Inversion of Deflection of the Vertical in the South China Sea Using ICESat-2 Sea Surface Height Data

Round 1

Reviewer 1 Report

1. Page 1, lines 20-22, “The RMSs of difference between gravity anomalies derived from the joint along-cross beam DOV and the above verification data are higher than those derived from the along-beam and cross-beam DOVs.” According to the table on page 17, the joint along-cross beam DOV method has the lowest RMS inversion among the three ways of obtaining DOV, along-beam, cross-beam, and joint along-cross beam results, which is inconsistent with the previous description.

2. On page 1, lines 38-47, the SSH algorithm used to calculate DOV, the algorithm has been proposed for a long time, is this method the latest algorithm to solve this problem? This paper also does not mention how to improve the algorithm and the lack of innovation.

3. Jason-3 data are used in this paper for unclear purposes. The experimental results show that the accuracy of (sea surface elevation) SSH of ICESat-2 is lower than that of Jason-3 SSH, and the accuracy of DOV of along-beam, cross-beam, and joint along-cross beam, and the accuracy of gravity anomaly of inversion is completely higher than that of the results obtained from Jason-3 data. The experiment cannot prove that ICESat-2 data is better than Jason-3.

4. On page 9, lines 290-292, the authors conclude that the ICESat-2 SSHs data are more reliable, but then on page 8, lines 274-276, the authors conclude that the overall accuracy of the ICESat-2 satellites is slightly lower than that of Jason-3, as also indicated in the table below. The authors must explain how they concluded that the ICESat-2 SSHs data are more reliable.

5. In the last two rows of Table 8 on pages 10-11, the number of sample points for Jason-3 is much smaller compared to ICESat-2, and the authors need to explain why. Does such a large difference in sample size have an impact on the results of the analysis?

6. On Page 11, lines 317-319, the authors state that the 8′ radii were chosen because the 8′ grid meridian component has the highest accuracy. However, the table below shows that the accuracy of the two components under a 10′ radius is higher.

7. Section 3 only introduces the calculation method of cross-beam DOV but not the other two DOV calculation methods, especially the detailed description of the joint along-cross beam DOV calculation method is missing.

8. The article is not innovative enough, as follows: a similar study has been conducted in the same study area in 2020; the method of calculating DOV from SSHs has been proposed in 1998; the standard model used for comparison with the inversion results is the commonly used standard model; the reliability of the ICESat-2 data used may not be comparable to other satellite data, at least from the experimental results of this paper, which cannot be proved.

Author Response

Reply to the Review Report (Reviewer 1) has been uploaded.

Author Response File: Author Response.docx

Reviewer 2 Report

Authors propose "  Inversion of Deflection of the Vertical in the South China Sea Using ICESat-2 Sea Surface Height Data". The topic is interesting and the paper is, in general, well-written. I, however, have the following comments: 

1. In the Introduction section specifically in related work section authors need to discuss some works in the other part of the world

2. Authors didn't show any map for their study area. They should add a map showing their study area

3. How did the authors deal with unbalanced dataset. 

4. Please conclude your manuscript in a more concrete way

Author Response

A point-by-point response to the reviewer’s comments has been uploaded.

Author Response File: Author Response.docx

Reviewer 3 Report

The inversion of surface height of marine gravity is great value in ocean remote sensing. In this manuscript, technical framework and experimental results are presented for deriving ICESat-2-derived deflection of the vertical. As the authors declare, ICESat-2 can effectively improve the precision of the gridded DOV by inverting the cross-beam DOV and the joint along-cross beam DOV. It seems that this manuscript focuses mainly on the technical aspects, while relative few attentions are paid to the inversion models and algorithms.

Specific comments are as follows:

1. As we know, the inversion accuracy of the marine gravity field inversion is very important. The accuracy and error analysis should be discussed.

2. It seems that the manuscript focuses mainly on the data processing. More discussions on models and algorithms of inversion techniques should be presented, if possible.

3. The advantages and disadvantages of the inversion techniques based on optical data ICESat-2 on microwave sensors data should be given, if possible.

4. English writing and grammar should be improved. For example, the first sentence in the Abstract “The traditional altimetry satellite, which is based on pulse-limited radar altimeter, only calculates along-track deflection of the vertical (DOV); hence, leads to poorer precision in the prime vertical component compared to the meridian component, which in turn limits the final accuracy of the marine gravity field inversion.”

Author Response

A point-by-point response to the reviewer’s comments has been uploaded.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The accuracy of the results obtained in this study is roughly similar to other studies, but the maximum accuracy that can be achieved is low. The highest accuracy of gravity anomaly in this study is 2.49 mGal. Despite the difference in the data used, the accuracy within 2 mGal has been achieved with swot in 2021, and several articles have shown that the accuracy of ocean gravity anomaly inversion by satellite altimetry can reach about 2 mGal. From this point of view, the study is not particularly remarkable, but it is worthwhile to study the potential of ICESat-2 data for ocean gravity inversion. The authors may consider adding the applicability of the technical approach of this paper, especially the advantages and disadvantages compared with other techniques, in the discussion section of the article as appropriate.

Author Response

We thank the reviewers for the time and effort that they have put into reviewing the previous version of the manuscript. 

Author Response File: Author Response.docx

Reviewer 3 Report

The revised paper addressed my concerns regarding the first version.

Author Response

We thank the reviewers for the time and effort that they have put into reviewing the previous version of the manuscript. Your suggestions have enabled us to improve our work.