This study conducted a comprehensive comparison with ICESat-2 data and CMIP6 sea ice model outputs. The writing and structure is general good. I have only a few minor comments here:

Figure 1: the quality of this fugure should be improved by using vectorized format.

Eqn 1: I suggest to use a table to include all model constants and variables so that we can be more clear which one is constant and which one is model outputs and which one is calculated in this study.

Eqn 3: use same math symbol format for h_s

L188: does the sea ice mass include snow?

L191: Table 1, not 2

L214: what is sea area percentage?

L240: Give the full name of NESOSIM if it appears for the first time

L243: remove the comma after et al.

L245: change vs to versus

Fig 2: (a-c) is the total freeboard? Please clarify; (g-i) I think it should be sea ice concentration as the unit of colorbar is %. In the caption, please change (top) to (a, b, c), (middle) to (d, e, f), (bottom) to (g, h, i), same for other figures.

L266: so in the IS2SITMOGR4 dataset, there are two density approaches, a constant and a J22 parameterization?

Fig 3: similar comments as for Fig 2

Table 2: use consistent unit of freeboard through out the paper. Here is cm, but it is m in the figures

Eqn 7: from this definition the plausibility index should be positive, then why there are negative values in Figure 9? and can we interprete this equation that the plausibility is big when \phi is small?

Section 3.1: I suggest to put the analysis of total freeboard prior to bulk ice density, as the inversion of ice density rely on the data of freeboard.

L407: Eq 4, not 5

L410: Eq 5, not 6

L416 and other places: use symbole ±

Fig 5: use a and b instead of left and right

Fig 7 and 8: have you tried the relative error instead of showing absolute errors? Also, add (a) and (b) for left and right subplots.

L545: I would suggest to use “Arctic” and “Antarctica” instead of “Arctic Ocean” and “Southern Ocean” or something different, “Southen Ocean” feels like a much broader region than sea ice actually exists.

Fig 9: why there are negative values while \phi is defined as only positive

Fig 11: put the explanation of those hatchings in the caption

L644: fontsizes in “there” not consistent

Section 4: Here I was expecting some more discussions of the difference between model outputs and observations, and also some suggestions to the modeling community. Afterall, there are quite a bit of implausible regions for both Arctic and Antarctica for some model results.

Review of the manuscript egusphere-2025-766 titled “Constraining CMIP6 sea ice simulations with ICESat-2” by Petty et al.

This manuscript describes an evaluation of CMIP6 sea ice simulations using observations from NASA’s ICESat-2, focusing on sea ice total freeboard and Arctic winter sea ice thickness—metrics not traditionally used in global climate model evaluation. The authors present a plausibility framework accounting for observational uncertainty and internal variability and explore its application in both hemispheres, ultimately suggesting the incorporation of altimetry data into model assessment pipelines and an increased focus on bulk sea ice density.

The manuscript clearly fits the journal’s scope: it develops and evaluates Earth system models (here, sea ice representation in climate models), integrates observational constraints, and offers methodological innovations of interest to the modeling and remote sensing communities.

Even though I am not an expert in global climate models, I found the manuscript generally easy to read and well structured. However, I think it will benefit from minor changes, which I shall detail below, and a thorough proof-reading. I find the manuscript ready for publication after minor (albeit a rather lengthy list of) revisions.

General comments:

• As is typical for a modelling study, and even more so for an intercomparison, there is an abundance of acronyms, abbreviations, and symbols. Perhaps a glossary or a table defining them could be helpful (in appendix)?
• As a non-expert, I think a conceptual flowchart early in the paper would guide (especially non-expert) readers through the multiple datasets, metrics, and model subsets used. I recommend adding such a figure.
• Journal’s guidelines are not completely followed, I will detail below those that caught my eye.

Detailed comments:

Abstract: Per journal’s guidelines, abbreviations need to be defined in the abstract and then again at the first instance in the rest of the text. However, abbreviations CMIP6 and GCM are not used at all in the abstract. CMIP6 is perhaps necessary, as it appears in the title (although abbreviations should be avoided there), but there’s no need for GCM. Furthermore, ICESat-2 is not explained in the abstract, but it is in the main text. 

L30ff: Throughout the manuscript, the reference to the very relevant paper Notz & SIMIP Community (2020) takes different forms (at least Community, 2020; Notz & SIMIP Community, 2020; Notz and SIMIP Community, 2020; Notz and Community, 2020). Furthermore, it appears wrong in the reference list under “Community, S” and not under Notz. 

L44ff: please reword “improvements … have been suggested, suggesting improvements…”

Fig. 1: I cannot find a single reference to this figure in the manuscript. Please increase the font size, it’s insufficient. The only thing I can see from it are the (undefined) abbreviations in capital letters.

L78: This is the only occurrence of CO₂, better to spell it out as carbon dioxide.

L96: Which spelling are you using, “(kilo)meter” or “(kilo)metre”?

L105 (and before): “their more limited temporal coverage”, I would have expected a sentence or two about the reasons here in the introduction. Issues concerning summertime altimetry data are not mentioned before L228

L141: Per journal’s guidelines, "data" is considered a countable noun --> CMIP6 data are

L147: Typo? ESGF, not ESGP. Also, OPeNDAP is not defined.

L185: Throughout the first half of the manuscript, the density units are formatted wrong. Per journal’s guidelines, units of physical quantities must be formatted with negative exponents. Please check this also in all figures.

L195: Earlier at L168 seawater density was 1024 kg m^-3, now 1026 kg m^-3. A bit confusing for the reader. Was the value 1024 kg m^-3 used anywhere in the study? What is the effect of this change?

Table 1: Caption should be situated above the table, please correct also for the other tables. Variables simass and sivol are not explained like the others are.

L211: Here and elsewhere, are embedded URLs necessary in the text, especially when they are followed by a citation with a DOI/permalink? This is a bit more personal preference, but I think they interrupt the flow of reading.

Fig. 2+: Per journal’s guidelines, “only the first word is capitalized in headers (in addition to proper nouns).” Please check throughout the manuscript.

L261-264: Per journal’s guidelines, common Latin phrases are not italicized nor hyphenated: in situ.

L267: The abbreviation EM is not defined nor used elsewhere. Furthermore, the sentence fails to acknowledge all instruments used in the derivation. I suggest “derived from coincident laser scanning, snow radar, and electromagnetic induction sounding data”.

Fig. 3 caption: Per journal’s guidelines, Figure --> Fig. when it’s not starting a sentence. Please check throughout the manuscript.

L280: Not EASE 2.0 like before? Also, this abbreviation should have been explained earlier (L208).

L290: Rewording needed “large-scale basin-scale”?

L294: NASA Team data has --> have

L300: Roach et al. (2020)

L321: No need for capitalization: upward looking sonar

L323: an --> the AWI CS2/SMOS product, or do they have several?

L344: Typo? 2014 --> 2024

L361: Abbreviation IS-2 not introduced. If you use such an abbreviation, do it consistently throughout the manuscript. Moreover, what is the Bessel correction?

L364: Typo? SSP-2.45 --> SSP2-4.5

L366: Typo? increasingly --> increasing

L379ff: The sentence starting “We then…” is too complex. Please rephrase.

L391: Per journal’s guidelines, Section --> Sect. (like you have written on the next line) when it’s not starting a sentence. Please check throughout the manuscript.

L392ff: I suggest Method 1) and Method 2) to avoid subsequent colons within a single sentence.

L395: The figure numbering in the Supplementary Information is broken, you have S4 twice. Thus, all the subsequent figure numbers are off. Please correct.

L409: Do you mean CNRM-CM6-1-HR? In Fig. 4 that model doesn’t have the black triangle marker.

Fig. 4: SI prefix for kilo is a lower case k, not K. Also in Fig. 10 and in the caption of Fig. 5.

L441: Fig. 5 has 16 models, as mentioned in its caption, too. 

L446: Why is “Annual” capitalized? It appears so multiple times in the manuscript, but not always. Please check thoroughly. Furthermore, in Fig. 5 the multi-model mean freeboard looks more like 22 cm, not 25 cm. Is the figure correct (version)?

L448: But ACCESS-CM2 is shown in Fig. 5?

L480: Typo? inter-modal --> inter-model

Table 3: Does the upper case M for sea ice area stand for the SI prefix mega, i.e. 10⁶? If so, please use the scientific exponent notation, because area in “mega kilometers squared” does not read well with two prefixes.

L539ff: Why do you want to highlight the models and metrics that are implausible? To me that’s thinking backwards, wouldn’t you want to highlight those that are plausible (/good/usable)? Think positive!

L542: highest --> best? Very high (and low) values are implausible, correct?

Fig. 9: The colorbar should extend at both min and max ends, as values beyond ~7 exist, please add arrows/triangles at the ends indicating that (like in Fig. 11). Additionally, abbreviations should be explained (SIA/SIT/TFB/NH/SH/IA).

L587ff: Again, does the upper case M for sea ice area stand for the SI prefix mega, i.e. 10⁶? If so, please use the scientific exponent notation, because area in “mega kilometers squared” does not read well with two prefixes. Furthermore, looking at Fig. 10, the volume changes are 10³, not 10⁶. Which one is correct?

L610: Typo? grid-scale

L611: uncertainties … was --> were

L614: Fig. --> Figure as it starts a sentence

L622: remove second “with”?

L642: Fig. --> Figure as it starts a sentence

L694ff/Discussion: I would recommend adding subsections to improve and clarify the structure. The subsections can be short, e.g. on sea ice bulk density, uncertainty quantification, altimetry datasets, etc. L691 could be removed completely.

L722: split infinitive, to better consider --> to consider better

L724: split infinitive, the ability of models to accurately capture regional internal variability --> the ability of models to capture regional internal variability accurately

L784ff: split infinitive, … to better constrain current and future variability in sea ice and their associated climate impacts. --> … to constrain current and future variability in sea ice and their associated climate impacts better.

L832ff: Is the citation correct, particularly the indicated time period?

L837: Wrong author

L933ff: No longer in press, please add DOI

L976ff: Accepted and published, please update

L981ff: Accepted and published, please update

L987ff: Page range or article number missing

L992: Incomplete citation, add journal, pages/article number, DOI

L994ff: Update article number

L1013ff: Accepted and published, please update

L1021ff: Update article number

L1025: Incomplete citation, add journal, pages/article number, DOI

L1027ff: This preprint was not accepted, but resubmitted as https://doi.org/10.5194/egusphere-2024-2821