Review of "The glacial systems model (GSM) Version 24G" by Tarasov et al.

Summary

Tarasov and coauthors describe the glacial system(s) model GSM, intended for simulating ice sheet evolution on the glacial-interglacial timescale. It covers a broad range of modules (climate, ice sheet flow, hydrology, solid Earth deformation, sediment transport, ... ) reflecting the complexity of relevant Earth system processes. I appreciate the effort of the authors to document this model and generally support publication of the manuscript while considering the comments below. 

General comments

1. The manuscript obviously struggles with the difficulty to document 30+ years of model development in one paper. Since many aspects are interlinked and grown together, it seems difficult to suggest a meaningful subdivision on the component level. Nevertheless, the idea to split the paper in two or more smaller parts seems obvious and should be considered. In particular in view of my comments below, which if properly addressed, would make the paper even longer. To give a concrete suggestion: some parts of the manuscript already tend to have the characteristics of a reference manual. One could envision to fully focus this manuscript as part 1 on that more technical description, and have a second part that fully explores model capabilities in a more applied sense.

2. It is difficult to suggest more content in a paper that is already 55 pages long, but while I find the descriptions both detailed and transparent, several parts of the model description are reduced to mere technical level. Generally, there isn't much opportunity to get a grip on what the model choices imply for actual simulations. I am thinking in particular about the different mass balance processes (2.7) and climate forcing options (2.10). The selection of what receives more detail with some examples and figures seems arbitrary. To give a better idea of the model's capabilities, all processes that are not already documented in separate publications should be fully described and illustrated.   

3. The introduction is rather short (effectively 1 paragraph, before going into model specifics) and doesn't give a good introduction to the science GSM is meant to address and the parts of the Earth system it tries to model. After reading the introduction, the reader should have gotten a basic idea of what GSM tries to model, how the different Earth system components interact and hang together, and what other approaches exist to do so. A schematic flow diagram or similar would be useful to support this part. 

4. I miss a better view on the approaches and processes that are celebrated in the introduction to be more complex, complete or otherwise superior to other models (englacial sediment transport, noise insertion). I feel these would require a more thorough analysis and comparison to show their relevance and justify these claims. Otherwise they should probably be toned down.   

5. Large parts of the Conclusions section actually read like a classical discussion section. Consider changing the title to Discussion and Conclusions. Also, the discussion should be extended to give a better view on model advantages, caveats and shortcomings compared to other ice sheet models used in the paleo context (resolution, grid refinement, approximations, ...) or why these are not relevant in the given context.

Specific comments

Title. How does this release relate to earlier (probably unpublished) version of a model with the same name, e.g. https://doi.org/10.5194/tc-7-1949-2013? This should be made clear in abstract and introduction. 

l2. Say what other components it includes to make it a glacial system model, rather than an ice sheet model? 

l2. could remove "glaciological" before "ice sheet model".

l29. Text from here already reads like part of the model description (Sec. 2). 

l29. "Glacial Systems Model" like in the title or "Glacial System Model" like in the abstract?

l30. "... not found as a set in any other ice sheet model"

I think it would be more useful to continue in the spirit of the first paragraph and discuss what this specific context requires as features, before stating how GSM addresses those, and only finally how that is an improvement over other models.  

l34. replace 'nor' by 'or' or 'and'?

l38. "GSM currently having 30 (Patagonia) to 53 (North America) ensemble parameters"

What about the other regions (Antarctica, Eurasia, ...)? Generally, the numbers by themselves or not so meaningful for the uninitiated. 

l41. "(uniquely to date) also has noise insertion options"

How is that different from https://doi.org/10.5194/gmd-15-8269-2022? Include a reference and explain.

Table 1. Include headings for 2.7 and 2.10 

l56. "SSA/SIA" 

Include reference to 2.4 and A1

l58. "appropriate" to do what? Remove "appropriate" or explain. 

l61. "basal drag laws for soft and hard beds" 

Include reference to 2.5

l67. Why not continue following the order of Table 1? 

l68. "geoidal deflection"

I get very few hits for this term on online searches. Is there a more common term for this, E.g. drawn from https://doi.org/10.1007/s10712-019-09525-z?

l75. "glacially-indexed GCM snapshots" 

Is this treated in 2.10.1?

l78. Include description of 2.10.4?

l79. Sec 2.12 is called "Ice margin nudging" in the  main text but "mass balance nudging" in Table 1. Make consistent. Include a short description and reference to 2.12 here.

l82. Include short description of 2.16 here. 

l82. In the end, refer again to Table 1.

l85. Remove one "(".

Table 2 caption "Non-climate forcing ensemble parameters" 

A bit difficult to parse. Maybe "Ensemble parameters (not related to climate forcing)"

 

Table 3

Greenland specific

latitudinal ramp width of added Holocene warming  "42.−40 ×(0.0 →1.0)"

Missing dot after 40?

l106. Sec 2.3 may be better placed as part of or directly after 2.1?

l114. Does the solver translate without adaptation to regular and lat-lon grids? 

l144. "Glenn" --> "Glen", also in l150.

l144. Several recent studies have suggested the Glen flow law exponent could be closer to 4. It seems like an obvious candidate to sample in the ensemble design. Is it easy in GSM to change n? Is that planned?

l165. "for a detail examination" --> "for a detailed examination of the"

l166. Consider using i), ii) , iii) after "accounts for:"

l170. "With the -DNeffDRAG compile flag"

Can this be motivated (physically)? What is the aim of this change?

l175 "exponent mb = 4"

Clarify is this is only for the Greenland domain or generally the case. 

l183. "contemporaneous sea level" 

In which vertical reference frame is the model operating?

l193. "fractional soft bed cover of the grid cell"

Where does this information come from? Is this from a dataset (which one) or dynamically computed based on sediment transport in the model?

l197. "in partial accord with a numerically self-consistent treatment for the setting of cell interface diffusion coefficients"

Not sure I can follow this. Have "cell interface diffusion coefficients" been introduced before?

l202. "input sediment fraction" 

Do we know how the sediment fraction enters the model?

l209. "their data input requirements are unlikely to be met"

Not clear to me what this is referring to.

l218. "A last motivation for this design choice"

While I agree with the arguments for paleo contexts, I wouldn't call that not-implementation a 'design choice'. Maybe just "choice".

l221. "subgrid pinning points under the ice shelf that aren’t presently active"

or not resolved.

l241. "grid cell resolution (∆xy)" 

How is that defined for a lat-lon grid? 

l250. insert "is" before "focused on surge cycling"

l263. "GSM ice and permafrost resolving bed thermodynamics"

Reads like this section is on bed thermodynamics. Maybe "ice thermodynamics and permafrost resolving bed thermodynamics"

Could remove "GSM", in line with other headings.

l267. "Heat source terms include full SSA and SIA contributions to deformation work (Qd) and the boundary heat flux"

Also describe all the other components of the equation.

l279. "Unlike many ice sheet models"

Not sure this is still the case. Energy conservation has been addressed in many ice sheet models these days. 

l307. "orbital changes in short-wave forcing"

Could refer to https://doi.org/10.5194/tc-8-1419-2014 for another approach to this concern.

l311. "Observationally, fitted PDD melt coefficients vary over a wide range"

I was confused by "fitted". PDD factors can be experimentally determined by measuring ice and snow melt and temperature at the same place. Where does the fitting come in? 

Also, wide range during the day, between different locations, or generally? 

l316. "only contributes to surface melt if the surface temperature is at 0o C"

Do you account for sublimation separately?

l324. "PDDs are computed for paleo modelling contexts based on a probabilistic distribution around mean monthly temperatures"

Calls for some references.

Figure 2 caption. "set to it’s nominally regressed value" --> "set to its nominally regressed value"

l345. "it has been common for paleo ice sheet models to determine PDD" --> PDDs

Calls for some references.

l359. "in the above supice equation"

Refer to equation number (20) instead.

l361. "RCMs" is not defined

l361. "This parameterization deviates from previous" 

Add "studies", give references.

l364. not clear what "ibid" points to.

l418. "submarine face melt" 

Maybe "submarine calving face melt"?

l477. insert "the" before "grounding line"

l488. "bergy bits"

Do you mean ice mélange or "sikkussaq"/"sikkusak"?

l500. "self-consistent DEM"

Is the DEM updated based on GIA and blocking of ice? If so, mention/explain here.

l520. "The change is ice thickness" replace "is" by "in"

l521. "surface runoff discharge calculation"

Is mass conservation imposed on the global level/per ice sheet domain? How does that work?

l522. "This ice remains subject to all the other mass-balance processes in the GSM."

Not clear to me what that means.

l526. and elsewhere. What is the logic of capitalising or not section titles? Make consistent.

l530. "August−February differences range up to 25%"

Is this a temperature difference? 25%/100% of what?

l533. "Ie is the mean monthly EBM temperature anomaly"

Could be useful to have a table of the different Is for a better overview, e.g. [I, purpose, equation, range] 

l584. "NCAR CSM general circulation climate model" 

Was called "CCM" at that time. See referenced article.

l643. Consider introducing a subheading for temperature and precipitation (I think bold without numbering is the level 4 heading)

l650. "as followings"

"as follows"

Eq 45. Resolve double subscript on T2m0.

l656. "Computed precipitation is then subject to the factor"

I presume this means moisture is not conserved? Could be good to mention.

l659. "climate index (Ic)", "dome elevation index Id"

I think using the longer form, like here, stating what the indices mean should be the standard throughout the paper. 

l664. 2.10.3 would make more sense for me to be a part of the section on precipitation above (at level 5), rather then a level 3. 

l665. "Paleo ice sheet modellers have traditionally relied"

Calls for some references.

l705. "the Eemian high-stand was inadequate"

add "sea level" to clarify what high-stand this is. 

What magnitude/range are you assuming and trying to match? Reference.

l715. Does this feature really need a separate subsection?  

l719. "is that module" --> "is that the module"

l730. Clarify in how far this means mass is not conserved.

l755. "activated basal hydrology component"

Add reference to sec 2.13.

l770. "as is typical for paleo ice sheet modelling"

Calls for some references.

l787. - l793. "To improve generalizability, ..." 

This text gets very specific and is difficult to understand without further instructions. Suggest to keep the description more general or make an example and get much more detailed. 

l794. "For ice sheets with extensive present-day ice cover"

What ice sheets do you have in mind? Are there more than two (GrIS and AIS)

l794. "the sensitivity of the correction to discrepancies in simulated 0 ka ice

thickness (compared to that observed) are too strong for such a correction approach"

What instead then? Is it not needed to correct those?

l796. "Geoidal deflection within the GSM ice sheet grid is computed using a linear approximation." 

Add a short introduction for the uninitiated. What is the purpose of this calculation?   

l811. " a future"

"the future" or "a future version".

l815. "inter-model comparison experiments"

Typically "model inter-comparison experiments"

l832. "This approximation will become more inaccurate"

While this statement gives some importance to this choice, it could be mentioned here that the choice of an initial temperature profile is arbitrary and mostly a question of convenience, to shorten the required equilibration time. In other words, this choice shouldn't really have a big influence on the final spun up temperature if the relaxation is done appropriately. Is this not illustrated in Fig 6?

l841. "For a set of not-ruled-out-yet"

This concept should be introduced early in the manuscript. E.g. in the introduction.

l846. "is the difference" --> "in the difference"  

Table 7. "key GSM fields"

Maybe "Key prognostic GSM variables"?

l875. "drainage solver verification again present-day drainage" --> "drainage solver verification against present-day drainage"

l898. "process noise injection for internal discrepancy assessment is also to date unique"

See comment l41.

l961. I find the verbatim font without any structuring elements is difficult to read. Could this be presented e.g. as a bulleted list with different symbols in addition to the indentation? 

Figure B1.  Difficult to make out differences. Consider zooming in or producing an inset, e.g. around the LGM, Eemian and/or present day.

l1035. Missing some dois throughout the reference list. Here e.g. https://doi.org/10.1007/s40641-017-0071-0

Supplement

All figures in the supplement.

- increase axes label sizes and y-tick label sizes

- parameter number (x-ticks) can be removed.

- increase x-axis range to fully include rightmost parameter 

- move parameter names up so they are not overlapping with the x-axis or outside of the figure to also avoid overlap of symbols and parameter names

- replace y-axis label "metric" by actual metric name and units.

Captions. Cryptical what the reference parameter vector is (e.g. an1600). Explain?

Figure 1. "for the parameter range in Tables2 and 3." ... "in the main manuscript".

In the manuscript “The glacial systems model (GSM) version 24G”, L. Tarasov and co-authors describe the glacial systems model, a software that simulates the evolution of ice sheets on paleoclimate timescales and that has been use for over two decades. The manuscript is very detailed, explaining both the physical processes included in the model, as well as their numerical implementation, and listed the different options available, including default configurations and parameter values.

The manuscript is usually well written and clear and the tables are useful, however I found the figures not very informative: it is unclear what these specific figures have been included as they are not very representative of the overall model, and additional the paper is missing some overall figures describing all the processes captured as well as typical runs including all the ice sheets and processes described (for example showing the ice extent for the different ice sheets at glacial maximum and interglacial times).

My main concerns are related to the need to be more clear about all the processes included, since people reading the manuscript are not necessarily familiar with all the physics, processes and numerical implementation described. For example, some terms are not common and should be better described (e.g., discretization of basal ice grid cell, etc.). I also noticed that many sections have very few references if any, and previous both performed both using this model as well as other models should be better referenced. So many of my comments below refer to the need to better and justify choices made in the code.

The discussion and conclusions are a little overlapping, and several aspects mentioned in the conclusions could be moved to the discussion. I also think that additional discussion about how this model compares to other existing models would be beneficial to highlight the similarities and differences with other ice models.

Finally, for the code and data availability, the configurations shown in this paper should be accessible and easily reproduced, on top of providing access to the GSM code.

Specific comments:

l.3: “evolved from” -> “evolved over”

l.3: “constrained last glacial” -> “constrained the last glacial”

l.4: “each major ice sheet”: these ice sheets should be listed

l.12: “traditional glacial indexed interpolation”: what does this mean?

l.23: “eg Drew …” -> e.g., Drew … (same in all the manuscript)

l.28: add examples

l.34: “nor subglacial” -> “and subglacial”

l.70: “The resolving of pro-glacial” -> “Resolving pro-glacial”

l.74: “the capture of marine ice” -> “to capture marine ice”

l.85: It is not clear how this is addressed: more uncertain parameters make it very difficult to constrain as observations are sparse/lacking for many aspects

l.91-94: What is the objective here? Would it be more clear to use physical values?

Table 2 caption: “depending on the whole parameter vector value” -> “combination”

Table 2: why use a range of 0 to 2 for the basal drag subgrid parameters? I thought these parameters represented a range from fully floating to fully grounded (so 0 to 1)?

Table 3: it would be better to include units in the table for as many parameters as possible

Table 4: Why use a different number of layers for the dynamics and thermodynamics, and how does it work in the code? How are the values transferred from one set of layers to the others and what is the impact?

l.96: what are implicit none?

l.102: “vertical layers”: are the layers fully vertical or terrain following?

Section 2.4: lots of references missing in this part (SIA, SSA, etc.)

l.116: What is the appropriate boundary condition here? Explain it clearly

l.122: Does it mean that it fails even when using the CFL condition? In this case what is the reason for that and what are the solution?

Eq.2: What is QL

l.144: “floating ice grounding onto ice free land”: this sentence is very confusing, try to explain that more clearly

l.153-158: how were all these values chosen and calibrated? What does the uncertainty look like?

l.162: add references for the sliding laws

l.165: “for basal drag” -> “with basal drag”

l.165: you should precise the form of this temperature ramp since this is a description paper

l.173: “scaling coefficients”: how are they chosen?

l.175: Greenland is mentioned here, is that valid for other regions?

Eq.8: What is \phi?

Eq.9: How do you calculate \frac{\partial \tau_b}{\partial U_b}? And why is it needed?

l.190: Which two coulomb options? Again more references are needed here

l.196: What does “numerically self-consistent” mean here, and how is that assessed?

l.212 and l.214: How are the regions with hard and soft beds determined? Can they evolve over time and if so under what conditions?

Eq.12: How is T_exp chosen and what are typical values?

l.242: Which “physical grounds” are referred to here? And what does that represent?

l.266: What is the horizontal diffusion not included? Also many coefficients and variables are not listed and explained in this equation.

l.267: Explain what the boundary heat flux is.

l.272: “discretization of the basal ice grid cell”: what does that mean and how is it discretized?

l.287: Why do that and not use all the terms? It should not be more complicated or expensive to run

l.293: Add references here

l.329: “mid to high latitude”: why use such a wide range? Could that be narrowed down?

Figure 2 caption: “Comparison GCM” -> “Comparison of GCM”

l.348: “non-Gaussian distribution”: describe what are the properties of the distribution

l.358: “for Greenland” -> can this change in other regions with different temperatures and precipitation?

l.374-379: What about increased melt in warm conditions? This paragraph is confusing and would deserve additional information.

l.384: “requisite” -> “require”

l.402: “horizontal advection dur to ice shelf ocean circulation”: Are you talking about the ice or ocean advection?

l.414: Why is needed to further reduce the friction beyond the ratio of grounded ice? What are the physical and/or numerical reasons for that? And how was a value of 0.5 estimated?

l.417: in the Rignot melt approximation, q is the freshwater discharge and its unit is in m^3/day. How is the value you are using related to the Rignot approximation? And what does it mean in terms of size of water channels?

l.420: the correct units should be described for a and \beta, with no units Eq. 24 is not consistent

l.424: What is “C_face”? What does it represent?

l.425: I thought ice shelves were necessarily marine. If “marine” is required, you want to explain why.

l.426: “For marine floating ice calving” -> “For ice shelf calving”

Eq.25: What are r and r_c?

Eq.26: What are d_a, d_t, and d_w? Mention what they represent just after the equation since the details are half a page later

Eq.28: What use such a form for this equation? What does it represent?

l.483: “heat to melt icebergs” -> Does it mean the icebergs or at least the amount of icebergs are tracked? How does it work?

l.497: What is. Marine depth?

l.527: remove “to date”

Eq.41: What is “scalarSealevel”? And where does the form of this equation come from?

l.609-610: Why use such a process for these components?

l.627: What do C_HTM and \Theta_wrm represent?

Eq.43: I don’t understand how I_c is calculated from I_c? Does it mean there are non-linear iterations? Or is it updated at different time steps?

l.647: What is the interpretation of the ensemble phase factor?

l.666: What are the typical lower and higher resolution? How are the fields interpolated or downscaled?

l.676: How is the weighting decided? What does appropriate mean in this context?

l.716: What is a “subgrid ice flow”?

l.723-724: I don’t understand this sentence. Is there always some nudging? What about the “regular” ice margin without nudging and its evolution? How about grounding lines?

l.731: What is fmgm?

l.747: How is the bed roughness scale found? What dataset or parameterization is used for that?

l.750: How is the location of the moulins decided?

l.757: Additional information regarding the sediment erosion, transport and deposition is needed, including processes that they represent, time steps of these computations, tracking (or not) the sediments deposited, and their integration with the rest of the model.

l.770: Why would not account for ice load changes in Greenland? Also some references to explain this further are needed.

l.775: Why store 30 kyr and not more or less? References needed here as well

l.817: Which ones are “poorly constrained”?

l.823: Does it mean you can choose on which variables you put the noise? Or is it simultaneously included for all variables?

l.825: How are they interpolated on the simulation grid?

l.837: I don’t understand this sentence.

l.845: “forcing as is evident is the difference” -> rephrase

l.856: How about the circular symmetry?

Table 7: How about the vertical velocity, the ice front and grounding line positions, and the surface mass balance and basal melt applied?

Figure 6: What is the difference between the last two lines? It cannot be figured out from the legend and figure caption.

l.863: what does “none-steady” mean?

l.875: “again” -> “against”

Code availability: make sure that the configurations shown in this paper and the code to reproduce the results shown (exact configuration) are also accessible.