the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Aug 2025
The DTU25 Mean Sea Surface: From and For SWOT
Abstract. We introduce a new Mean Sea Surface model (MSS) that incorporates the wide-swath altimetry obtained from the Surface Water and Ocean Topography (SWOT) satellite, along with long timeseries of conventional altimetry. The DTU25MSS constrains long wavelengths (> 20 km) from a suite of conventional altimeters while utilizing almost 2 years of SWOT observations to reduce the short wavelength noise and incorporate previously unmapped geodetic features into the MSS. Parametric long wavelength corrections of the SWOT data in order to compensate for the short time-scale is presented, and the resulting MSS model is compared with contemporary MSS models as well as data from the SWOT Cal/Val orbit. The MSS is available on http://doi.org/10.11583/DTU.29412275, and includes an experimental MSS which has the reference period moved to 2023 as opposed to 2003, to compensate for sea level rise. To extend the MSS into the coastal zone, the high-resolution 250 m SWOT data is used close to the coast (< 40 km), and resolves complex features previously not included. Using an updated MSS with better resolved short wavelength signals is seen to be a large benefit for interpreting the detailed SWOT observations with reduced leakage of geodetic features into the oceanographic signals, as well as ~30 % increase in spatial resolution. Due to incorporating complex novel features in the coastal zone that have been resolved by SWOT the full effect on other more coarse observations is a potential for further studies. The SWOT data and utilization of it is only expected to be improved with time and further development of methods for utilizing this new dataset will move it closer to its full potentials.
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Interactive discussion
Status: closed
- RC1: 'Comment on essd-2025-404', Anonymous Referee #1, 28 Sep 2025
-
RC2: 'Comment on essd-2025-404', Anonymous Referee #2, 16 May 2026
- The study has modeled a MSS in different frequency bands, including long wave (>20km), intermedium wave (4-20km), and short wave (<40 km from coasts). How to define the long wave (>20km), intermedium wave (4-20km), and short wave (<40 km from coasts) of MSS? The scale around 20km in oceanography should be sub-mesoscale.
- Multiple sources of satellite altimeter data were used in the manuscript. Detailed information on all satellite altimetry data used should be provided. How to unify the spatiotemporal datum of different altimetry data?
- Is the MSS model constructed using 20 years of satellite altimeter data?
- The MSS in Eq. (1) corresponds to t0. But t0 is 2003, why? t0 should be the middle time of the data period.
- There are a large number of data gaps in the SWOT/KaRin altimetry data for one cycle. How to handle these data gaps?
- The signal-to-noise ratio of the 250m SWOT/KaRin altimetry data is very low. How to improve the quality of these data?
- The SWOT/KaRin altimetry data has a relatively short time. What corrections have been made to the short-term SWOT/KaRin altimeter data in order to obtain data that is consistent with the observation time required for MSS modeling?
- How to determine the weights in Eq. (13)?
- Why is the PSD of DTM25 the smallest in the high-frequency part in Figure 7 (c)?
- How are the data gaps handled in Figure 8?
- What corrections are made to Sentinel-3 altimeterry data in Table 1? How to define short wavelength?
- Why is the PSD of DTM25 the smallest in the high-frequency part in Figure 15?
Citation: https://doi.org/10.5194/essd-2025-404-RC2 -
AC1: 'Comment on essd-2025-404', Bjarke Nilsson, 26 Jun 2026
The comment was uploaded in the form of a supplement: https://essd.copernicus.org/preprints/essd-2025-404/essd-2025-404-AC1-supplement.pdf
Interactive discussion
Status: closed
-
RC1: 'Comment on essd-2025-404', Anonymous Referee #1, 28 Sep 2025
General comments
The manuscript proposed by Nilsson et al. aims providing a new Mean Sea Surface from satellite altimetry including features from recent observations of the wide-swath altimeter SWOT. Depending considered wavelengths, the use of SWOT altimetry allows improving Mean Sea Surface at intermediate and short wavelengths including coastal regions.
The manuscript is detailed, clearly explained and all limitations of the product are explicitly discussed. Even if the product will most probably evolve with improvements in SWOT processing and the increasing length of observed time series, the provided dataset is an important step in the Mean Sea Surface estimation.
Specific comments
Figures are particularly well designed. However, following ESSD publishing recommendations, it would be important to double check with provided tools to improve the accessibility of colour figures (https://www.earth-system-science-data.net/submission.html#figurestables) mainly for uncertainties.
Minor and technical corrections
Figure 4
As for figures 2 and 3, it would improve the readability to see the location where are show the coastal comparisons.
Figure 8
The chosen colormaps are not perceptually uniform. It could make sense for MSS to track large features but I would suggest to use a perceptually uniform colomap for uncertainties.
Figure 14
Please revise the figure caption to improve readability as several a,b,c appears on the figures and are not described in the caption.
p.6 / l. 127 – please replace “The parameters is” by “The parameter is”
p.14 / l. 276 – please corrct “latitudional” by “latitudinal” or “meridional”
Citation: https://doi.org/10.5194/essd-2025-404-RC1 -
RC2: 'Comment on essd-2025-404', Anonymous Referee #2, 16 May 2026
- The study has modeled a MSS in different frequency bands, including long wave (>20km), intermedium wave (4-20km), and short wave (<40 km from coasts). How to define the long wave (>20km), intermedium wave (4-20km), and short wave (<40 km from coasts) of MSS? The scale around 20km in oceanography should be sub-mesoscale.
- Multiple sources of satellite altimeter data were used in the manuscript. Detailed information on all satellite altimetry data used should be provided. How to unify the spatiotemporal datum of different altimetry data?
- Is the MSS model constructed using 20 years of satellite altimeter data?
- The MSS in Eq. (1) corresponds to t0. But t0 is 2003, why? t0 should be the middle time of the data period.
- There are a large number of data gaps in the SWOT/KaRin altimetry data for one cycle. How to handle these data gaps?
- The signal-to-noise ratio of the 250m SWOT/KaRin altimetry data is very low. How to improve the quality of these data?
- The SWOT/KaRin altimetry data has a relatively short time. What corrections have been made to the short-term SWOT/KaRin altimeter data in order to obtain data that is consistent with the observation time required for MSS modeling?
- How to determine the weights in Eq. (13)?
- Why is the PSD of DTM25 the smallest in the high-frequency part in Figure 7 (c)?
- How are the data gaps handled in Figure 8?
- What corrections are made to Sentinel-3 altimeterry data in Table 1? How to define short wavelength?
- Why is the PSD of DTM25 the smallest in the high-frequency part in Figure 15?
Citation: https://doi.org/10.5194/essd-2025-404-RC2 -
AC1: 'Comment on essd-2025-404', Bjarke Nilsson, 26 Jun 2026
The comment was uploaded in the form of a supplement: https://essd.copernicus.org/preprints/essd-2025-404/essd-2025-404-AC1-supplement.pdf
Data sets
DTU25MSS Mean Sea Surface Ole Baltazar Andersen and Bjarke Nilsson http://doi.org/10.11583/DTU.29412275
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General comments
The manuscript proposed by Nilsson et al. aims providing a new Mean Sea Surface from satellite altimetry including features from recent observations of the wide-swath altimeter SWOT. Depending considered wavelengths, the use of SWOT altimetry allows improving Mean Sea Surface at intermediate and short wavelengths including coastal regions.
The manuscript is detailed, clearly explained and all limitations of the product are explicitly discussed. Even if the product will most probably evolve with improvements in SWOT processing and the increasing length of observed time series, the provided dataset is an important step in the Mean Sea Surface estimation.
Specific comments
Figures are particularly well designed. However, following ESSD publishing recommendations, it would be important to double check with provided tools to improve the accessibility of colour figures (https://www.earth-system-science-data.net/submission.html#figurestables) mainly for uncertainties.
Minor and technical corrections
Figure 4
As for figures 2 and 3, it would improve the readability to see the location where are show the coastal comparisons.
Figure 8
The chosen colormaps are not perceptually uniform. It could make sense for MSS to track large features but I would suggest to use a perceptually uniform colomap for uncertainties.
Figure 14
Please revise the figure caption to improve readability as several a,b,c appears on the figures and are not described in the caption.
p.6 / l. 127 – please replace “The parameters is” by “The parameter is”
p.14 / l. 276 – please corrct “latitudional” by “latitudinal” or “meridional”