the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Mar 2025
Brief Communication: Correction of Fundamental Errors in the EVP Sea Ice Dynamics in ICON
Abstract. Since its implementation, EVP sea ice dynamics in the ice-ocean model ICON-O contained severe errors and thus all ICON simulations coupled to it. Two errors prevented convergence of the EVP solver, while a third miscalculated the quadratic drag law using an incorrect relative ice velocity. This caused excessive ice drift, overly mobile ice, and large open water patches, distorting ocean-atmosphere exchanges—worsening at higher resolutions. Correcting them improved the sea ice drift, aligning it with observations and yielding a realistic ice cover. This study marks a turning point in ICON's sea ice representation, ensuring significantly improved simulations at all resolutions.
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RC1: 'Comment on egusphere-2025-906', Anonymous Referee #1, 14 Mar 2025
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Review of Correction of Fundamental Errors in the EVP Sea Ice Dynamics in ICON by Oliver Gutjahr, egusphere-2025-906
The manuscript describes bugs in the sea ice dynamics code of the sea ice component of the general circulation model ICON-O, and how fixing them improves the sea ice simulations with this model. The manuscript is well written and the figures are clear.
My main concern is the scope of the manuscript and the format. As it is, I do not learn anything from the manuscript other that there were bugs in the sea ice part of ICON-O (and possibly also in FESIM) and that they have been fixed with a beneficial effect. This is interesting only for a very specific group (scientists using ICON or ICON results) and I don’t see why this should be the subject of a peer reviewed paper, but it should show up in the bug-reporting system of ICON, and disseminated to the uses. Therefore, I do not recommend to publish this manuscript in the present form.
To me, it would make a lot of sense to make the manuscript more interesting to a larger group of sea ice modeller. With some small additions the author could give the manuscript a different spin to make it more general (and hence publishable). I have these suggestions:
- There are two bugs: (1+2) The EVP solver could not converge because the dynamics variables velocity and stress were reset to zero after each iteration, and (3) there was a bug in the formulation of the ice-ocean stress that lead to an overestimation of the ice-ocean stress. The effects of these totally different bugs are assessed together. I think that it would have been more instructive to isolate these effects, i.e. by comparing additional simulations where only one of the two bugs are fixed. I assume that the solver error (1+2) is less important than the ice-ocean stress error (3).
- The manuscript claims that the EVP solver now converges, but there’s no “proof” or illustration of this. Kimmritz et al (2015, doi:10.1016/j.jcp.2015.04.051 and 2016, doi:10.1016/j.ocemod.2016.03.004) and many others found that the convergence rates of the EVP solver are always very slow, and that true convergence is only reached with O(1e5) iterations. It’s important to check the convergence and if the solver would converge if enough iterations are allowed (see, e.g. Kimmritz 2016, but also others).
- It would be interesting, if the ice-ocean stress is treated explicitly or semi-implicitly in the ICON code. If it is the later, then it is much easier to make this mistake of forgetting the implicit sea ice velocity and that would be instructive for others to find similar mistakes, but the manuscript does not describe this aspect.
Minor comments:
page 1
l13: “internal rheology determines its deformation” rewrite, because “sea ice” as used here is not a realistic medium, but we parameterise it as a quasi-continuous non-normal fluid with a nonlinear (here viscous plastic) rheology connecting stress and strain rates, i.e. here you need to talk about the rheology of the quasi-continuum. “Sea ice” does not have a rheology but consists of floes that may have a rheology (probably very different from what we use for sea ice models).
page 3
l38: “The last error is also present in the recent version of FESOM2.5 (Rackow et al., 2025) and originated in ICON-O when the FESIM ice dynamics (Danilov et al., 2015) were adapted.”
I hope that the authors of FESIM etc have been informed.
page 4
l50: “Since the EVP solver did not converge in previous versions, a tuning was necessary. Increasing the number of subiterations NEVP = 500 was found to be an appropriate balance between computational time and solution quality.”
See also main comments: Can you show convergence rates? According to Kimritz etal 2015/2016, convergence is linear and generally slow. What was the criteria for N=500?
l52: “The constant ice-ocean drag coefficient was raised to Cw = 12 ×10−3 (up from 5.5×10−3)”
Not clear why this is necessary. Wouldn’t it make more sense to reduce the atmospheric drag on ice if too strong winds are the problem? What about the atmospheric drag on the ocean? That should be too strong, too. How was that handled?
l56: “The first impact is on the sea ice concentration, which is exemplarily shown in Fig. 1 for a date (22. March 2012 at 6 UTC) near the end of the simulation.”
See also main comments: It would have been very instructive to understand if the unconverged EVP solver or the ocean stress error are more important. I.e. have a simulation only with error (3), and (1+2) correct and vice versa. This would add more information than just the bug report.
l66: “friction” -> I believe the technical term is ice-ocean stress
page 4
l67: “Since ERA5 overestimates the wind stress over sea ice, using a higher ice-ocean drag coefficient further slows the drift, which is in better agreement with NSIDC (Fig. 2c).”
Again, since ERA5 has been identified as the culprit with too high wind speeds, either correcting the wind speed (no clue how) or reducing the atmosphere-ice stress seems to me to be a better method than counteracting the high momentum input into the ice by a large ice-ocean stress. In this way I would expect that the too fast ice (see figure 2c) imparts too much momentum to the ocean and the too high wind speeds lead to too high ocean surface velocities.
Citation: https://doi.org/10.5194/egusphere-2025-906-RC1
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