Review of:

Effects of Arctic sea-ice concentration on surface radiative fluxes in four atmospheric reanalyses  

This study uses four different atmospheric reanalyses to analyze the impact of sea-ice concentration on surface radiative fluxes in the Arctic. The analysis reveals that surface temperature explains most of the variance of upward LW radiation in the inner ice pack, while sea ice does so for the marginal ice regions. The decadal decrease in upward SW radiation is explained by the decline in surface albedo stemming from the sea-ice decline, while the decline in downward SW radiation is mainly due to increases in cloudiness. The decrease in sensitivity to sea ice for upward LW radiation is attributed to the increase in surface temperature in the inner ice pack and the decline of sea ice in the marginal ice regions. Overall, the study is sound and well done, but there are issues that need to be addressed before I can recommend the manuscript for publication.

Major Comment:

The variance of upward LW radiation to sea ice is greatest at the 100% sic level (Fig. 2). This, however, is to be expected because this is a limiting value for sea ice, as long as it is cold enough for sea ice to remain at 100% the conditions can vary greatly, so this value should not be included in a sensitivity analysis as it no longer reflects a sensitivity to sea ice. If included in the results, it will steepen the slope artificially. I recommend the authors redo the analysis including only sic values from 1-99% and test whether this changes any of the results presented in the manuscript. Furthermore, the sensitivity results (e.g. Fig. 2) reflect a sensitivity that is larger at high values of sic than low values. This makes sense as low values of sic tend to have thinner sea ice and a greater percentage of open ocean. For smaller values of sic, the open ocean serves as a moderating influence that lowers the variance of the sea ice temperature and upward LW flux due to its high thermal inertia. For thinner sea ice, the ocean below also moderates the variance. The greater the sea ice thickness (SIT) the weaker the influence the ocean below has on the surface temperature and upward LW flux. Therefore, I suggest the authors attempt to include an analysis of SIT for the sensitivity analysis, which admittedly might be cumbersome as SIT observations are still lacking but a reanalysis such as PIOMAS might be helpful. This would provide a more insightful analysis of the sensitivity of upward LW fluxes to sea ice, while providing for a more complete physical explanation of the results.

Additional Comments:

The results section 3, discussion section 4 and conclusions section 5 have some repetitive information. I suggest a revision to streamline the paper.

Lines 88-89: Sentence is unclear. Improve the clarity of the definitions for R1 and R2, respectively.

Lines 100-101: Why is the open ocean ‘usually’ and not always warmer than the sea ice surface? If open ocean is colder than sea ice, wouldn’t we expect the ocean to freeze into sea ice?

Lines 135-136: Did the authors mean “cannot be applied where no SIC is present”? I suggest a revision.

Line 206: Suggest change to “we also noted”.

Line 224: Suggest change to “plays an undeniable role” and from “also decadal …” to “decadal changes in DSW must also be…”.

This study aimed to compare the affects of sea ice concentration on the surface radiative fluxes from 4 reanalysis and this sort of a companion paper to their previous study looking at turbulent fluxes from these reanalysis. They found that the upward longwave radiation was most sensitive to SIC in the winter, and upward shortwave radiation was most sensitive to SIC during the summer. They found that the relationship between SIC and upward longwave radiation has decreased from the first 20 years compared to the last 20 years in the 1980-2021 record and attributed this to the thinning ice and warming surface temperatures.

I found this study to be very insightful, and the methods and results were clearly explained and easy to follow. The figures were also clear and easy to understand. I feel like these types of analysis are essential to the scientific community to better understand the uncertainties and limitations posed by these global reanalysis products when studying the Polar regions like the Arctic Ocean. I feel like this paper should be published after my minor comments are addressed.

Sentence beginning on line 30: Needs a citation

Line 70: why do you not also include surface downward longwave radiation? I am sure you would want to do this to account for the full radiative fluxes.

Table 1: can you elaborate more on how the albedo is parameterized for NCEP/CFSR

Is it coupled to the ocean or atmosphere or both? Please elaborate. Any information on the sea ice model that they use?

It might be more specific if you actually spelled out what the albedos were for the other reanalysis rather than just stating the citations, and include this information in the table.

Figure 3, I think that makes sense because we can probably assume that the ocean surface temperature in the marginal ice zones is likely above -1.8C and the ice is less compact so the ice temperature that was calculated was probably off some.

Line 194: Yes this makes sense that the CCC would be so different between all of the reanalyses because they all have differing cloud schemes (one moment, two moment, etc). It might be nice to reference these differences or add their parameterizations to the table 1.

Figure 6

Any idea why MERRA2 has such a large change in CCC, especially in the North Atlantic, compared to other reanalysis?

General Comment:

Since the SIC is so important for the energy budget of the lower atmosphere and ocean in the Arctic, it might be good to compare the SIC with passive microwave SIC observations to determine which SIC is most realistic? Then what conclusions can be made towards your other results which are so highly SIC dependent/driven?