Comment on tc-2021-232

In this study the authors seek to find out what GIA model resolution is required to accurately capture viscoelastic deformation in response to present-day and future ice loss in the Amundsen Sea Embayment of West Antarctica. They conduct several experiments to determine this: 1) the elastic response to an idealised cylindrical load; 2) the elastic response to realistic ice loss; 3) viscoelastic response to realistic ice loss with a 1D earth structure and three 3D earth models. The results are given in terms of percentage error for each grid resolution tested and the authors find that errors converge at a resolution of 3.75 km, or three times the radius for the idealised experiment. Furthermore, the results show that the error from neglecting viscoelastic deformation over short time scales, or from adopting different 3D earth models, is far larger than the error from grid resolution.

In this study the authors seek to find out what GIA model resolution is required to accurately capture viscoelastic deformation in response to present-day and future ice loss in the Amundsen Sea Embayment of West Antarctica. They conduct several experiments to determine this: 1) the elastic response to an idealised cylindrical load; 2) the elastic response to realistic ice loss; 3) viscoelastic response to realistic ice loss with a 1D earth structure and three 3D earth models. The results are given in terms of percentage error for each grid resolution tested and the authors find that errors converge at a resolution of 3.75 km, or three times the radius for the idealised experiment. Furthermore, the results show that the error from neglecting viscoelastic deformation over short time scales, or from adopting different 3D earth models, is far larger than the error from grid resolution. This topic is timely given the efforts within the GIA community to improve models. Highresolution models are very computationally expensive, prohibitively so for most, and quantifying the error for different grid resolutions is very valuable. The paper is well written and clearly organised but would benefit from some small amendments to the text as suggested below.

General Comments
1) The main conclusions from this study state that the GIA model grid resolution needed for an isolated cylindrical load is three times its radius, and that for realistic ice loading in the ASE a grid resolution of 3.75 km is required to achieve an error of 2%. I think the paper could be improved by adding a qualitative discussion of how these findings might apply in other areas. Line 344 states "for most applications, errors of less than 5% can be achieved with a 7.5 km grid, and errors of less than 2% with a 3.75 km grid". It would be beneficial to have a short section in the discussion expanding on this, discussing whether this rule of thumb is limited to ASE, or marine grounding lines, whether it might apply in other areas of Antarctica where present-day ice change is occurring, or how different spatial scale of ice loading change might affect this general rule.
2) In this study the authors find that representation of the ice load on the GIA model grid leads to higher error than the grid resolution itself, however, there is no discussion of how representation of the Earth structure within the model grid might impact the results. Particularly in Section 2.2 and 2.3, what is the resolution of the seismic tomography data that is used? How does this compare with the vertical resolution of the grid with depth, how much is it being down sampled? Perhaps the results from the tests mentioned on line 158/159 could be included in the supplementary information.

Specific Comments
Line 21: The authors repeatedly refer to a "spatially isolated load". The use of this term is particularly important since once of the main conclusions of the paper -a grid resolution needed of 3 times the radius of the load -only holds for cylindrical loads. For clarity, consider changing this term to "spatially isolated disc/cylindrical load". Line 57: please quantify short spatial scales, e.g. 10s of km, or 100s of km.   Line 260: "grid points" do you mean nodes? Is the load applied at the nodes or the centre of the elements?
Line 292: "serve as a guide…. appropriate grid resolution for a given problem" I think its important to include here that this guide is restricted to isolated cylindrical loads.
Line 335-344: There is reference to Figure 5    Line 560: add "in the ASE" to the first sentence to clarify the region the conclusions apply to.

Technical Comments
Line 158: missing word "limited to the surface (and?) a few layers down to 10km" Line 211: incorrect spelling of adopted Line 475: reflect rather than reflecting