An Extended Quasi−Analytical Algorithm for Retrieving Absorption Coefficient Using 510–620 nm Bands from OLCI and MERIS Satellite Data
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsPlease see the following comments.
Line 20: What is ‘GSM’???
Line 20-21: Again, what is ‘QAA-v5’, and ‘QAA-RGR’?? Please consider using the List of Abbreviations for the manuscript.
Line 23: What do you mean by’~20%’, is it approximately? Please revise this unprofessional usage!
Line 24: What are your criteria for reasonable? Please use the number to tell the truth.
Line 46: Is Rrs(λ) the same with ‘Water Leaving Reflectance’?
Line 106: Please enlarge the Figure1-b&c, I tried to enlarge it to 200% to see the details inside these two subfigures. Please add the scale, north arrow, and coordinate grids to all the subfigures here!
Line 99-100: Whas was the minimum distance between your field measurements? At each site, what was your sampling strategy. How many data points did you collected.
Line 142: Why did you include the SeaWiFS, MODIS, VIIRS, and GOCI in the Figure 2. Did you make Figure 2 all by yourself?
Line 171: Please enlarge the font size of the Figure-4.
Line 175-186: Were you indicating that those two locations representing type I and type II waters?
Line 191: Why the N is 93 here, as we can see you had 127+144 samples in total (Line 90-102)??
Line 196: What is the distribution for the independent variable in this regression. There are clearly less data points in the high y-value region. Why didn’t you use the piece wise linear regression?
Line 199-200: Did you have any references for the numbers of the parameters inside Table 1, or did you derive all of them by yourself?
Line 201: Capitalize the ‘p’ for the first in this subtitle.
Line 203: Once again, what was ‘GSM’? Obviously, you used this ‘traditional’ method as your comparison to how the advance for your proposed QAA-GRI method, but you missed the introduction of this GSM method. Neither did you cite the reference for the GSM method!
Line 218: Please add the regression lines to all the subfigures. Please add the sub caption as a,b,c… to this figure.
Line 208: Where is the description of the other two QAA based methods??
Line 222: Is the dash line inside Figure 7 representing the Identical Line? Please enlarge the font size in all the subfigures. Add the sub captions to these figures!
Line 235 & 238: Please see previous comments on Figure 6 & 7 for the revision of Figure 8 & 9.
Line 268: Where is Figure-10a???
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsReview of the Article: "An Extended Quasi-analytical Algorithm for Retrieving Absorption Coefficient Using 510-620 nm Bands from OLCI and MERIS satellite Data"
The article presents an extended quasi-analytical algorithm (QAA-GRI) for retrieving the total absorption coefficient of water bodies using satellite data from the Ocean and Land Color Instrument (OLCI) and the Medium Resolution Imaging Spectrometer (MERIS). The study uses data from the East China Sea (ECS) and Lake Qiandaohu (QDH) to develop and test the algorithm. The authors demonstrate that the QAA-GRI algorithm outperforms other models like GSM, QAA-v5, and QAA-RGR in terms of accuracy. They applied this algorithm to OLCI imagery and observed promising results in the ECS region, including insights into seasonal and annual variability in total absorption coefficient influenced by the Yangtze River discharge and coastal currents.
In general the strangths and weaknesses points are listed below.
The strangths:
· The development of the QAA-GRI algorithm, which extends the use of green-red bands in satellite imagery for absorption coefficient retrieval, is a significant advancement in remote sensing of aquatic environments.
· The use of extensive field measurements and satellite data for algorithm validation ensures a robust and reliable assessment.
· The study effectively compares the new algorithm with existing models, providing clear evidence of its superior performance.
· The application of the algorithm to real-world scenarios, such as observing seasonal and annual variations in the ECS, demonstrates its practical utility.
The weaknesses:
· The study primarily focuses on data from the ECS and Lake QDH. Expanding the data scope to include a wider range of water bodies could enhance the algorithm's applicability.
· The algorithm shares the limitations of the original QAA in highly turbid waters, which could restrict its use in certain aquatic environments.
· There's a mention of possible inaccuracies due to atmospheric correction methods, especially in inland waters, which might affect the algorithm's reliability.
I also do suggest the following points for further research, that the authors may consider eithe in the current resrach or may be stated in the discussion as a kind of linitation and give oppertunty for other researchers. They are:
· Applying the algorithm to a broader range of aquatic environments would test its versatility and robustness further.
· Addressing the limitations in highly turbid waters could enhance the algorithm's usefulness across more varied water types.
· Developing or integrating more accurate atmospheric correction methods could improve the algorithm’s performance, particularly in inland waters.
Below, a detailed line-by-line of some issues that need to be reconsidered by the Authors:
Lines 34-43: The introduction effectively sets the context for the study, emphasizing the importance of inherent optical properties (IOPs) in aquatic and optical environments. However, it could benefit from a more detailed explanation of how these properties influence the retrieval of biogeochemical parameters.
Lines 59-70: The review of existing algorithms is comprehensive, providing a clear rationale for the development of the QAA-GRI algorithm. The distinction between empirical and semi-analytical algorithms is well articulated.
Lines 95-105: The field measurements are well-documented, covering different water bodies. However, the selection criteria for these specific sites should be clarified to strengthen the study's methodology.
Lines 109-117: The measurement techniques and calculations for remote sensing reflectance (Rrs) are well detailed. The choice of average value for surface Fresnel reflectance (r) could be further justified with references to similar studies.
Lines 118-127: The laboratory measurements of the absorption coefficient are methodologically sound, but the study could benefit from a brief discussion of potential limitations or uncertainties associated with these laboratory techniques.
Section 3.1:
The optical variability analysis is insightful. However, the interpretation of Rrs spectra could be more robust if supplemented with statistical analysis to quantify the observed differences.
The data analysis regarding the ternary plot and the statistical results of a_p_c (560)-(620) is clear, but the implications of these results on the algorithm's performance in different water types could be more explicitly discussed.
Section 3.2:
The enhancement of the QAA-GRI algorithm is well-described. The use of a power fitted method and its equation is a key strength. However, a deeper statistical analysis of the fitting process (e.g., goodness-of-fit metrics) would strengthen the findings.
Discussion: The discussion candidly addresses the uncertainties and limitations of the extended algorithm, especially in highly turbid waters and the challenge of atmospheric correction. This transparency is commendable, but suggestions for mitigating these limitations would enhance the section.
Conclusions: The conclusions effectively summarize the study's contributions and implications. The mention of further refinements and validations is appropriate, acknowledging the ongoing nature of research in this area. It would be beneficial to propose specific directions for future research based on the findings.
Comments on the Quality of English Language
Minor editing of English language required
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsBased on Sentinel-3A Ocean and Land Color Instrument (OLCI), an extended green-red band quasi-analytical algorithm (QAA-GRI) for retrieving the absorption coefficient was developed in this research to analyze the total absorption coefficient of two field measurements from the East China Sea (ECS) and Lake Qiandaohu (QDH). The study basically achieves the target of proposing a new algorithm based on new data and proving its effectiveness. However, some structural issues still need to be clarified.
1. Considering that the overall study only discusses the improvement of the estimation of the total absorption coefficient, it is recommended to directly review the estimation of the total absorption coefficient in the introduction instead of the overall discussion of IOP.
2. The introduction could benefit from a more precise explanation of the value and purpose of the study. The analysis of existing research in the introduction does not sufficiently highlight the identified gap of algorithms to evaluate the total absorption coefficient.
3. In Line 75, the authors highlight that “few studies have taken advantage of the Ocean and Land Color Instrument (OLCI) on Sentinel-3A” without an explanation of the data resources. The authors should discuss more about the advantages of the OLCI on Sentinel-3A and compare its characteristics with other data sources, such as VIIRS and Landsat.
4. The procedure of data preprocessing is missing.
5. Since the wavelength is crucial for the performance of retrieving the absorption coefficient, the reason for choosing wavelengths 443, 490, and 510 to compare the performance should demonstrated more clearly.
6. The section of discussion seems more like a list of findings instead of the discussion of the methodologies.
Comments on the Quality of English LanguageExtensive editing of English language required.
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Reviewer 4 Report
Comments and Suggestions for Authors===========================================================================
24-11-2023
Dear Editor and Authors,
The manuscript “An Extended Quasi-analytical Algorithm for Retrieving Absorption Coeffient Using 510-620 nm Bands from OLCI and MERIS satellite Data” is generally well written and supported but it needs some improvements in abstract, keywords, introduction, results and conclusions. The manuscripts present an average to high originality-novelty theme with good significance of content and scientific soundness. It needs improvement in some parts, like abstract, introduction, results, conclusions and in quality of presentation.
So, my Overall Recommendation for the manuscript is:
“Accept after minor revision (corrections to minor methodological errors and text editing).”
I hereby send you the 20 most important corrections/suggestions/questions for the manuscript entitled “An Extended Quasi-analytical Algorithm for Retrieving Absorption Coeffient Using 510-620 nm Bands from OLCI and MERIS satellite Data”.
Kind regards
Reviewer #
===========================================================================
The 20 most important comments/suggestions/questions for the manuscript are:
1. Reviewer: Authors should read carefully the “Instructions for Authors” of the water MDPI Journal.
Authors should read very carefully the “Instructions for Authors” and especially the “Abstract” and “Materials and Methods” sections.
2. Lines 11-30: Authors abstract.
Reviewer: Authors should revise the abstract properly in good, grammatically correct English and make it more concise.
I suggest authors should revise and rewrite the abstract.
3. Lines 25-26: Authors wrote: “Seasonal and annual variability in total absorption coefficient was investigated using seasonally (2010) and annual (2002-2012) MERIS satellite imageries.”
Reviewer: The above it is written in results part of the abstract. This is not a result.
Please move it upward in line 19, after the text “… The key model between absorption coefficient at 510 nm a(510) and GRI was reconstructed using power function based on the extensive datasets.”, and before the text “The results indicate that…”.
4. Lines 19-30: In abstract.
Reviewer: Although authors investigated Sentinel 3 OLCI data and MERIS instrument data, in the results section of abstract they present only results for the Sentinel 3 OLCI data. It would be better for authors to add their most important finding for the MERIS instrument data, in a concise matter.
5. Lines 21-22: Keywords. Authors wrote “absorption coefficient; quasi-analytical algorithm; green-red bands; OLCI and MERIS”.
Reviewer: Authors should improve and revise their keywords.
A suggestion is given below:
quasi-analytical algorithm for retrieving absorption coefficient;
Sentinel 3 OLCI and MERIS instrument data.
6. Lines 33-91: Introduction.
Reviewer: The introduction must be improved, in order to provide more sufficient background.
7. Section Materials and Methods:
Reviewer: Please check again the section Materials and methods and revise where it is needed in order to be more clearly written.
8. Lines 100-101: Authors wrote: “Another field measurement was collected in
the ECS on 1–12 January and 25 May–2 June 2017.”.
Reviewer: Authors should revise the above sentence properly in good, grammatically correct English and make it more concise.
A suggestion is given below:
Two field measurements were carried out in the ECS area on 1–12 January and 25 May–2 June 2017.
9. Lines 100-105: Authors wrote: “There are 127 sampling sites were collected during the ECS cruises, among which a total of 47 stations (excluding those sites located in coastal waters with high suspended sediments, since the QAA algorithm are not suitable for high turbid waters) have a full suite coincident remote sensing reflectance and total absorption coefficients (Figure 1c).”
Reviewer: Authors should revise the above sentence properly in good, grammatically correct English and make it more concise.
A suggestion is given below:
“In the present study a total of 127 sampling sites were used during the ECS area cruises, among which a number of 47 stations (excluding those sites located in coastal waters with high concentrations in suspended sediments, since the QAA algorithms are not suitable for high turbid waters [please insert a citation to support this]) have a full suite coincident remote sensing reflectance and total absorption coefficients (Figure 1c).”
10. Figure 1: Authors wrote: “(a) Field sampling sites' geographical locations; (b) Sampling sites in the QDH, Zhejiang Province. (c) Sampling sites in the ESC.”.
Reviewer: Authors should improve the quality of figure 1, and also, use bigger text in (b) and (c) legends.
Moreover, it would be better to revise the text.
A suggestion is given below:
“(a) Field sampling sites' geographical locations; (b) Sampling sites in the QDH area, Zhejiang Province. (c) Sampling sites in the ESC area.”.
11. Lines 125-127: Authors wrote:
Reviewer: The α(λ) was calculated according to [46] as authors firstly write, or according to [47]. Please clarify and revise the citation.
12. Figure 4:
Reviewer: (a) and (b) symbols are missing in Figure 4. Please insert (a) and (b) symbols in Figure 4.
13. Figure 6:
Reviewer: (a)… (f) symbols are missing in Figure 6. Please insert (a) ….(f) symbols in Figure 6, and also insert and explain them in the caption of the figure.
14. Figure 7:
Reviewer: (a)… (c) symbols are missing in Figure 7. Please insert (a) ….(c) symbols in Figure 7, and also insert and explain them in the caption of the figure.
15. Figure 8:
Reviewer: (a)… (f) symbols are missing in Figure 8. Please insert (a) ….(f) symbols in Figure 8, and also insert and explain them in the caption of the figure.
16. Figure 9:
Reviewer: (a)… (c) symbols are missing in Figure 9. Please insert (a) ….(c) symbols in Figure 9, and also insert and explain them in the caption of the figure.
17. Figure 10:
Reviewer: Authors mention and explain (d) in the caption of the Figure but there is not a (d) symbol and image in the Figure 10. Please insert image (d) in Figure 10 or remove (d)… from the caption of the figure.
18. Figure 11:
Reviewer: (a)… (d) are missing in Figure 11. Please insert (a) ….(d) symbols in Figure 11, and also insert and explain them in the caption of the figure.
19. Figure 12:
Reviewer: (a)… (k) are missing in Figure 12. Please insert (a) ….(k) symbols in Figure 12, and also insert and explain them in the caption of the figure.
20. Lines 327-345: 5. Conclusions
Reviewer: Authors should check again the conclusions section of the manuscript and revise properly in good, grammatically correct English and make it more concise.
Kind regards
The Reviewer #
-END of REVIEW-
Comments for author File: Comments.pdf
Minor editing of English language required
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
Comments and Suggestions for Authors
The completeness of the paper has been improved after the revision. Some minor problems that need to be addressed include:
1. The word “Materials” in the title of Chapter 2 is hard to understand. Data might be a more appropriate word. The explanation of the methods is still way too short as the main contribution of this paper is the proposition of a new algorithm.
2. In Line 139, the authors mentioned that “the OLCI level-2B WFR product acquired from the European Space Agency (ESA) Copernicus Open 140 Access Hub was chosen as a representative demonstration 141 to exhibit the performance of the extended QAA-GRI algorithm.” The utilization of this data and the resulting reduction in data preprocessing should be clearly stated.
3. Simple processing on the satellite images, such as stitching and data format conversion should be included in either the introduction of data or methods.
4. The reason for choosing specific wavelengths to compare the algorithm's performance should be presented by citing relevant references.
Author Response
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Author Response File: Author Response.pdf
