|Re-Review of TC manuscript 2015-224 now titled "Application of GRACE to the assessment of model-based estimates of monthly Greenland Ice Sheet mass balance (2003-2012)" by Schlegel et al.|
The authors have addressed my comments from an earlier revision and the manuscript has clearly improved from its last version. It is, however, still not ready for publication. I have found many issues of imprecision in the language and in the descriptions that need to be addressed before publication. While the manuscript has gained in clarity from the restructuring, it needs another round of careful revision by the authors to clean up remaining problems. I have listed some detailed comments below, but my estimate is that it needs a bit more work than checking off that list.
The use of minus signs in product names (e.g. ISSM-GrIS+P) is confusing, because they also appear in difference calculations. You could use underscores instead, or find another solution.
For me Figure 8-11 are clearly supplementary material. A move would help to make the main paper a bit lighter.
While not crucial for the interpretation, some attempts to make plots more comparable seem in place. Projections and aspect ratios e.g. differ considerably between figures. The GrIS appears in 5-6 different "projections". S2, S3, S6 and S7 appear compressed, same for S9-S13. For projected grids 1:1 aspect ratio should be the standard.
The choice of colour schemes for 2D plots in general seems poor to me and should be improved. The standard red-blue gives a good idea what is positive and what is negative but not much more. The jet colour scheme does not work well when the full colour range is not used. See also detailed comments per figure below.
P1,L16 add "and other" before related.
P1,L17 Maybe "predicted by the SMB models". SMB doesn't predict.
P2,L8-9 "accurate understanding of ice flow sensitivity to future changes in surface mass balance" is only one aspect. Sensitivity to ocean forcing and bed geometry needs to be mentioned here as well.
P2,l15 Shannon et al. (2013) is not the right reference to allude to "model-based estimates [...] plagued with significant spread and large uncertainties". What may be true for the SEARISE experiments (own spinup procedure, different treatment of boundary conditions/model forcing) does not apply here. Instead of removing the reference I would suggest to attempt a more balanced description of the state of large-scale ice-sheet modelling.
It is also misleading that the first missing process listed just below is basal lubrication, which was explicitly addressed by Shannon et al. (2013).
P2,L21 Remove one closing bracket.
P2,L26 "ice sheet surface deformation" sounds unusal. Please reformulate.
P2,l27 Move (Enderlin et al., 2014) to the end of the sentence to avoid consecutive brackets.
P2,l32-33 Reformulate "onto high-resolution grids". Maybe "with high spatial resolution".
P3,L1 There are many examples of references that show a good agreement between SMB models. A recent example is Vernon et al. (2013). I reiterate my call for a balanced view on the achievements of the modelling community.
P3,l3 New sentence after "ice discharge"
P3,l20 "offer an opportunity *to improve* ... ", as estimates of GrIS MB have existed before.
P3,l26 "conclusions" seems too strong of a wording for the amount of uncertainty. Same applies to P3,L34. Please reformulate.
P4,l28 "GAE and GAF" are not defined.
P5,l19 "solution" instead of "formulation"
P5,l24-28 BMB is not included in your model! Remove it from equation in l25, it's confusing.
P5,l29 insert "ice" before thickness.
P5,l31 insert "The" before SMB.
P6,l2 There is an inconsistency in the argumentation here. You write between 1971-1988 the ice sheet had on average zero mass balance, but the reference period is a different one (1979-1988). Clearly, the ice sheet did not have a MB of zero each year between 1971-1988 but on average. Please clarify.
P6,l3 insert "net" before mass balance. This change may be needed in other places as well.
P6,l6 I miss a clear motivation why the ice sheet relaxed to 1979-1988 SMB may serve as a staring point for 1840. My understanding is that there may not be many good physical reasons (like assumed similarity between 1840 and 1979-1988) but better technical reasons. This should be clarified.
P6,l6 The causality implied by the use of "Therefore" escapes me here. Is the relaxation you talk about the steady-state relaxation to 1979-1988 SMB or further relaxation between 1840 and present? You should quantify the effect of both the 140 year simulation and of the original relaxation.
P6,l9 Not clear now what "For this" relates to. Reformulate.
P6,l9-10 "topography" is also not a "surface property". With surface property I would refer to the albedo or the type of snow cover but not a geometrical quantity like topography.
P6,l9-10 It is not correct to say that you "best capture [...] surface velocities at the beginning of our simulation". You may best capture velocities for the period 1979-1988 but not for 1840. For 1840 you make some kind of ad-hoc decision to use the present state. See also comment P6,l6 above.
P6,l24 What is a "relative steady state"? Reformulate.
P6,l30 It should be "we restart from the 1978 model state and force with SMB anomalies ..."
P6,l32 Confusing that the SMB product time ranges are given as [1979-2014], when you only use 1979-2012 as in the title and all the plots.
P7,l1 Include "lateral" after "their"
P7,l3 Include "change" after "thickness"
P7,l11 For me, "mascon elements" should be used throughout the manuscript, but since you have argued against it, for consistency, it should be be called "mascons" here instead.
P7,l14 Contributions to what? Clarify.
P7,l19 Since you say projected, mention which projection is used.
p7,l32-33 Sentence "Note ..." should be mentioned earlier when masking is discussed (around p7,l18)
p8,l6 Add "in those regions" after "mass trend"
p8,l28 "output from RCMs that are forced"
p9,l5 insert "of" before "RCM-derived"
p9,l19 Clarify over which area the thickening has been averaged.
p9,l29-31 Reformulate and avoid ,(), in the sentence.
P10,l8ff This whole paragraph needs to be substantiated and reworked. Main problem: The ice sheet model is treated as a grey box that does something largely unknown to the SMB forcing.
Some suggestions are given here.
P10,l8 I believe the section title should be "Dynamic contribution" or similar.
P10,l10 "decreasing the rate of ice discharge" has to be much better explained. It is a reason for the difference between SMB and ISSM not the result. References have to be included that have described this effect before (e.g. Huybrechts and de Wolde, 1999; Gillet-Chaulet et al., 2012; Goelzer et al., 2013)
Please describe what is actually happening physically and remove the following statements.
P10,l11 "effects of the ISSM model"; l12 "use of the ice sheet model"; l13 "the model mutes"
P10,L13 "suggesting"; l15 "likely related to" are vague statements given that a process is physically modelled and can be studied looking at the output.
P10,l22 I find this interpretation confusing. The better agreement you refer to is likely referring to the reduced error range. However, the error range is only compressed relative to the SMB results because of the dynamic response, which is non-linear. There may be a problem with the error propagation.
P10,l24 "close match" suggests that ISSM-GrIS and SMB-GrIS should match, which is not the case. Better to reformulate to "close similarity" or similar.
P13,l9 Why are problems with the initial velocity raised in this context? Not clear how the background state is expected to change the results here.
P13,l11 Not clear what the continuation refers to, maybe "the Northwest loses mass".
p17,l25 "differences in" instead of "differences between"
p19,l13-16 Details about what numbers are used to encode the masks are irrelevant. Please remove.
p19,l28-31 Details about what numbers are used to encode the masks are irrelevant. Please remove.
Figures and Tables:
The colours still appear muted in the figure compared to the legend and are not easily distinguished.
Include a black dashed line for reference total SMB of the BOX model
Include "cf." or "see" before "Sect. 5"
Add a note for the different vertical scale in A compared to B in the caption.
The figures are not of good enough quality. Zooming in to distinguish overlapping lines doesn't help. --> Panel A has too many lines to be legible. Maybe you could omit outlines of the error ranges to improve this. Otherwise, consider creating a new figure or split content with S5.
Shouldn't ISSM-GrIS have the same error bars as SMB-GrIS? The error range is shown to be smaller, which can not be the case. Please clarify.
The standard jet colour scale makes it difficult to distinguish positive and negative trends, especially as the upper 30 % of the scale are not used. Using the same scale for the absolute and the difference, while elegant for the display may be too much of a limitation. Please consider revising the colour scales.
Same comment as for Figure 4. Here the colour scale saturates at the top end, while the lower half is not used for A and B (all positive values). I suppose this was chosen to have a zero-centred bar for the differences. Please consider using two different scales for absolute values and differences and revising the colour scales.
Same comment as in Figure 5. Trend difference is mainly positive, and amplitude difference mainly negative. Half of the colour information is wasted here.
Similar to colour problems in Figure 6.
Hard to read this table.
Start caption with "Cumulative mass trends (Gt/yr)".
Consider exchanging right and left side.
Why not include a first row of informative categories in the table?
Why refer to Fig 3A for the left entries, aren't they corresponding averages of the right entries?
Indicate the inset area of B in A.
Consider non-linear colour scale in B.
Consider more informative colour scale in B.
See similar problem with colour scale and map as in figure 4 and 5.
Too many lines. This figure is largely similar to Figure 3. The added value is the different SMB products. Maybe remove GRACE, ISSM-GrIS and SMB-GrIS for better legibility. Otherwise rename GRACE to GRACE-JPL to make legend in line with text and figure 3.
It should be pointed out in the text that BOX is very similar to the three-model average except for the last year.
Consider more informative colour scheme for A. Delta velocity should probably be displayed on a non-linear scale. I suppose the Northern outlets exceed -10 m/yr difference. How was the limit motivated, to fit with scale in B?
Consider more informative colour scheme for B.
Consider more informative colour schemes and harmonise (A is different from B and C for no apparent reason)
If I understand correctly, the trends should not be zero, but undefined for interior mascons. If so, this should be represented in the figure (maybe grey or white instead of green).
Gillet-Chaulet, F., Gagliardini, O., Seddik, H., Nodet, M., Durand, G., Ritz, C., Zwinger, T., Greve, R. and Vaughan, D. G.: Greenland ice sheet contribution to sea-level rise from a new-generation ice-sheet model, The Cryosphere, 6(6), 1561–1576, doi:10.5194/tc-6-1561-2012, 2012.
Goelzer, H., Huybrechts, P., Fürst, J. J., Nick, F. M., Andersen, M. L., Edwards, T. L., Fettweis, X., Payne, A. J. and SHANNON, S.: Sensitivity of Greenland ice sheet projections to model formulations, J Glaciol, 59(216), 733–749, doi:10.3189/2013JoG12J182, 2013.
Huybrechts, P. and de Wolde, J.: The dynamic response of the Greenland and Antarctic ice sheets to multiple-century climatic warming, J Climate, 12(8), 2169–2188, 1999.
Vernon, C. L., Bamber, J. L., Box, J. E., Van Den Broeke, M. R., Fettweis, X., Hanna, E. and Huybrechts, P.: Surface mass balance model intercomparison for the Greenland ice sheet, The Cryosphere, 7(2), 599–614, doi:10.5194/tc-7-599-2013, 2013.