Reduction of Spatially Structured Errors in Wide-Swath Altimetric Satellite Data Using Data Assimilation
, Nora Poel 1,2, Jean-Michel Brankart 1, Jacques Verron 1,3 and Laura Gómez Navarro 1,4Round 1
Reviewer 1 Report
This manuscript examines approaches to reduce spatially structured errors for the upcoming Surface Water and Ocean Topography (SWOT) satellite mission. The authors propose an error reduction method to remove part of the SWOT signal associated with structured errors. This produces a detrended SSH signal is used in ensemble data assimilation with information from an independent ensemble of scenarios. These scenarios are taken from an experiment of a very high resolution model simulation. The results is significant reduction in the large scale SWOT errors. Filtering also reduces small scale errors and allows energy to be recovered down to a scale of 25 km.
Given the desire to use the data from the upcoming SWOT mission, approaches to reduce errors are needed and valuable. The approach here makes sense and suggests an ability to significantly reduce errors. Thus papers such as this are worth publishing. Although generally well written, there are a few places where the manuscript could be improved. Thus I suggest minor revisions. Details are provided below.
Line 15: are not is
Introduction: It would be nice to have a bit more background on the SWOT mission to begin the introduction, including why ssh is needed, and the importance of mesoscale and submesoscale information.
Line 24: …in two characteristics…
Line 27: remove ‘allow to’
Lien 40: What exactly do the authors mean by a ‘strong hypothesis’ – maybe some better wording can be used
Line 68: …in the simulation…
Line 68: Justify that a method using a model without tides will give reasonable results. Come back to this point during the discussion as well.
Line 92: Which version of NEMO?
Line 95: suggests represents instead of observe
Line 98: Might it be useful to have a regional map, with a circulation schematic of the region in comparison to the Gulf Stream location?
Line 132: de-noise
Figures: For figures such as 3, the x-axis and y-axis are not defined. I assume they are latitude and longitude (in which case it might be better to indicate longitude W rather than using negative values)
Figure 1 caption: The top middle panel is not defined
Figure 4: What are the x and y-axis?
Line 165: Not sure what “…substituted to an SSH information” means.
Section 3.1: Why are the randomly picked static ensemble from a different time period than the main model simulation?
Figure 5: What are the units of the colour bars? Assume m, but it is not stated.
Figure 5 vs 6: Why do some figures use m for ssh, and others (such as 6) use cm? Be consistent throughout.
Line 242: approximately
Line 272: similar to…
Line 320: depends
Conclusions: Would like to see some ending paragraph about what the authors hope SWOT will be able to provide after the successful use of a scheme such as their’s. I.e. Why is work such as this important.
Author Response
We would like to thank Reviewer 1 for the constructive comments. Please find below the point-by-point response to the review. Most of the minor comments have been addressed directly in the manuscript (highlighted in red). In the following, we address (in italic font) some of the minor comments of Reviewer 1 (in bold font).
Introduction: It would be nice to have a bit more background on the SWOT mission to begin the introduction, including why ssh is needed, and the importance of mesoscale and submesoscale information.
A wider introduction on the importance of mesoscale and submesoscale observations for oceanography and how SSH data can provide them has been added. (lines 21-26)
Line 68: Justify that a method using a model without tides will give reasonable results. Come back to this point during the discussion as well.
Reviewer 1 brings up a crucial point about the impact of internal tides on the SWOT data. It has been shown in multiple studies that this impact should be substantial. However, this study does not yet take into account this impact and only focuses on the noise-signal separation. This is now more clearly stated in the introduction. (lines 70-74)
As suggested by Reviewer 1, a discussion on the behavior of our method in the context of internal tides is now proposed. This discussion is speculative so, as mentioned in the manuscript, the impact of internal tides on the method should be assessed in further studies. (lines 345-353)
Note also that several members of the SWOT Science Team are developing deterministic (Arbic) and empirical (Dushaw, Ray, Zaron, Zao, Ubelmann) methods aiming at filtering the coherent part of the internal tides from the signal. These methods may natably reduce the impact of the internal tides on observed SSH signal.
Line 92: Which version of NEMO?
The NEMO version used was version 3.5. It is now stated in the manuscript. (line 98)
Line 98: Might it be useful to have a regional map, with a circulation schematic of the region in comparison to the Gulf Stream location?
A map of the full North Atlantic simulation NATL60 SSH field is now provided including the OSMOSIS region highlighted. (Figure 1)
Figures: For figures such as 3, the x-axis and y-axis are not defined. I assume they are latitude and longitude (in which case it might be better to indicate longitude W rather than using negative values)
The x-axis and y-axis of the figures are now given in latitude and longitude in North and West coordinates respectively. (Figure 1, 4, 6 and 9)
Section 3.1: Why are the randomly picked static ensemble from a different time period than the main model simulation?
The ensemble is randomly selected outside the experimental time period in order to avoid consanguinity between the ensemble and the artificial observations. This procedure is common in twin experiment settings. It is now clearly stated in the manuscript. (lines 213-215)
Figure 5 vs 6: Why do some figures use m for ssh, and others (such as 6) use cm? Be consistent throughout.
The color bars in Figure 6 and 9 (now 7 and 10) were given in cm in order to be able to have the SSH RMSE, in Figure 6a and 9a, at the same order of magnitude as the other variables hence making it possible to display all variables on the same histogram. To maintain that feature while being consistent, the SSH in Figure 7 and 10 are now given in 10-2meters. (Figure 7 and 10)
Conclusions: Would like to see some ending paragraph about what the authors hope SWOT will be able to provide after the successful use of a scheme such as their’s. I.e. Why is work such as this important.
An ending paragraph, which clarifies the novelty and the contribution of the method to the SWOT mission scientific objective, has been added to the conclusion.(lines 375-381)
Reviewer 2 Report
The focus and the novelty of this paper is not clear. The third paragraph in the introduction is considered to be the objective. However readers will not understand what the author wants to describe in subsequent paragraphs. Therefore, the novelty has not been clarified in the conclusion.
Author Response
We would like to thank Reviewer 2 for the constructive comments. Please find below the point-by-point response to the review. The minor comments have been addressed in the manuscript (highlighted in red). In the following, we address (in italic font) the minor comments of Reviewer 2 (in bold font).
The focus and the novelty of this paper is not clear. The third paragraph in the introduction is considered to be the objective. However readers will not understand what the author wants to describe in subsequent paragraphs.
The focus and the novelty of the paper are now clearly stated at the beginning of the third paragraph in the introduction. (lines 45-47)
Therefore, the novelty has not been clarified in the conclusion.
An ending paragraph, which clarifies the novelty and the contribution of the method to the SWOT mission scientific objective, has been added to the conclusion. (lines 375-381)
Round 2
Reviewer 2 Report
Thank you for addressing my concerns.
Minor comment:
The last paragraphs (outline part) in the section 1 should be deleted, because it is not a book.
