The DTU21 global mean sea surface and first evaluation

Andersen, Ole Baltazar; Rose, Stine Kildegaard; Abulaitijiang, Adili; Zhang, Shengjun; Fleury, Sara

doi:https://doi.org/10.5194/essd-15-4065-2023

Articles | Volume 15, issue 9

https://doi.org/10.5194/essd-15-4065-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/essd-15-4065-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 15, issue 9

Data description paper

|

13 Sep 2023

Data description paper |

| 13 Sep 2023

The DTU21 global mean sea surface and first evaluation

Ole Baltazar Andersen, Stine Kildegaard Rose, Adili Abulaitijiang, Shengjun Zhang, and Sara Fleury

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Final revised paper (published on 13 Sep 2023)
Preprint (discussion started on 08 May 2023)

Interactive discussion

Status: closed

RC1:
'Comment on essd-2023-160', Anonymous Referee #1, 14 May 2023
Satellite altimeter data of 2 Hz instead of 1 Hz are used to construct DTU21MSS in the manuscript. The diameter of general footprint for radar altimeter is commonly about 4-10 km. So the correlation of 2-Hz SSHs should be considered in the study.

The time of DTU21MSS spans about 20 years (1993.1.1 to 2012.12.31). You know, 19-year is very better for the MSS time span (Yuan et al. 2020. https://doi.org/10.1016/j.csr.2019.104009). Many satellite altimeter data are collected since 2013, and it is very important to reduce them to 1993.1.1~2012.12.31 with the SSH time change correction method.

There are many satellite altimetry missions in table 1 with different SSH resolution and precision. How to evaluate effects of resolution and precision on MSS models? Here GM data are all since 2010, and how to reduce the time reference to 1993.1.1 to 2012.12.31?

The uncertainty of MSS for the low frequency of over 150 km is very serious. And the uncertainty of MSS over the polar area and the coastal area is very great. How to evaluate and reduce these uncertainties?

What about the mechanism for the large change of MSS over the serious sea currents?

Please increase practical applications of MSS model.
Citation: https://doi.org/10.5194/essd-2023-160-RC1
- AC1: 'Reply on RC1', Ole Baltazar Andersen, 06 Jul 2023
  
  Thank for the very useful comments to the manuscript. I have replied to the following comments as they appear in the review. The review comments are shown in bold.
  Satellite altimeter data of 2 Hz instead of 1 Hz are used to construct DTU21MSS in the manuscript. The diameter of general footprint for radar altimeter is commonly about 4-10 km. So the correlation of 2-Hz SSHs should be considered in the study.
  Inded we have retracked the 20/40 Hz data in the derivation of the DTU21MSS using physical retracking. The radar altimeter footprint is larger for Ku band than Ka band. The trailing edge of the waveform which corresponds to distances larger than around 2.5 km from the center point of the radarpulse is mainly used to derive the SWH parameter. Hence there might be an effect this parameter. We do however use two-pass retracking which decouples the possible correlation.
  The time of DTU21MSS spans about 20 years (1993.1.1 to 2012.12.31). You know, 19-year is very better for the MSS time span (Yuan et al. 2020. https://doi.org/10.1016/j.csr.2019.104009). Many satellite altimeter data are collected since 2013, and it is very important to reduce them to 1993.1.1~2012.12.31 with the SSH time change correction method.
  There has been a lot of focus on the Accuracy of MSS models recently (e.g. Pujol et al., 2019) and particularly in the preparation for SWOT. I agree with the reviewer that we will soon need to update the MSS to encompass 30+ years of data. However we decided to keep the reference period of 20 years for the current MSS for the time being as this enables directly comparion between the DTU, CLS and SIO MSS models At line 66: we added: We kept the averaging period for DTU21MSS to be able to validate the MSS directly with other MSS models. Changing the averaging period by as little as 3 years will change the mean by 1 cm as well as the spatial pattern due to ongoing sea level change (Veng and Andersen, 2019).
  There are many satellite altimetry missions in table 1 with different SSH resolution and precision. How to evaluate effects of resolution and precision on MSS models? Here GM data are all since 2010, and how to reduce the time reference to 1993.1.1 to 2012.12.31?
  We updated the desciption how we deal with this at line 230.The fine-scale computation is done in small tiles of 1º x3º with a 0.5 º boundary to parallelize the computation process. As all wavelength longer that the size of the tiles are removed (roughly 200 km) we found that there was no need to adjust the period of the GM data to the MSS averaging period (1993-2012)
  The uncertainty of MSS for the low frequency of over 150 km is very serious. And the uncertainty of MSS over the polar area and the coastal area is very great. How to evaluate and reduce these uncertainties?
  I think that we present the differences wiht other MSS models and indeed the difference is significant in Polar region. This is mainly due to the choice of retrackers as we also mention. This is also the reason why we investigate hugely computer ressources in the project to retrack all data in these regions. This has significantly reduced the errors when comparing with independent altimetry like S3A/B.
  What about the mechanism for the large change of MSS over the serious sea currents?
  In the computation of the mean profiles, we correct for the ocean variability as described in Pujol et a., 2017. I added the following sentence to explain at line 173: In the computation of the mean profiles we apply the DUACS daily gridded sea level anomalies to remove ocean variability (Pujol et al., 2017)
  Please increase practical applications of MSS model.
  We have added the following sentence to the introduction Mean sea surface models are increasingly used as vertical offshore reference surfaces for offshore operations (e.g., dredging, windfarms, bathymetry surveys)
  
  Citation: https://doi.org/10.5194/essd-2023-160-AC1
RC2:
'Comment on essd-2023-160', Anonymous Referee #2, 07 Jun 2023
The mean sea surface (MSS) plays an important role in geodesy, marine geodesy, and other disciplines. This paper introduced a new MSS called DTU21MSS and a suite of evaluations are performed. It should be said that this paper is very helpful to improve the accuracy and spatial resolution of an MSS model construction. I support its publication in ESSD after properly addressing the questions raised by previous reviewers and making a major revision. Please find detailed comments on the current MS below.
There are multi-satellite altimetry data from 1993 to now. Why is the DTU21 MSS established by the altimetry data from 1993.01.01 to 2012.12.31?

The mean profile of TOPEX/J1/J2 is the reference for the DTU21 MSS. While the sea surface heights derived from TOPEX/J1/J2 are within the 66° parallels, those derived from other satellites, such as Cryosat-2 and Saral AltiKa, are outside the 66° parallels, so how do you unify the references between the two?

Is the ERM data used for the DTU21 MSS construction also 2-HZ, and if so, how is it obtained, it is processed in the same way as the GM data?

Are both ERM and GM data processed by waveform retracted? If not, how is the ERM data corrected for the ocean sea state bias? If yes, where does TOPEX waveform data come from?

What is the “correlation length”, and why is it different in the longitude and latitude directions?

How can ERM and GM data be unified within the 66° parallels in the space-time reference?

For Figures 4 and 6, why not compare DTU21 with DTU18?

Whether Figure 6 gives the mean profile from Sentinel-3A along track 719 or 471? Which is inconsistent with the statement on Line 260.

The vertical coordinate to the right of Figure 6 should be SLA.

Figure 9 can only see the difference between DTU21 and DTU15 in the Baltic Sea and the Aleutian trench zone in Alaska, but cannot see that DTU15 is worse than DTU21.

How did you get the long wavelength of the DTU21 MSS within the 66º parallels?

I don't understand the meaning of Figure 3.

From the overall DTU21 evaluation, it is not clear how much DTU21 has improved in the short wavelength band within the 66° parallels. It is also not clear how much the satellite altimeter data of 2 Hz instead of 1 Hz will improve the accuracy of the DTU21 MSS. Therefore, additional experiments should be needed to further evaluate DTU21.

Technical corrections:
Line 32-33: It should be “…………. (Pujol et al., 2018; Yuan et al., 2023)”.
Line 98: It should be “…………. (Raney, 2011).”
Line 115: It should be “Maus et al. (1998) and Sandwell and Smith (2005) ………….”.
Line 123: It should be “…………. (Garcia et al., 2014; Zhang and Sandwell, 2017).”
Line 170: This is not a complete sentence.
Line 184: It should be “…………. Andersen and Knudsen (2008).”
Citation: https://doi.org/10.5194/essd-2023-160-RC2
- AC2: 'Reply on RC2', Ole Baltazar Andersen, 21 Jul 2023
  
  Thank you for the detailed review and constructive comments. The comments help revising the manuscript and I have put the review comments in bold and the changes to the manuscirpt in italic in the following:
  Please find attached a pdf with my comments and explanations. I tried to paste text into the online window but you could not read the tex properly.
  
  Citation: https://doi.org/10.5194/essd-2023-160-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Ole Baltazar Andersen on behalf of the Authors (21 Jul 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (24 Jul 2023) by Giuseppe M.R. Manzella

AR by Ole Baltazar Andersen on behalf of the Authors (31 Jul 2023) Manuscript

Short summary

The mean sea surface (MSS) is an important reference for mapping sea-level changes across the global oceans. It is widely used by space agencies in the definition of sea-level anomalies as mapped by satellite altimetry from space. Here a new fully global high-resolution mean sea surface called DTU21MSS is presented, and a suite of evaluations are performed to demonstrate its performance.