the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Jan 2025
The surface mass balance and near-surface climate of the Antarctic ice sheet in RACMO2.4p1
Abstract. This study presents a new near-surface climate and surface mass balance (SMB) product for Antarctica for the historical period (1960–2023) using the updated version of the Regional Atmospheric Climate Model (RACMO), version 2.4p1 (R24). We study the impact of the major updates implemented in R24 on the climate of Antarctica, and show that the SMB, surface energy budget, albedo, pressure, temperature and wind speed compare well with observations. Compared to preceding RACMO versions, the advection of snow hydrometeors and alterations in the blowing snow parameterization affect the SMB, resulting in more precipitation in the mountains of West Antarctica and the Antarctic Peninsula, and an alternating pattern of SMB changes in the coastal zones of East Antarctica. Integrated over the ice sheet, including ice shelves, the modeled SMB is 2546 Gt yr-1, with an inter-annual variability of 133 Gt yr-1. Melt fluxes are small in Antarctica, at 124 Gt yr-1 and an inter-annual variability of 31 Gt yr-1, but can be significant on ice shelves, locally exceeding 500 mm water equivalent yr-1. The presence of melt water in snow compares remarkably well with remote sensing observations and has improved compared to the previous operational RACMO version, 2.3p2. Temperature, shortwave radiative fluxes and albedo are modeled well compared to in-situ observations. Longwave radiative and turbulent fluxes, however, require further model developments.
Competing interests: At least one of the (co-)authors is a member of the editorial board of The Cryosphere.
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RC1: 'Comment on egusphere-2024-3728', Anonymous Referee #1, 26 Feb 2025
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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.
Citation: https://doi.org/10.5194/egusphere-2024-3728-RC1
Data sets
Monthly RACMO2.4p1 data for Antarctica (11 km) for SMB, SEB and near-surface variables (1979-2023) Christiaan van Dalum, Willem Jan van de Berg, and Michiel van den Broeke https://doi.org/10.5281/zenodo.14217232
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