Polar Ocean Tides—Revisited Using Cryosat-2

Round 1

Reviewer 1 Report

This is a very nice work that improves tide models in the polar regions, utilizing  9 years of C2 data reprocessed by a new retracking algorithm. The authors give a detailed description of their methods from the physical retracker to the response formalism. This work is definitely acceptable to publish. I just have a few minor suggestions for your consideration in preparing the final draft.

L15 and other places, 'pseudo-' is not necessary

L15 'monthly' and '28.33 days' the former should be removed

L19 and other places, 'Polar' lower case is ok

L20 'reciprocal' can be understood in different ways

L30, 'a'

L42, 'spatial' is not necessary

L45, '82\circ, degrees", where does 82 come from?

L49, 'large'?

L50, 'Polar Regions'?

L59, missing 'is'

L60, 'ice sheets'.

L61, Is abbreviation 'SIN' accepted by all?

L69, 'large footprint' pleas give numbers here

L81, new term 'DTU22', stands for what?

L123, 'Msub{2}' and other places, it is hard to see '2'

L129, '20Hz', better be '20-Hz'?

L187, 'see seen'

Figure 3, the '40 S' can be changed to '50 S'?

Figure 5, M2, Msub2?

Figure 7 and other places, you compare cosine and sine components both. It is ok but it takes more space. I think you can just show their absolute differences. Thus, only two panels are needed. It is because we care much about their MVD as in Table 2. In particular, Figure 8 now has 8 panels. Of course, you can also compare their phase differences.

Author Response

We generally corrected all very constructive comments by reviewer 1.

L15 and other places, 'pseudo-' is not necessary

Toitally correct. Removed throughout

L15 'monthly' and '28.33 days' the former should be removed

indeed

L19 and other places, 'Polar' lower case is ok

Thanks

L20 'reciprocal' can be understood in different ways

L30, 'a'

L42, 'spatial' is not necessary

L45, '82\circ, degrees", where does 82 come from?

L49, 'large'?

L50, 'Polar Regions'?

L59, missing 'is'

L60, 'ice sheets'.

L61, Is abbreviation 'SIN' accepted by all?

L69, 'large footprint' pleas give numbers here

L81, new term 'DTU22', stands for what?

L123, 'Msub{2}' and other places, it is hard to see '2'

L129, '20Hz', better be '20-Hz'?

L187, 'see seen'

Figure 3, the '40 S' can be changed to '50 S'?

Figure 5, M2, Msub2?

Figure 7 and other places, you compare cosine and sine components both. It is ok but it takes more space. I think you can just show their absolute differences. Thus, only two panels are needed. It is because we care much about their MVD as in Table 2. In particular, Figure 8 now has 8 panels. Of course, you can also compare their phase differences.

We also reduced the tables to MVD which is more informative.

Reviewer 2 Report

This paper revisited polar ocean tides using an improved retracked Cryosat-2 dataset. It seems that their results are reasonable, meaningful and useful. However, several aspects could be further improved in order to having it published in this journal. Therefore, my recommendation is major revision. The main questions I encountered when reading the manuscript are as follows:

Major concerns:

1、Since this work revisited polar ocean tides following the work of Zaron, I think it is necessary to compare the performance of DTU22 proposed by this paper and Zaron’s model in the  Antarctica. The performances of Zaron’s model can be added in Table 2. Also, more constituents should be added, such as N2, K2, Q1, P1 tides, which are also important.

2、When using ArcTiCa dataset, tide gauges in fjords and rivers should be excluded. Moreover, tide gauges less than 1-year should be also excluded. Then, the authors can re-calculate Table 3 with edited ArcTiCa dataset, otherwise, Table 3 is meaningless.

Minor concerns:

1. Line 9, change ‘polar tides’ to ‘polar ocean tides’

2. Line 30, change ‘is’ to ‘are’

3. Line 74, can you explain what are pseudo-sub-cycles? I do not know much on this and cannot understand it.

4. Line 83, delete ‘the’ before most

5. Figure 2 is not clear enough

6. Alias periods can be more accurate in Table 1. For example, M2 alias periods for ERS and Jason missions are 94.487 and 62.076 days. Also, sample period for T/p-Jason missions is 9.9156 not 9.916 days.

7. Line 349, change ‘polar tides’ to ‘polar ocean tides’

Suggestions: I am interested in tidal response method, however, it seems that there is no publicly accessible codes for the response method. I send emails to several scientists for codes of the response method but they do not reply me. I wonder whether you can make your code publicly accessible, which may be beneficial for the whole tidal community. Thanks a lot!

Author Response

We thank the reviewer for very nice an constructive comments. Our comments are in bold but we generally followed the comments

This paper revisited polar ocean tides using an improved retracked Cryosat-2 dataset. It seems that their results are reasonable, meaningful and useful. However, several aspects could be further improved in order to having it published in this journal. Therefore, my recommendation is major revision. The main questions I encountered when reading the manuscript are as follows:

Major concerns:

1、Since this work revisited polar ocean tides following the work of Zaron, I think it is necessary to compare the performance of DTU22 proposed by this paper and Zaron’s model in the  Antarctica. The performances of Zaron’s model can be added in Table 2. Also, more constituents should be added, such as N2, K2, Q1, P1 tides, which are also important.

Thanks for the comment. We decided to compare with Zaron’s model in the Weddell sea which is the only location where this model covers. This reduced the comparison to 20 gauges with this model.

2、When using ArcTiCa dataset, tide gauges in fjords and rivers should be excluded. Moreover, tide gauges less than 1-year should be also excluded. Then, the authors can re-calculate Table 3 with the edited ArcTiCa dataset, otherwise, Table 3 is meaningless.

We also felt that this table was not very good so we followed the suggestions and improved the editing of the tide gauges and also removed many outliers. This reduced the efficient number of gauges to 67 which is similar to what has been published before.

All minor concerns were corrected following the suggestiong below.

Minor concerns:

1. Line 9, change ‘polar tides’ to ‘polar ocean tides’
2. Line 30, change ‘is’ to ‘are’
3. Line 74, can you explain what are pseudo-sub-cycles? I do not know much on this and cannot understand it.
4. Line 83, delete ‘the’ before most
5. Figure 2 is not clear enough
6. Alias periods can be more accurate in Table 1. For example, M2 alias periods for ERS and Jason missions are 94.487 and 62.076 days. Also, sample period for T/p-Jason missions is 9.9156 not 9.916 days.
7. Line 349, change ‘polar tides’ to ‘polar ocean tides’

Suggestions: I am interested in tidal response method, however, it seems that there is no publicly accessible codes for the response method. I send emails to several scientists for codes of the response method but they do not reply me. I wonder whether you can make your code publicly accessible, which may be beneficial for the whole tidal community. Thanks a lot!

Reviewer 3 Report

Dear Authors,

This study is useful and interesting, it provides the tide information in the polar area, which is difficult to obtain in traditional studies. I have three comments.

1, how to validate the results, do you have some comparison with in-situ data.

2, in the coastal zones, how to ensure the accuracy of your results due to the size of footprint.

3, the importance and usage of your study should be highlighted in abstract.

Author Response

We thank the reviewer for the comments and have updated the manuscript.

Dear Authors,

This study is useful and interesting, it provides the tide information in the polar area, which is difficult to obtain in traditional studies. I have three comments.

1, how to validate the results, do you have some comparison with in-situ data.

All minor concerns were corrected following the suggestiong below.

2, in the coastal zones, how to ensure the accuracy of your results due to the size of footprint.

Indeed this is considered and mentioned in the manuscript

3, the importance and usage of your study should be highlighted in abstract.

This was improved in the revised version

Reviewer 4 Report

This paper achieved a substantial advancement of tidal estimates at high latitudes thanks to the physical retracking of Cryosat-2 data, which the authors demonstrate through comparison of their new model with FES2014b and tide gauge data. The explanation of their approach and results is very clear.

There are two areas which could be improved:

(1) The authors mention the aliasing of M2 and the annual cycle. Are there any sources of data with which to compare their estimate of the annual cycle?

(2) The authors show improvement of their model compared to FES2014b with respect to tide gauge data. However, as they state, the tide gauge data are problematic for this purpose because many were assimilated in constructing the FES2014b model. A comparison of variance reduction statistics for some altimeter data which was not used in the development of either FES or their model might be useful.  Would it be feasible to illustrate variance reduction statistics when their model is used for tidal corrects to high-latitude S3a or S3b data?

Other than the above, all of my comments are just minor wording & typos:

l30: omit "a", or replace with "understanding"

l39: "to have" --> "with"

l44: add "into an orbit with"

l47: "have been" --> "were"

l50: "near-by" --> "nearby"

l52-53: The sentence illustrates a recurring issue with the writing:
with the phrase, "improving these regions". Of course, a "region" cannot
be imporoved. The authors' meaning is that the knowledge of tides
(or the precision of tidal predictions, etc.) could be improved.

l55: "that" --> "in that"; change "Cryosphere" to "cryosphere"

l59: "optimized" --> "is optimized"

l92: period needed after "[20]"

l93: omit "basically"

l100: "and more data will be available" ?

Fig 2: Need to use the same grid style and plotting style in the
right panel as used in the left panel.

l162: "of" --> "off"

l164-165: Hopefully this assertion will be verified later?

S2.2 is very clear

S3 Results: Some discussion of the annual cycle would be interesting.

see above

Author Response

We thank reviewer 4 for the very nice and constructive comments. Again our comments are in bold. We decided not to publish comments on he annual signal as it was hard to find an independent model to compare with. 

This paper achieved a substantial advancement of tidal estimates at high latitudes thanks to the physical retracking of Cryosat-2 data, which the authors demonstrate through comparison of their new model with FES2014b and tide gauge data. The explanation of their approach and results is very clear.

There are two areas which could be improved:

The authors mention the aliasing of M2 and the annual cycle. Are there any sources of data with which to compare their estimate of the annual cycle?

The annual cycle is available from https://ftp.space.dtu.dk/pub/DTU10/ANNUAL_SSH_V2

But we did not find it useful to compare with an internal product as this was not guaranteed to be independent.

(2) The authors show improvement of their model compared to FES2014b with respect to tide gauge data. However, as they state, the tide gauge data are problematic for this purpose because many were assimilated in constructing the FES2014b model. A comparison of variance reduction statistics for some altimeter data which was not used in the development of either FES or their model might be useful.  Would it be feasible to illustrate variance reduction statistics when their model is used for tidal corrects to high-latitude S3a or S3b data?

Indeed this is a very valuable point. We tried to contact the authors of FES2014b to get the tide gauges without success. Unfortunately the S3a and 3b are very problematic in the polar oceans due to the very problematic alias periods as also mentioned in table 2.

Other than the above, all of my comments are just minor wording & typos:

These were all corrected accordinly

l30: omit "a", or replace with "understanding"

l39: "to have" --> "with"

l44: add "into an orbit with"

l47: "have been" --> "were"

l50: "near-by" --> "nearby"

l52-53: The sentence illustrates a recurring issue with the writing:
with the phrase, "improving these regions". Of course, a "region" cannot
be imporoved. The authors' meaning is that the knowledge of tides
(or the precision of tidal predictions, etc.) could be improved.

l55: "that" --> "in that"; change "Cryosphere" to "cryosphere"

l59: "optimized" --> "is optimized"

l92: period needed after "[20]"

l93: omit "basically"

l100: "and more data will be available" ?

Fig 2: Need to use the same grid style and plotting style in the
right panel as used in the left panel.

l162: "of" --> "off"

l164-165: Hopefully this assertion will be verified later?

S2.2 is very clear

S3 Results: Some discussion of the annual cycle would be interesting.

Round 2

Reviewer 2 Report

This paper can be accepted.

Reviewer 3 Report

Dear Authors,

The revised manuscript can be accepted as the current form.

Best wishes,