Assessing the Effect of Tandem Phase Sentinel-3 OLCI Sensor Uncertainty on the Estimation of Potential Ocean Chlorophyll-a Trends

Round 1

Reviewer 1 Report

Overall: When you mention trend in result section, please specify that trend is for chlorophyll or sensor uncertainty. Everywhere, it's written just trend and sometimes hard to follow.

Keywords: Please do not include the words used in title of the manuscript here.

Lines 100-101: covering a two-month period (7th 101 June 2018 - 16th October 2018). June to October is not two month period. Please revise.

Figure 5. Please revise the caption from Figure 4 to Figure 5. . Also, mention in figure caption that black areas have missing data (probably due to cloud cover, right?)

Figure 5. Please include similar Figure (Mean chlorophyll and standard deviation) for OLCI-A to compare the result with OLCI-B. 

Lines 183-185: “The first is the actual measurement uncertainty of the two sensors, which may respond differently to different surface and near-surface conditions including contributions from coloured dissolved organic matter (CDOM), and thus have a spatial pattern.” Is CDOM only factor affecting? What about Total Suspended Matter?

 

Author Response

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Reviewer 2 Report

This technical note assesses the impact of the uncertainty of ocean chlorophyll derived from Sentinel-3A and Sentinel-3B OLCI sensors during their tandem phase. The analysis shows that if future aging can be well quantified and mitigated, Sentinel-3A and Sentinel-3B should provide a consistent trend. This manuscript is well written and organized but some minor aspects need to be considered before its final acceptance.

 

1. Figure 2. Why does the trend increase after adding random noise? Is it possible decrease?

2. Figure 4. Why is the trend Std of OLCI-Av small than that of OLCI-Aa in some high chlorophyll areas, for example, the coast of Arabian Peninsula?

3. Line 178. Figure 4 should be Figure 5.

Author Response

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Reviewer 3 Report

The technical note entitled “Assessing the effect of Tandem Phase Sentinel-3 OLCI sensor uncertainty on the estimation of potential ocean chlorophyll trends” by Hammond et al. assessed the use of Sentinel 3A and 3B in tandem phase for the retrieval of chlorophyll a trends.

My main concern about this technical report is the use of a level 3 product for the comparison of 2 sensors. Maybe it would be better to compare performances for level 1, level 2 and level 3 products. Additionally, the use of an algorithm which uses a band ratio reduces the noise, so it would be interesting to see the trends of an algorithm that is not based on a ratio, for example, the FLH. Another concern is the lack of statistical work, I believe that some statistical test could be included to compare the performance of the two sensors.

Additionally, it would be interesting to describe the atmospheric correction used and to compare the products from the atmospheric correction (level 2 products).

Here are some minor comments:

L45 – Climatic record? I think it should be more related to water quality.

L54 – Maybe you should add some products from MERIS as well, or even citing MODIS that have been almost 20 years collecting ocean color data. I think that there are information missing in this paragraph.

L78 – What type of atmospheric correction was used?

L95 – For band ratio-based algorithms the noise is reduced, however, what about when algorithms use more than a band ratio where geometric effects of the light source are not cancelled out? Maybe you should use other algorithms for chl-a estimation which is not based on a band ratio, not only OC4MEe.

L97 – Not sure if L3 products are the best for a sensor comparison because you will be adding the sum of errors from L1, L2 and L3 processing (especially when generating a global product). Maybe it would be better to also compare L2 products since they are the source for L3 products.

L148 – the text within the Figure is too small, please change the font size.

L197 – Maybe it would be better to show more statistical analysis to show this.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

Dear authors, 

 

Thank you for providing a revised version of your manuscript. The manuscript improved and there are only few minor issues that need to be corrected.

Usually the "a" in chlorophyll-a is in italic "a" to have an "alpha" shape like. Additionally, it is typical to see chlorophyll-a as chl-a not only chl (since if could be chl-B or chl-c). Then only chl is usually the total chl content.

L198 - Can you please add some of the statistical results (quantitative) here instead of only referencing it to Appendix C (those maps are more qualitative)?

The conclusion session has a different font size compared to the rest of the manuscript. 

Best Regards,

Author Response

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