-GC3.1 model as used in CMIP6 HighResMIP experiments

Malcolm J. Roberts1, Alex Baker2, Ed W. Blockley1, Daley Calvert1, Andrew Coward3, Helene T. Hewitt1, Laura C. Jackson1, Till Kuhlbrodt2, Pierre Mathiot1, Christopher D. Roberts4, Reinhard Schiemann2, Jon Seddon1, Benoît Vannière2, and Pier Luigi Vidale2 Malcolm J. Roberts et al.
  • 1Met Office Hadley Centre, Exeter, UK
  • 2National Centre for Atmospheric Science (NCAS), University of Reading, Reading, UK
  • 3National Oceanography Centre, Southampton, UK
  • 4European Centre for Medium Range Weather Forecasting (ECMWF), Reading, UK

Abstract. CMIP6 HighResMIP is a new experimental design for global climate model simulations that aims to assess the impact of model horizontal resolution on climate simulation fidelity. We describe a hierarchy of global coupled model resolutions based on the HadGEM3-GC3.1 model that range from an atmosphere-ocean resolution of 130 km-1° to 25 km-1/12°, all using the same forcings and initial conditions. In order to make such high resolution simulations possible, the experiments have a short 30 year spinup, followed by at least century-long simulations with both constant forcing (to assess drift and the focus of this work), and historic forcing.

We assess the change in model biases as a function of both atmosphere and ocean resolution, together with the effectiveness and robustness of this new experimental design. We find reductions in the biases in top of atmosphere radiation components and cloud forcing. There are significant reductions in some common surface climate model biases as resolution is increased, particularly in the Atlantic for sea surface temperature and precipitation, primarily driven by increased ocean resolution. There is also a reduction in drift from the initial conditions both at the surface and in the deeper ocean at higher resolution. Using an eddy-present and eddy-rich ocean resolution enhances the strength of the North Atlantic ocean circulation (boundary currents, overturning circulation and heat transports), while an eddy-present ocean resolution has a considerably reduced Antarctic Circumpolar Current strength. All models have a reasonable representation of El Nino – Southern Oscillation. In general the biases present after 30 years of simulations do not change character markedly over longer timescales, justifying the experimental design.

Malcolm J. Roberts et al.
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Status: final response (author comments only)
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Malcolm J. Roberts et al.
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Short summary
We investigate the role that horizontal grid spacing plays in global coupled climate model simulations, together with examining the efficacy of a new design of simulation experiments that will be used by the community for multi-model comparison. We found that finer grid spacing in both atmosphere and ocean-sea-ice models leads to a general reduction in bias compared to observations, and that once eddies in the ocean are resolved several key climate processes are greatly improved.
We investigate the role that horizontal grid spacing plays in global coupled climate model...
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