the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Aug 2024
Understanding uncertainties in coastal sea level altimetry data: insights from a round robin analysis
Abstract. The satellite radar altimetry record of sea level has now surpassed 30 years in length. These observations have greatly improved our knowledge of the open ocean and are now an essential component of many operational marine systems and climate studies. But use of altimetry close to the coast remains a challenge from both a technical and scientific point of view. Here, we take advantage of the recent availability of many new algorithms developed for altimetry sea level computation to analyze the sources of uncertainties of this procedure when approaching the coast. To achieve this objective, we did a round robin analysis of radar altimetry data, testing a total of 21 solutions for waveform retracking, correcting sea surface heights and finally deriving sea level variations. Uncertainties associated with each of the components used to calculate the altimeter sea surface heights are estimated by measuring the dispersion of sea level values obtained using the various algorithms considered in the round robin for this component. We intercompare these uncertainty estimates and analyze how they evolve when we go from the open ocean to the coast. At regional scale, complementary analyses are performed through comparisons to independent tide gauge observations. The results show that tidal corrections and mean sea surface can be significant contributors to sea level data uncertainties in many coastal regions. However, improving quality and robustness of the retracking algorithm used to derive both the range and the sea state bias correction, is today the main factor to bring accurate altimetry sea level data closer to the shore than ever before.
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Status: open (until 17 Oct 2024)
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RC1: 'Comment on egusphere-2024-2449', David Cotton, 17 Sep 2024
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Summary
Interesting work, new results that give insight on uncertainties in measuring Sea Level Anomaly from satellite altimeter data.
It is recommended that the paper is accepted for publication but with minor changes, to be subject to further review.
General points
The aim of the paper is to analyse the source of “uncertainties” in sea level measurements from radar altimetry data, based on the dispersion of sea level anomaly values obtained by the different algorithms and corrections.
The paper is well written, with all aspects of the analysis clearly explained.
The paper should make it clear that this analysis is of uncertainties in the measurement of sea level anomaly resulting from different processing algorithms and with different sources of corrections, as distinct from uncertainties due to random errors in the measurement of sea level, e.g. due to small scale variability.
The discussion should make clear this analysis applies specifically to LRM data, and not to SAR Altimetry data.
I would like to have seen some discussion on the reasons behind the differences in performance between the different algorithms and corrections. Are there potential physical reasons?
OS questions:
- Does the paper address relevant scientific questions within the scope of OS?
Yes – the main question – to gain a better understanding of uncertainties in satellite altimeter measurement of sea level anomaly is relevant to, and lies within the scope of, OS
- Does the paper present novel concepts, ideas, tools, or data?
The paper introduces and compares results from new processing algorithms. Thus the data and analysis are new.
- Are substantial conclusions reached?
The conclusions provide new insight into the uncertainties in different aspects of the calculation of sea level anomaly from satellite data. There are no new insights into the physics of variability in sea level anomaly
- Are the scientific methods and assumptions valid and clearly outlined?
- Are the results sufficient to support the interpretations and conclusions?
- Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)?
Yes - the methods are valid, clearly explained and support the conclusions. The approach can be reproduced, and the data accessed, on the information provided
- Do the authors give proper credit to related work and clearly indicate their own new/original contribution?
Yes – the references are relevant and appropriate
- Does the title clearly reflect the contents of the paper?
I would prefer that the title made it clear that it was the uncertainties in the measurement of sea level anomaly that was being assessed, e.g.:
Understanding uncertainties in the satellite altimeter measurement of coastal sea level altimetry data: insights from a round robin analysis.
- Does the abstract provide a concise and complete summary?
- Is the overall presentation well structured and clear?
- Is the language fluent and precise?
The abstract and main text are well written and clear. Some specific recommendations for clarification of language have been provided.
- Are mathematical formulae, symbols, abbreviations, and units correctly defined and used?
Yes
- Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated?
Some specific recommendations for clarifications have been provided
- Are the number and quality of references appropriate?
In general yes – some references are missing / incorrect.
- Is the amount and quality of supplementary material appropriate?
Not applicable
Specific comments
Section 2.1 General Goals.
P3 Line 70 Re the focus on LRM data. Should note then that this analysis of uncertainties is specific to LRM data. SAR Altimeter Delay Doppler processed data will have different characteristics.
P3 Line 93 Laignel et al, 2022 – not in references.
P4 Line 105 “uncertainties in sea level data”. Need to be careful not to generalise – this analysis only provides information on variability in sea level data from different processing approaches, not directly on variability in the original sea level data.
Section 2.2 Overview of the selected algorithms
P5 line 127. Some missing words: “were also discarded because they are considered as very accurate …..”
P5 line 134 “aforementioned article” – replace with Cazenave et al., 2022 to avoid any uncertainty.
Section 2.3 Tide Gauge Data
Line 166, 168 – I get an error for the refmar URL – just give the main French URL
Section 3 Methodology
P9 Line 212: “as the density of altimetry points is higher close to the coasts due to the presence of islands and to the tracks configuration.”
I do not understand this statement. The density of points cannot be higher due to the presence of islands. If anything it should be lower. Please rewrite this sentence.
Section 4.1 Results / Ionospheric Correction
P11 Figure 3. Caption refers to blue and green lines. Figure has blue and orange lines.
P11 line 259. Add “in SLA” to the end of this sentence – “as are potentially the uncertainties in SLA”
Section 4.2 Wet Tropospheric Correction
P12 line 268 – “Due to the large space-time variability of this correction (0-50 cm), the latter is considered the best option. Should provide a reference for this statement – also, is this specifically over ocean?
P13 line 283 Use “pair” instead of “couple” in “…differences of STD of SLAs for each couple of SLA solutions…”
Section 4.3 Ocean Tide Correction
P16 343-344. I find this sentence confusing: “In reddish (blueish) regions, FES2014b (the regional solution) decreases the STD of SLA more significantly.” Does the regional model (FES2014b) decrease the STD of the SLA more significantly in both red and blue coloured regions?
Please rewrite to make the point clearly.
P16 line 353. Suggest to replace “if” by “although”, and add “such” i.e. : “These results illustrate that althoughvery significant progress has been made since studies such as Ray (2008), large uncertainties remain…”
P17 line 363. “However, if this result can probably be extrapolated to the whole Mediterranean Sea, which is characterized by small tidal amplitudes except in a few areas (Adriatic Sea, Gulf of Gabes), it should be qualified for the Australia region, as it strongly depends on the tide gauge stations.” It is not clear to me what this sentence means. What is “the result” that can be extrapolated? Is it that the FES2014b regional is the TG model that gives the highest correlation and lowest RMS? If so, this should be explicitly stated.
Section 4.4 Mean Sea Surface Height
P18 line 376 Andersen and Knudsen, 2009 is not included in the references.
P18 line 382 Pujol et al., 2016 not in the references
P18 line 382 Schaeffer et al., 2022 not in the references
Section 4.5 Altimeter Range and SSB
P20 line 419 “For the Adaptive and ALES retracking algorithms, it is the 2D SSB solution directly computed at 20 Hz.” Replace “it is” by “we consider”
P20 line 426 Could an additional panel be added to Figure 10 to illustrate the point that the MLE4 retracker provides fewer valid data solutions than the other retrackers?
P20 line 428-9 “Here again, the spread between the SLA solutions obtained with these three retracking algorithms (Figure 10.c) clearly increases when approaching the coast, reflecting an increase in the SLA uncertainty associated to the range-SSB couple.” It is not clear what “couple” means here?
Maybe rephrase, e.g.
“Here again, the spread between the SLA solutions obtained with these three retracking algorithms (Figure 10.c) clearly increases when approaching the coast, reflecting an increase in the SLA uncertainty associated to uncertainties in range and SSB.”
Section 4.6 Synthesis of the results
P23 Line 479-480 “Assuming that the spread of SLA values obtained by changing the calculation algorithms provides an estimate of the associated SLA uncertainty,…”
This text could be interpreted to read that the spread of SLA values from the different calculations is representative of geophysical variability in SLA. To avoid this interpretation I suggest to remove the first part of the sentence so that it starts “We summarize in Table 2 the main results…..
P23 line 481. And similarly I’d suggest to change
“Beyond the near-coastal region, the biggest contributors to SLA uncertainties are the SSB and the range, both associated with the retracker algorithms, generating an uncertainty of about 1 cm.”
to
“Beyond the near-coastal region, the biggest contributors to uncertainty in the LRM altimeter estimate of SLA are the SSB and the range, both associated with the retracker algorithms, generating an uncertainty of about 1 cm.”
Rest of this section.
In general I would recommend to replace “SLA uncertainties” with “uncertainties in the estimated SLA”
P25 line 512
I think some more precise language is required.
“Of course, we cannot be sure that these results reflect the estimate of how far the SLA obtained may be from the true SLA value because no measure of truth exists.”
The basis of the discussion is that the analysis provides an estimate of the range of possible values of SLA using different approaches to the calculation. Thus, the argument of this paper goes, this is a proxy for an estimate of precision. It is not aiming to assess accuracy (how close the measurement is to the actual value – which is only available at tide gauges).
The discussion should make the distinction between precision and accuracy, and also note the range of different estimates is due to different approaches to the calculation and not necessarily representative of the individual precision of any single measurement, or of the natural variability of SLA within the footprint of the radar measurement.
Am I happy about the final sentence:
“Note that even if this work was carried out with LRM altimetry data, part of the conclusions should also contribute to modern altimetry techniques such as SAR and SARin, as all satellite altimetry missions share some common correction terms, such as tidal and MSSH models for example. Even with their increased observational capabilities, which are favorable for monitoring coastal zones, the way these new types of altimetry observations are processed and the methodologies used to calculate the various geophysical corrections remain critical steps to derive accurate and precise geophysical information.
References
Missing References
- Laignel et al, 2022
- Andersen and Knudsen, 2009
- Pujol et al., 2016 (there is a Pujol et al., 2018)
- Schaeffer et al., 2022
References listed but not referenced in the text
- Egbert and Erofeeva, 2002
- Legeais et al., 2018
Peng and Deng, 2018 should be Peng et al., 2018
Citation: https://doi.org/10.5194/egusphere-2024-2449-RC1
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