Review of “The surface mass balance and near-surface climate of the Antarctic ice sheet in RACMO2.4p1” by Christiaan T. van Dalum et al. 

The Cryosphere: egusphere-2024-3728

General comments:

The authors introduce and evaluate the state-of-the-art version of the Regional Atmospheric Climate Model (RACMO), version 2.4p1 (R24), applied in Antarctica in this paper. Because RACMO is widely used in the cryosphere community to investigate ice sheet surface mass balance, knowing the current state of the state-of-the-art version of RACMO must be interesting for readers. Also, this paper is informative for readers interested in climate model development in polar regions. I have found that the model development is solid, and the evaluation processes are reliable. I would suggest that this paper can be published after revisions. 

One major issue for me is composition of this paper (all the major specific comments are related to this point): The authors want to show comparison results between R24 and the previous version R23p2 before showing evaluation results of R24 with respect to in-situ measurements, which I could not fully understand. I think the model evaluation results with respect to in-situ measurements should always come first in the result section for this kind of model development paper. This is because, if the model performance with respect to in-situ measurements is not good, comparisons against the previous model versions and/or model data analyses have no meaning. Therefore, confirmation of the suitable model performance is the first priority for this kind of paper in my humble opinion. Please recall that the authors’ research group always does like this when they perform SEB (surface energy balance) analyses, which I think is the solid scientific approach.

Specific comments (major)

Section 3: I believe the authors should start by presenting model evaluation results with respect to in situ measurements (in particular, Sect. 3.3 and Table 2) before showing interannual variations of model-simulated SMB and its components. The current format mixes evaluation results and analyses of interannual variations, which was a bit difficult for me to follow the content. Related to this point, in Sect. 3.1 (around L. 221 ~ 222), I wanted to know about the impacts of enhanced horizontal resolution in R24, which was explained later in Sect. 3.3. If the current contents in Sect. 3.3 come before the current contents in Sect. 3.1, it will be easier for readers to understand the authors’ thoughts on the model performance. 

Section 4: Like Sect. 3, I think the contents in Sects. 4.2 and 4.3 should be presented before that in Sect. 4.1. I think model evaluation with respect to in-situ measurements should always come first. 

Section 5: The same as my comment on Sect. 4.

Specific comments (minor)

L. 105 “Rain and snow are now also prognostic and separate variables, allowing them to have a fall speed and to be moved by advection of the ambient air flow.”: Is it true? How did the authors calculate rainfall and snowfall amounts in RACMO2.3?

L. 121 (Sect. 2.1.3): Please indicate the time interval of the RACMO output data. 

L. 217 ~ 218 “where exposed blue ice lowers the albedo (Tuckett et al., 2021).”: Is it an observed result or what R24 simulates? Please clarify. 

L. 251 ~ 252: The authors indicate that a change of 5.9 Gt yr-1 is insignificant and a change of 5.8 Gt yr-1 is significant, which sounds strange. Please reformulate.

L. 340 ~ 342 “In the Wilkins ice shelf, these aquifers exist for several months before refreezing during winter, and are typically located at least 1 m below the surface, thus undetectable by the remote sensing technique used here.”: Is it an observed result? If so, please add a reference for this argument.

L. 466 (and L. 180) and Fig. 12: White-sky albedo is defined as albedo in the absence of a direct component when the diffuse component is isotropic (e.g., https://lpdaac.usgs.gov/products/mcd43d61v006/). What is the definition of the RACMO2 albedo here? Is it white-sky albedo? Please clarify. 

Technical corrections

L. 11 “shortwave radiative fluxes”: Net or downward or upward? Please specify.

L. 12 “Longwave radiative fluxes”: Same as the technical comment on L. 11

L. 32 “In Antarctica”: This can be removed because it is already said in L. 30. 

L. 58 ~ 60: Almost the same information is introduced at the beginning of Sect. 2.1. I think this part in the introduction section can be removed.

Figure 2 caption and L. 213 “Yearly-average SMB”: I feel this is a bit confusing. At first, I felt it referred to the value of monthly (or daily?) accumulated SMB averaged over a year. But maybe it's “average annual (accumulated) SMB”? If so, for the Figure 2 caption, I would write like “Annual accumulated SMB of R24 (a) averaged for 1979-2023 ~”. If the authors agree with this point, please check other parts of this paper, e.g., Fig. 3 caption. 

Table 1 caption: The difference should be introduced together with Δ before the standard deviation. 

L. 229 “Fig. 2c with d”: “Fig. 2c with Fig. 2d”?

Table 2 caption: I think it is better to rephrase “~AntSMB observations (1979-2017) with respect to (w.r.t.) R24 and R23p2” -> “~ R24 and R23p2 with respect to (w.r.t.) AntSMB observations (1979-2017)”. Please also check the header information in the table. 

L. 298: Suggest adding “in R24” at the end of “The interior of the EAIS thus seems to be too dry.”

L. 318: “Radiative transfer” -> “Radiative transfer in snow and ice”?

Figure 9b: The intervals in the x and y axes can be changed from 50 to 100. The current presentation of x-axis is not good. 

Figure 10b: The interval in the color bar should be improved. At present, there are three “0.00”, two “0.01”, and two “-0.01”.

Figure 12a: Suggest adding “Modeled” before “Melt with respect to the QSCAT melt product regridded to ~”

L. 513: It is helpful for readers to indicate the horizontal resolution of R23p2 again here. It is also informative if the authors list some technical updates achieved in R24 with respect to R23p2 here.

General Comments

The manuscript by van Dalum et al. presents a comprehensive evaluation of the new RACMO2.4p1 regional atmospheric climate model applied to the Antarctic Ice Sheet over the 1960–2023 period.

The results demonstrate improvements over the previous RACMO2.3p2 version, particularly in simulating key components of the SMB and near-surface climate. The integration of updated physical parameterizations—such as a revised blowing snow scheme or a spectral snow albedo model represent a step forward in polar regional modelling. Further, the manuscript provides robust validation using a multi-source approach (AntSMB, AntAWS, IMAU AWS, MODIS, QSCAT), while also acknowledging existing limitations, such as sublimation-related uncertainties in the ablation area.

This work fits well within the scope of the journal, provides valuable climate data for the scientific community, and represents an important contribution to the field. I recommend minor revisions to further improve clarity and interpretability.

There are a few specific areas where additional clarification or discussion could be considered to enhance the manuscript:

1. Sublimation and longwave radiation biases: The manuscript acknowledges the underestimation of downward longwave radiation (~20 W/m²), dry biases in the AIS interior and uncertainties related to sublimation, particularly in marginal zones. A more detailed discussion of these aspects would improve the understanding of model limitations. If available, consider including sensitivity tests or comparisons with alternative physical parameterizations to evaluate these biases more thoroughly.
2. Early-period output (1960–1979): As noted by the authors, outputs from the early period are less reliable due to limited satellite data availability. It could be interesting to considering the exclusion of this timeframe from mean statistics, or including uncertainty estimates to better reflect its lower reliability.
3. Comparison with other models: While the comparison with RACMO2.3p2 is well presented, a brief discussion of how RACMO2.4p1 compares with other regional climate model outputs over the AIS (e.g., MAR or CRYOWRF) would provide additional context, even if a direct evaluation was not conducted. In addition, It should be stated whether the AWS data used for validation are assimilated into ERA5, as this could affect the independence of the evaluation.

Specific comments

• L28: Add a citation to support the statement “…which is the root cause of recent net mass losses of the AIS.”
• L32: Add a reference for “…In Antarctica, ablation is typically the result of sublimation or drifting snow erosion.”
• L67 (End of Introduction): Consider deleting this paragraph, as it appears unnecessary.
• L105: “This results, for example, in a more realistic representation of mixed-phase clouds…” Consider splitting this sentence or rephrasing for better readability.

• Figure 1: I suggest to add a legend showing measurement locations (e.g., AntSMB, IMAU AWS in red squares). This would allow removal of related description in the text (e.g., L157, L164) and improve figure clarity.
• Figure 2a: The 0 value is currently shown in blue. Why not changing to white for consistency with other figures (e.g., white = zero, blue = negative, yellow/red = positive)?
• L215–L219: This is interesting, however, consider adding regional averages or specific values with standard deviation, rather than descriptive text. This approach, done in Section 3.2, improves the quantitative interpretation.
• Figure 4: The legend in panel (b) overlaps important details during a period of high-SMB uncertainty. I suggest moving it outside the plot in the right side. Also, consider replace symbols in figure legends with line’s instead of points.
• L280–L290: Clarify how regionalization of the AIS is done in the methods sections, If it is based on IMBIE conventions, I suggest including this detail in the Methods section and not repeat it hereafter.
• L490: In the sentence starting “At the end of December…”, please specify exact dates.

Conclusion

The paper may be extensive for readers who are not familiar with the topic. I suggest including a brief list with bullet points of the main findings in the conclusion to help readers recap before the final remarks.