Satellite observations of snowfall regimes over the Greenland Ice Sheet

McIlhattan, Elin A.; Pettersen, Claire; Wood, Norman B.; L'Ecuyer, Tristan S.

doi:https://doi.org/10.5194/tc-14-4379-2020

Articles | Volume 14, issue 12

https://doi.org/10.5194/tc-14-4379-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/tc-14-4379-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 14, issue 12

Research article

|

04 Dec 2020

Research article |

| 04 Dec 2020

Satellite observations of snowfall regimes over the Greenland Ice Sheet

Elin A. McIlhattan, Claire Pettersen, Norman B. Wood, and Tristan S. L'Ecuyer

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Final revised paper (published on 04 Dec 2020)
Preprint (discussion started on 21 Nov 2019)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'anonymous comments', Anonymous Referee #1, 27 Dec 2019
- AC1: 'Authors’ Response to RC1 and RC2', Elin McIlhattan, 13 Mar 2020
RC2: 'Review of tc-2019-223 "Satellite Observations of Snowfall Regimes over the Greenland Ice Sheet"', Anonymous Referee #2, 08 Jan 2020
- AC1: 'Authors’ Response to RC1 and RC2', Elin McIlhattan, 13 Mar 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (further review by editor and referees) (16 Mar 2020) by Louise Sandberg Sørensen

AR by Elin McIlhattan on behalf of the Authors (19 Mar 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (30 Mar 2020) by Louise Sandberg Sørensen

RR by Anonymous Referee #3 (25 May 2020)

Suggestions for revision or reasons for rejection

This study uses satellite observations from CloudSat and CALIPSO to investigate snowfall regimes across the Greenland Ice Sheet. The classification of snowfall events across the ice sheet into ice-phase and mixed-phase cloud processes is novel and there are some interesting findings that may be useful for the ice sheet surface mass balance community. However, the manuscript requires major revisions before it can be considered for acceptance. The main problem is that the manuscript is poorly structured which makes it difficult to understand what literature gap the study aims to fill and how the authors did it. In the methods section, there is text about topographical basins in the “Satellite Data” section and text about averaging snowfall observations in the “Reanalyses” section. To fully understand the methods, the reader has to scroll up and down to find the relevant sentences. The manuscript is also lacking key details and there are many instances where the writing style should be more precise. Finally, one of the major novelties of the study is to divide the snowfall events in topographical basins. But this ignores extreme spatial gradients in air temperature. I think a better version of this study would be to separate the ice sheet by elevational as well as topographical regions. More detail is provided in my specific comments below.

P1L22-P2L4: The first few lines of the introduction are muddled. The authors state that the Greenland Ice Sheet is important for the Earth’s energy budget but cite a paper that finds that Greenland’s impact on the cryosphere radiative forcing has not changed since 1979. Only later in the paragraph do the authors state the importance of the ice sheet for global sea-level rise. I recommend that the authors remove the first few sentences and focus the first paragraph on describing Greenland’s recent and alarming contributions to global sea levels (i.e. from P2L4 onwards).

P2L11: Only humans “build”, recommend changing to “adding” or similar.

P2L27: What is meant by “looked”, please be more specific.

P2L31-33: The lack of observational constraints is just one of many difficulties that have models have when modeling snowfall across the Greenland Ice Sheet. A better justification for this study would develop this statement more, especially given the prevalence of models used for Greenland SMB studies.

P2L34: Again satellites do not “look” at snowfall, please be more specific. I noticed Anonymous Referee #1 also raised this point.

P3L1: This sentence does not make sense, how can “variations in surface emission” introduce errors into passive microwave estimates?

P3L5-7: The authors state that CloudSat has an advantage because it provides information on snowfall and precipitating clouds. But the previous paragraphs provided no information about why the latter should be important. The authors should consider moving P3L13-14 upwards to make this point.

P5L20-21: Would be useful to clarify the study period here? 2006 to 2016?

P5L21-22: 0.94 x 1.25 seems a strange choice of grid spacing, please justify. Why not a more conventional 1x1 degree grid spacing?

P5L23-31: This paragraph appears to be about the satellite products and drainage basins that are used to analyze the data which makes it confusing to read. Please separate.

P6L13-14: What is meant by the “bin above the lowest for all profiles”? Please clarify.

P7L17: Does CloudSat/CALIPSO pass directly over Summit? If not, at what distance were CloudSat/CALIPSO data considered representative of Summit?

P7L20: Why did you choose 5 m/s? Is it the average wind speed at Summit?

P8L1-3: Why is this text in the section titled “Reanalyses”? Please consider re-structuring this methods section.

P8L4: What is meant by the “nearest hourly ERA5 output”? Do the authors just take one grid cell from ERA5 for each basin or do you average across the basin?

P8L8: What is meant by “ice habit”?

P10L10-11: Please define “summer months”? May- September?

P10L18-20: How do these figures compare to other studies?

P10L20-21: Defining two time periods of different length is very confusing for presenting the results. Suggest using the more conventional seasonal time periods.

P16L30: The annual accumulation value (399 Gt yr-1) identified in this study is associated with some serious bias due to missing snowfall events. The authors should state this here or remove this figure from the introduction because it does not represent a realistic estimate of Greenland snowfall.

Figure 1: More information required. What does the numbering refer to? I presume the star is the location of Summit? Please clarify in the figure.

Figure 2: Again, I do not understand why summer and winter represent different time spans. Either clarify this oddity or change the time periods to represent six months.

Figure 3: The divergent color-scale for tor panels A, D and G does not work well. Suggest using a sequential color-scale.

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ED: Reconsider after major revisions (further review by editor and referees) (25 May 2020) by Louise Sandberg Sørensen

AR by Elin McIlhattan on behalf of the Authors (04 Aug 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (22 Aug 2020) by Louise Sandberg Sørensen

RR by Anonymous Referee #3 (16 Sep 2020)

Suggestions for revision or reasons for rejection

This is my second review of “Satellite Observations of Snowfall Regimes over the Greenland Ice Sheet” by McIlhatten et al. The authors have generally done a good job of responding to my previous comments. The writing is more precise, the methods are mostly well-structured, and the division by elevation adds some additional insight. On my second reading I did still find a few minor things which could be improved. Note that my line numbers refer to the tracked changes version.

Specific comments

P1 L11: Suggest adding a “We find” to start this sentence so the transition to the results is clear

P1 L19-21: The abstract finishes with one of the least interesting results of the manuscript. Suggest removing and adding a concluding sentence that brings all these findings together. Perhaps something about implications for modeling Greenland Ice Sheet mass balance (which is what this study claims to do in the third sentence) or future behavior of the ice sheet.

P2 L4-18: Good scientific writing usually has some sort of order (from most to least important would work in this case). But in this introduction the importance of snowfall for accumulation is mentioned almost as an afterthought. This would be very confusing for readers with less expertise in this field. I’d argue that the role of snowfall for adding mass to the ice sheet is THE most important role in Greenland surface mass balance. Consider re-ordering. Having said that, I commend the authors for improving the rest of the introduction which now reads much better with the added text.

P2 L12: Suggest adding “and bare ice” after “old snow”. Also the Enderlin et al. (2014) paper does not appear to have investigated drivers of albedo on the Greenland Ice Sheet. A better reference would be: Ryan et al. (2019). Greenland Ice Sheet surface melt amplified by snowline migration and bare ice exposure. Science Advances.

P2 L33-35: Suggest adding the following recent reference which also identified modeled snowfall bias over Greenland: Ryan et al. (2020). Evaluation of CloudSat's Cloud‐Profiling Radar for Mapping Snowfall Rates Across the Greenland Ice Sheet. Journal of Geophysical Research.

P6 L10-11: The methods are generally better structured now but this sentence seems a little out of place. I would add it next to the sentence P7 18-21.

P7 L11-12: Suggest adding a sentence that outlines how 2BCCL retrieves cloud phase

P9 L7-8: Again this first sentence doe not fit with the rest of the paragraph which is actually about CloudSat’s CPR. Then the next paragraph is about ICECAPS. Suggest removing mention of CloudSat in this first paragraph.

P10 L19-23: Please also add discussion about how these results compare to Ryan et al. (2020). “Evaluation of CloudSat's Cloud‐Profiling Radar for Mapping Snowfall Rates Across the Greenland Ice Sheet” who did a similar comparison with almost identical results.

P19 L23: Unless I am missing something, the word “conditional” is meaningless here. Consider removing.

P28: “the numbering of the basins corresponds to their numbering system” is unnecessary because already implied.

P31: Ah so this what conditional refers to. Why don’t you just say, “mean snowfall rate for given regime”? If you are going to stick with “conditional” please also clarify in P7 L7-13.

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ED: Publish subject to minor revisions (review by editor) (30 Sep 2020) by Louise Sandberg Sørensen

AR by Elin McIlhattan on behalf of the Authors (06 Oct 2020) Author's response Manuscript

ED: Publish as is (19 Oct 2020) by Louise Sandberg Sørensen

AR by Elin McIlhattan on behalf of the Authors (21 Oct 2020) Manuscript

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Short summary

Snowfall builds the mass of the Greenland Ice Sheet (GrIS) and reduces melt by brightening the surface. We present satellite observations of GrIS snowfall events divided into two regimes: those coincident with ice clouds and those coincident with mixed-phase clouds. Snowfall from ice clouds plays the dominant role in building the GrIS, producing ~ 80 % of total accumulation. The two regimes have similar snowfall frequency in summer, brightening the surface when solar insolation is at its peak.