the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Multi-year simulations at kilometre scale with the Integrated Forecasting System coupled to FESOM2.5/NEMOv3.4
Abstract. We report on the first multi-year km-scale global coupled simulations using ECMWF’s Integrated Forecasting System (IFS) coupled to both the NEMO and FESOM ocean-sea ice models, as part of the Horizon 2020 Next Generation Earth Modelling Systems (nextGEMS) project. We focus mainly on the two unprecedented IFS-FESOM coupled setups, with an atmospheric resolution of 2.8 km and 4.4 km, respectively, and the same spatially varying ocean resolution that reaches locally below 5 km grid-spacing. This is enabled by a refactored ocean model code that allows for more efficient coupled simulations with IFS in a single-executable setup, employing hybrid parallelisation with MPI and OpenMP. A number of shortcomings in the original NWP-focussed model configurations were identified and mitigated over several cycles collaboratively by the modelling centres, academia, and the wider nextGEMS community. The main improvements are (i) better conservation properties of the coupled model system in terms of water and energy balance, which benefit also ECMWF’s operational 9 km IFS-NEMO model, (ii) a realistic top-of-the-atmosphere (TOA) radiation balance throughout the year, (iii) improved intense precipitation characteristics, and (iv) eddy-resolving features in large parts of the mid- and high-latitude oceans (finer than 5 km grid-spacing) to resolve mesoscale eddies and sea ice leads. New developments made at ECMWF for a better representation of snow and land use, including a dedicated scheme for urban areas, were also tested on multi-year timescales. We provide first examples of significant advances in the realism and thus opportunities of these km-scale simulations, such as a clear imprint of resolved Arctic sea ice leads on atmospheric temperature, impacts of km-scale urban areas on the diurnal temperature cycle in cities, and better propagation and symmetry characteristics of the Madden-Julian Oscillation.
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Status: open (until 07 Jun 2024)
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RC1: 'Comment on egusphere-2024-913', Anonymous Referee #1, 24 Apr 2024
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This is a report on an ECMWF contribution to the nextGEMS project consisting of a 4.4 km and 2.8 km version of IFS coupled to two eddy-permitting ocean models, NEMO and FESOM. Accurate coupled 5-year simulations, especially with IFS-FESOM, are a nice achievement on their own and a step toward the nextGEMS goal of multidecadal km-scale ocean-coupled simulations using seamless models that can skillfully span weather and climate timescales. A variety of improvements over ECMWF’s operational IFS-NEMO model (9 km atm/0.25 degree ocean) are implemented, including global water and energy fixers, cloud tuning for radiation balance, a scale-aware parameterization of convective mass flux, ocean cooling from snow melt, Antarctic meltwater runoff, better land, urban and snow schemes, etc., in combination producing an impressively realistic climate simulation. Some phenomena that require km-scale to simulate are highlighted, including atmosphere-ocean interaction around leads, urban heat islands, and a QBO of realistic period driven primarily by resolved-scale gravity waves.
Overall, the paper is well written and the results are state-of-the-art. I appreciated the illuminating discussion of the model development process, proceeding through three major iterations. It would have been nice to see a bit more analysis of the simulated ocean state, including mesoscale eddy statistics and vertical structure. The 2m temperature bias over ocean in the 5th year of the IFS/FESOM simulation looks remarkably small in Fig. 10, which is encouraging, but were other characteristics such as eddy kinetic energy, mean currents, thermocline depth, etc. examined? These characteristics do evolve on sub 5-year timescales and are relevant to the 30-year performance of a coupled model, so they seem in scope for this paper.
It is sometimes claimed that km-scale climate modeling is fundamentally simpler than 100 km grid climate modeling due to less reliance on poorly constrained parameterizations of subgrid variability, e.g. orographic gravity waves/drag, deep convection, subgrid cloud heterogeneity, etc. The IFS experience seems to be that this is only partly true - the simulations are best if those subgrid parameterizations are still active at this scale, even though resolved-scale motions are doing more of the work. This point might be made a bit more clearly in the conclusion to the paper.
Citation: https://doi.org/10.5194/egusphere-2024-913-RC1
Model code and software
FESOM2.5 source code used in nextGEMS Cycle 3 simulations with IFS-FESOM Thomas Rackow et al. https://zenodo.org/doi/10.5281/zenodo.10225419
Source code changes to the Integrated Forecasting System (IFS) for nextGEMS simulations Thomas Rackow et al. https://zenodo.org/doi/10.5281/zenodo.10223576
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