Winter Arcticc sea ice thickness from ICESat-2: upgrades to freeboard and snow loading estimates and an assessment of the first three winters of data collection
Petty, A. A.
Summary: With the ICESat-2 satellite laser altimeter launched in September 2018 the sea ice community has access to very high resolution observations of the sea ice topography and hence novel means to retrieve the sea ice thickness distribution. This manuscript details the impact of recent changes in the processing of the ICESat-2 data relevant for such retrieval. It details further the impact of updates of an important data set of ancillary information required for this retrieval: the snow loading. The manuscript convinces with comparably clear messages of these impacts mentioned and comes up with a presentation, interpretation and discussion of the seasonal development of the freeboard, snow and sea ice thickness values at basin scale.
The present manuscript falls into the category of work typically connected to new satellite missions when the (raw) processing is still ramping up and updates in the derived products are issued at a comparably high frequency. It is therefore of a quite technical nature - albeit it has a considerable fraction of geophysical interpretation. I was wondering therefore, whether the authors have considered to hand in this manuscript also into the journal "Earth System Science Data"? I'd say it is at the verge between both journals.
I should note that I read this manuscript in the second round of reviews - which explains why I refer to comments of the previous review at a few occasions.
GC1: The authors need - throughout the manuscript - clarify and correct the usage of "total (sea ice plus snow) freeboard" and "sea ice freeboard". Currently, these terms appear to be mixed and they are in part misleading the discussion of the results. A laser altimeter observes the total freeboard. A radar altimeter is supposed to observe the sea ice freeboard. This must be corrected in the manuscript both in the text as well as in all figures and tables where this applies. You will find repeated notion of this in my specific comments. This is the main reason why I suggest that this manuscript requires another round of major revisions because it might require some time to correct this in an appropriate way.
GC2: Despite a high fraction of technical details there are a few technical and/or methodological issues that, to my opinion, are not laid out sufficiently well. To these belong i) the calibration of the parameter gamma, ii) the evaluation of the NESOSIMv1.1 product for other months than April, and iii) the description of how you derived the sea-ice thickness uncertainties.
L19-21: Is an enhanced agreement with CS-2 based sea-ice thickness data that desirable given the fact that penetration depth of the radar signal into snow plus ice-snow interface processes and properties play a much larger role therein?
L35-46: In this paragraph you speak of "sea ice freeboard" in the context of laser altimetry. I strongly recommend to clearly differentiate between sea ice freeboard and total (sea ice plus snow) freeboard which is the quantity derived from the ICESat-2 elevation measurements first. Any conversion into sea ice freeboard (if need be) requires knowledge of snow thickness and density. It could be the that product you are using is named "sea ice freeboard" but physically it is not.
- What I am missing in front of this paragraph is that classical paragraph that tells us why satellite altimeters are such an important tool to observe the polar regions and why this is important. This would move your paper away from the impression one of the reviewers had that this is merely a technical report. In other words: So far the paper is not put into a larger context sufficiently well.
L51-53: "Additional .... procedure" --> You could stress perhaps, that this is a high skill for daylight, clearsky conditions.
L108: "Sea ice freeboard" --> I assume this is in fact the total (sea ice plus snow) freeboard and should be referenced like this in the paper.
Section 2.1.1: I can understand that one of the previous reviewers got the impression to read a technical report rather than a scientific paper. This section appears to be quite long given the comparably little information that appears to be relevant for the content of the paper. To my opinion, this section could be condensed such that the main changes that determine the differences between rel002 and rel005 are highlighted while the reader is referred to the (regularly updated) technical documentation and change log associated with every release for the less relevant changes.
L195: How is the calibration of gamma carried out?
L212/213: "Here we choose instead ..." --> This recalibration has the disadvantage that is is only valid for April snow conditions. Is that correct? How reliable can NESOSIM v1.1 then be for the rest of the freezing season. How is this period evaluated?
L232-241: It is still not clear to me after these lines what the initial snow thickness on the sea ice on September 1 is.
L232: It is still not clear how you tuned gamma.
L251: "declining trend" --> Is suggest to either write "decline" or "decrease" or "negative trend". A declining trend is a trend which is not stable over time but where the change of the parameter with time is decreasing over time.
- I am sure you are aware that this decline in snow thickness has several causes, beginning with the change in ice age, over changes in snow accumulation (period), actual ice drift vectors and retrieved ice drift vectors, to potential inconsistencies and spurious trends in the input data to NESOSOM v1.1. Because of these it might be a good idea to not overinterpret this decline. In addition: The tuning / evaluation of the NESOSIM v1.1 is limited to April ... and hence the time series shown for October could be less credible / reliable than the time series shown for April.
L272-276 / Figure S3: Please check these lines; it seems they contain two times almost the same sentence.
- How much ice is in the Kara Sea in October? I am not overly convinced that this particular region is well suited looking at October snow thickness on sea ice. I am wondering where the ice edge drawn in the Kara Sea for October comes from.
- Figure S3 units are cms. Is this correct?
- The color table used in the snow thickness maps on the left does not convince me that there is "measurable" snow thickness on Kara Sea sea ice. It looks white.
- Apart from these comments, I guess I have a conceptual problem with on the one hand striving to produce a temporally reasonably fine resolved snow thickness data set for proper sea-ice thickness retrieval and on the other hand generating (again) kind of a climatology.
Shouldn't the strategy be to use the auxiliary data sets when these are fully available? I mean, you also switched to a different daily sea-ice motion product for the time period that NSIDC drift is not available. You are not computing a climatology. Also, since you don't show these NRT products in your paper and only deal with the winter season 2020-2021 as the most recent, I believe you could condense this part of the investigation considerably. Perhaps you could mention at the side that a climatology based on NESOSIMv1.1 based on the 2010s (which would exclude 2020 by the way) provides an October snow thickness which is about 1 cm smaller than mW99 and an April snow thickness which is about 5 cm larger than mW99 for the inner-Arctic region you selected. You could then argue that most likely - if one still wants to use a climatology - then the NESOSIMv1.1 climatology might be a better choice ... but your results in fact only show the difference but do not indicate which of the two data sets is the more accurate one. I therefore find this comparison between mW99 and the 2010s snow thickness climatology not overly useful and I cannot recommend to keep this in the manuscript. To my opinion this sets the wrong signal.
- Did you try to compute a similar time series as shown in Figure 4 using W99 / mW99 and available multiyear ice areal fractions for "your" Inner Arctic region to see whether the negative trend in NESOSIM snow thickness is also visible in W99 snow thickness (simply via the change of the multiyear ice versus first-year ice partition)?
L279-298: Given the coarseness of the NESOSIM snow thickness data I am wondering whether from the view point of spatial scales involved it would make much more sense to discard the interpolation from 100 km x 100 km over more than 4 orders of magnitude to 30 meters and instead work with 10 km along track mean freeboard estimates. With that the interpolation would just be about one order of magnitude - at least into one direction.
But lets see what you will write about this in your discussion section.
- I note that your description of the error estimation in Lines 284/285 is not overly specific. If there are no further details given elsewhere I recommend to specify better how you carried out this step.
L307: So in the monthly sea ice thickness data you use the CDR version 4 but in NESOSIMv1.1 you use version 3. This difference in versions is possibly not dramatic, isn't it?
L326-332: I am sorry, I don't get why you prefer to compare ICESat-2 sea ice thickness data based on rel005 of ATL10 with the four CS-2 sea ice thickness products using THEIR snow loading. What is the motivation? What are you aiming to show here? It seems you are using the CS-2 sea-ice thickness products as a benchmark against which you would like to reference your product. Is this a viable approach given the difficulties / assumptions these radar altimeter products need to deal with? If you find a large bias / RMSD ... I'd say this is fine ...
In addition, you only use data of strong beam #1 here - to be consistent with your previous work. Are you not interested in how your updated sea-ice thickness product (see the many updates you wrote about so far in the manuscript!) compares to all these other products?
Section 2.6: This section appears to need some more work at it seems not to be complete. Also, it is not clear what the ERA5 data are for here. "To assess the winter Arctic atmospheric conditions" reads as if you made an investigation of the atmospheric conditions such as comparing ERA5 with rawinsonde and in-situ observations or the like. You need to work on your wording. Also, the last sentence does not fit quite well. I guess you merely perform a consistency check whether ERA5 2-m air temperatures support your assumptions about the physics and conditions but the nature of what you do is far away from an "assessment". It is an inter-comparison.
L362-365: The derivation of individual freeboard values from surface heights and approximated sea surface level implies that there are negative freeboard values as well. What is their fraction and what did you do with these?
L366: The results shown in Figure 6 are for your inner Arctic region or the entire Arctic?
L399: I see the "performance" of the v1.1_2010s snow thickness climatology in a slightly different way than you. I would state that in April it appears to be a good representation of what Nv1.0 and Nv1.1 provide. But in January and particularly in November the distributions differ considerably, casting doubts about the credibility of this climatology; these doubts are justified looking at the sea-ice thickness distributions where usage of the climatology provides a substantially smaller modal (1st mode) sea ice thickness. If used for model initialization, assimilation or intercomparison studies the sea ice thickness data based on the climatology are certainly more problematic - especially at this critical time of the freezing season. I cordially invite you to tone your statements about the climatology into this direction rather than saying, well, all data sets look just fine with some minor differences.
L403/404: "In general ..." --> I suggest to refer to a table or figure in your manuscript which supports this very general comment.
L468-470: How do the W99 snow densities compare to your densities? Can you identify hotspots in space and/or time where the W99 snow density would be considerably off compared to your results?
L478-481: "For example: thinner ... mean thickness." --> consider revisiting and perhaps correcting these statements in light of so far failing to adequately discriminate between total (sea ice plus snow) freeboard and sea ice freeboard in your manuscript.
L486-510: I am sorry, but I do not see the added value of anomalies computed from three winters of data. To my opinion this Figure 12 and this paragraph can be deleted without substantially changing the relevance of the manuscript.
If you decide to keep both, then I recommend to condense the text considerably and focus on those highlights that seem most obvious to be related with each other. Please keep in mind that a positive snow thickness anomaly in April does not need to coincide with a negative sea ice thickness anomaly - simply because the snow thickness during earlier in the winter determine how much the ice had the chance to grow thermodynamically. There is a temporal dimension involved which cannot be interpreted from the maps shown. Also consider to mention the retrieval noise of the parameters presented to foster readers to disentangle what is noise from what is a real signal.
L514-516: "In general ... thicker" --> Does this intercomparison result fit to other results where, e.g. PIOMAS data were compared to in-situ, sub-marine and ICESat data? In my mind there is this result from somerwhere in the published literature that PIOMAS over-estimates thinner ice and under-estimates thicker ice - a result that is not that well confirmed by your results - particularly not for the second winter period.
L545-551: "These ice type .. this limited record" --> What I am clearly missing here in the discussion is the role of different snow thickness conditions. And this discussion could be also linked better with the 2m-air temperature and longwave radiation data from ERA5.
- What is interesting to see, for instance, is that the sea ice thickness increase between November and February is larger in 2018/19 than in 2019/20 despite a) the sea ice itself being thicker and b) the snow thickness being larger by about 5 cm. Wouldnt' one expect that thicker ice with a thicker snow load grows less thermodynamically than comparably thin ice with a thinner snow load - provided the oceanic and atmospheric forcings are roughly the same?
- Another observations is that from November through January the freeboard values are the same for 2019/20 and 2020/21. At the same time snow thickness is about 5 cm larger for 2020/21. This means the sea-ice freeboard which is your total (sea ice plus snow) freeboard minus snow thickness is actually smaller by 5 cm for 2020/21 than 2019/20 which would point to about 40 cm thinner sea ice during these months in 2020/21 than 2019/20 and is actually confirmed by the respective panel. So here your observations are consistent in themselves. The fact that over these 3 months there is 20 cm more ice growth in 2020/21 compared to 2019/20 despite a more or less constant snow load could be discussed more specifically in light of the balance between thicker snow isolating better while thinner sea ice isolates less and the ancillary data used (drift, ERA5).
- I guess my recommendation is to discuss specific observations in your data time series in a comprehensive way, taking ERA5 and drift information of Fig. 11 into account instead of kind of listing what is shown in the respective panels without connecting the information well to your observations.
- Did you check, by the way, how ERA5 treats sea ice and its snow cover? How well is the snow thickness on top of sea ice resolved and is the sea ice allowed to grow and melt? Has it a variable thickness that would support discussions about feedback between 2m-air temperatures and ice and snow thickness?
L562-564: "For example ... " --> As always the interpretation of such maps offers potential for subjectivity and might depend stongly on the observer. I for my part rather see that the 2019/20 winter is different in terms of the strength and extent of the Beaufort Gyre - including the drift speed of its southern limb - as compared to the other two winters considered. Since the anomaly maps of these two other winters do not provide consistent information, i.e. exhibit different spatial patterns and signs of the anomalies, I am not convinced that the possible relationship between these anomalies and the ice drift pattern should be discussed the way you did. I note in this context, that the anomalies you are referring to here are mostly below 0.5 m in magnitude. How large an anomaly needs to be to be larger than the retrieval noise?
L615-617: This statement is not sufficiently well supported by the results given in the manuscript.
L619/620: "although ... scales." --> Also this part is not sufficiently backed up by the results presented in your manuscript.
L638-644: I am wondering whether you perhaps could be a bit more specific here because there are snow depth observations from several sovjet drift stations and in addition the N-ICE2015 and the MOSAiC campaigns provided quite a lot of useful snow thickness data to be used for a better calibration and/or evaluation of NESOSIM.
L556-674: I am not sure this "sea ice thickness reconciliation" paragraph should be kept as is. My impression is that it could be potentially misleading. What I am missing is i) a more clearly formulated statement that ICESat-2 could perhaps be the benchmark sensor for sea-ice thickness retrieval rather than CryoSat-2 - simply because of the more well defined main reflection horizon and the smaller number of snow processes and properties influencing already the freeboard retrieval. ii) What could also be more in the focus of future developments is a better handling of the different spatio-temporal scales involved - both in the retrieval process but also in the evaluation of the products. All parameters, freeboard, snow and ice thickness have their specific distributions. Two issues that I do not find solved satisfactorily is the error propagation of the downscaling approach from 100 km to 30 m and the propagation of the influence of substantial difference in the acquisition time of ICESat-2 data within one grid cell. These (and others) would be enough material for further improvement and I currently do not see the need to write that much about the potential of radar altimetry. But this is clearly my personal view based on this manuscript and on the results presented in similar papers.
Figure 1: I don't find this figure particularly well developed. Neither is clear that sea ice comprises level and ridged parts nor is clear that a radar measures sea ice freeboard while a laser measures total (sea ice plus snow) freeboard. The fact that the laser signal might also penetrate into the snow a bit is neglected. The role of the "internal ice stresses" is not clear, neither is mentioning of keel depth for satellite altimetry of sea ice. Not represented well is that radar waves may in fact be reflected at the snow-ice interface or even from within the sea ice but that they may also not reach to that interface at all. While the figure is for winter, which is good, the caption refers to the key challenges and one of these definitely is to measure freeboard during summer in the presence of melt ponds. So there is in fact a lot more one could and potentially should include into this schematic figure.
Figure 2: Particularly in the bottommost row the white circular area is not a well defined circle. What is the reason for that? Is there a higher probability for data drop-outs near the pole?
The main aim of the paper is to show the differences between rel002 used Petty et al. 2020 and rel005 used in this paper. I am wondering what can learn in terms of science from the many other panels and whether it wouldn't be sufficient and more straightforward to concentrate on the differences between rel002 and rel005 only.
Figure 3: What about the median biases for both cases? I note that (b) shows more cases than (a). Why?
- Personally, I would make the lengths of x- and y-axes the same when the data range is the same - here 80 cm for both quantities shown.
Figure 4 c) What is the reason for the violins for Oct. through Jan. to look cut off at the side facing higher snow thickness values?
Figure 5: This map contains many regions that are not used in the paper. It might make sense to color all regions not used in the same color as open water, only explain the acronyms / names of the regions actually used, and add that this map is "adopted" from a region mask provided by ...
Figure 6: The x-axis title needs to be changed into "total freeboard". Please provide the meaning of the dashed lines in the caption.
- I strongly recommend to be consistent in the notation between the figure and the caption. You write r002, r003 and so forth in the figure but write "rel002" and "rel005" in the caption text; this is inconsistent. Similarly, in the figure you write "bnum1", "bnum3" and so on, in the caption text you write "beam #1, #3" and so on. This is inconsistent as well.
Figure 9: I note that the sea ice thickess uncertainty seems not to be influenced by the substantial differences in the "mean day of the month" varying over a very short distance the ICESat-2 data. Hence in one 25 km grid cell ICESat-2 might come (on average) from the first few days of a month while in the adjacent grid cell this data might stem from the end of the month. I would expect that thermodynamic sea ice growth and dynamic processes can add substantially to the uncertainty - most likely especially during months October through January. Did you check this?
Figure 14: I guess it would make sense to not show values for September but rather begin with October as this is when the freezing season commences. There is room for one more panel in which you could put the fraction of the FYI relative to the total sea ice area of your Inner Arctic region. The same comment applies to Figure 15, but here for the MYI fraction of course.
Pure typos / editoral comments:
A general editoral comment upfront: Since you advertize at the end that basically all your results and the processing is available as jupyter notebook you might want to scan you paper and reduce the number of repeated mentioning of this. I think you overdo it a little bit.
L28/29: "Mean first-year ... negligible" --> possibly you refer to changes or differences?
L139: "2we" --> "2, we"
L218: "consensus" --> In the modeling world one would speak of an ensemble mean - or in your case apparently an ensemble median. I would find this a better expression because "consensus" is something that usually involves some discussion, some weighing perhaps, some additional constraints. What you do, however, is taking mean and median, i.e. use a statistical tool. Hence, I would recommend to change the wording accordingly - here and later in the text.
L231: "halving the blowing snow open water coefficient" --> would you mind to show the respective equation in this paper as well so that everybody immediately understand the effect of what you stated here?
L247-249: You are providing enough information about this region map in the context of Figure 5 I guess and can delete the sentence "Out Inner ... Figure 5" and instead refer to this figure at the end of the previous sentence.
L304-306: "Our monthly ... ATL10 data" --> namely what ancillary data?
L309: "on to" --> "onto"
L311-317: This reads a lot like again being in a technical report. Have you thought about publishing this manuscript in the journal Earth System Science Data? Would that be a more adequate journal for your manuscript being comparably rich in processing and product details, bug fixes, novel masks, and so forth? I slowly begin to second that other reviewer who had difficulties to see the merit of this manuscript in The Cryosphere.
L374-376: Since you do not consider June I suggest to remove June and the respective value from this sentence.
L407: Here you write rel003, in L413++ you write rel005. What is correct?
L413-424: I am wondering whether you need to list all ranges for all parameters shown in Figure 8. Do you see a chance to simply let speak Figure 8 for itself and perhaps only put the highlights here - perhaps along the lines that correlation coefficients in general increased by around 0.1, that biases change (not necessarily reduce) by about 30 cm and that standard deviations reduce by, on average, 0.2 m. This might make this part of the paper easier to read.
L417: "58" --> "0.58"
L434: "i-j" --> "i-k"
L450/451: "The data within this domain ... Figure 10e)" --> You need to put labels a) to f) to the panels in Figure 10 if you want to refer to them.
- I cannot see any sea-ice concentrations within the range given here (40-60%) in your Figure 10 e; something needs to be corrected here.
L533: "snow depth in November ... winters" --> you could denote in addition, that in Feb to April, however, differences in the snow thickness on the sea ice are between 2 cm and 4 cm.
L611: I suggest to use "significant" only in the context of statistical tests which I could not see in your manuscript. Hence "considerable", "notable", or "substantial" might be a better wording here.
L654/655: "The extension ... estimates" --> While this is true it would require first and foremost that we have (more) accurate freeboard estimates during summer from ICESat-2?
L920: "now depths" --> "snow depths"
L958/959: Perhaps better (and more correct): "The background dark grey shading in panels a to k is the CDR sea ice concentration shown in panel l."