the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Aug 2023
The sea ice component of GC5: coupling SI3 to HadGEM3 using conductive fluxes
Abstract. We present an overview of the UK’s Global Sea Ice model configuration version 9 (GSI9), the sea ice component of the latest Met Office Global Coupled model, GC5. The GC5 configuration will, amongst other uses, form the physical basis for the HadGEM3 (Hadley Centre Global Environment Model version 3) climate model and UKESM2 (UK Earth System Model version 2) Earth system model that will provide the Met Office Hadley Centre/UK model contributions to CMIP7 (Coupled Model Intercomparison Project Phase 7). Although ocean model configurations have been developed for many years around the NEMO (Nucleus for European Modelling of the Ocean) ocean modelling framework, the GSI9 configuration is the first UK sea ice model configuration to use the new native NEMO sea ice model, SI3 (Sea Ice modelling Integrated Initiative). This replaces the CICE (Community Ice CodE) model used in previous configuration versions. In this paper we document the physical and technical options used within the GSI9 sea ice configuration. We provide details of the implementation of SI3 into the Met Office coupled model and the adaptations required to work with our ‘conductivity coupling’ approach, and also provide a thorough description of the GC5 coupling methodology. A brief evaluation of sea ice simulated by the GC5 model is included, with results compared to observational references and a previous Global Coupled model version (GC3.1) used for CMIP6, to demonstrate the scientific credibility of the results.
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RC1: 'Comment on egusphere-2023-1731', Anonymous Referee #1, 13 Oct 2023
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In this paper, Ed Blockley and his co-authors provide a description of the sea-ice model GSI9 and its coupling within the Coupled Model GC5.
The paper is clearly structured, provides a helpful summary of the functioning of the model, describes in detail the technical aspect of the coupling of GSI9 to the ocean and to the atmosphere, and is very well written. I therefore generally recommend its publication, but I think that the following comments should be addressed in a revised version. They are in particular geared towards increasing the usefulness of this paper for a general sea-ice modelling audience who would like to draw insights from such paper for their own work.
Overall, I felt that the paper provides too little guidance to the reader regarding the motivation of the shift to SI3 and to the new coupling scheme. A description of the advantages and disavantages of SI3 vs CICE (if possible) and of flux coupling vs. standard coupling would be helpful. The reader could then infer for themselves whether such shift is considered useful for scientific / numerical / strategic reasons.
I also would have liked to see a broader discussion of the performance of the new model setup. How computationally expensive are these runs relative to the ones with the previous model version (or: Which percentage of the ocean computations happen within the sea-ice module in this version and in the previous version)? How well does the sea-ice model scale in this setup with the number of CPUs compared to the previous model version, unless this is documented elsewhere?
For the coupling, more information would be helpful regarding the standard coupling frequency and standard time step for the atmosphere and the ocean. I was surprised to read that coupling occurs at 2-3 ocean and atmosphere timesteps, as this seems to imply a similar time step in the atmosphere and the ocean. Is this indeed the case? Most models that I am aware of use a much longer time step in the ocean than in the atmosphere. Are there any drawbacks for a lower coupling frequency in the flux-coupling approach compared to the standard approach, given the somewhat unphysical development of internal sea-ice temperature for a fixed surface temperature between the coupling intervals? Or is the flux coupling of advantage, as the surface temperature and the atmosphere interact physically more realistically, and this is considered more important in a coupled setup?
Regarding model evaluation, the current analysis does not allow too much insight regarding the source of the potential model improvement, and in particular little insight into potential improvements related to the sea-ice component. For the Southern Ocean, improvements in the ocean component rather than in the sea-ice component are mentioned as the primary reason for the improved sea-ice simulation. It remains unclear whether improvements e.g. in the atmosphere component are the primary reason for the improved sea-ice thickness distribution in the Arctic. Is there any indication that the sea-ice component itself has contributed to these improvements in either hemisphere? I know this is hard to show, but maybe an analysis of more sea-ice inherent properties would be more helpful (e.g., melt-pond fraction, small-scale variability, lead fraction, or the like).
Finally, I was not sure why HadISST was used for the sea-ice concentration comparison rather than one of the standard direct satellite products. Using HadiSST 2.0 can be misleading, as they use a very generous ocean grid with many islands etc being water. "This results in much larger sea ice extents in HadISST.2 for all calendar months, unless the same mask is applied. We recommend that the same grid and mask are used when comparing any sea ice concentration data set." (quote from HadISST 2.2.1.0 website)
Minor comments:
l.69: What does "largely similar" mean? Shouldn't it be either "similar" or "largely equal"? ;)
l.76: Are the uppermost ocean grid cells completely turned into ice as the ice approaches very large thickness? Which coordinate system is used in the ocean model and how is the ice incorporated into it?
l.89: Is this a different convection scheme compared to previous model versions? If so, what was used before? Is third-order necessary and/or helpful?
l.96: Not sure what "this" refers to
l.128: Would be helpful to briefly indicate how the heat-flux calculation by Maykut and McPhee (1995) works
l.136: What is "the beta coefficient"?
l.147: Is salinity also used to calculate energy content / heat capacity in the thermodynamics?
l.226: How relevant is passing these velocities to the atmosphere, as they should be relatively small compared to the wind speed. Is the numerical overhead of passing them over negligible? Or would setting them to 0 in the atmosphere suffice? (Maybe nothing to be examined for this paper, but maybe this is known)
l.302ff: I would recommend to leave this out, or to move it to somewhere else. It's neither a conclusion, nor really helpful at this stage, I find. Maybe it'd be better to integrate the individual future plans into the relevant sections of the paper, with a brief motivation.
Typos / Grammar
l.47: Drop "is"
l.48: "are discussed in the GC5 paper"
l.80: Drop comma
l.90: 'or "to" open water' is somewhat easier to read, I find
Citation: https://doi.org/10.5194/egusphere-2023-1731-RC1
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