the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Sep 2024
Integrating wide swath altimetry data into Level-4 multi-mission maps
Abstract. Real-time observation of ocean surface topography is essential for various oceanographic applications. Historically, these observations relied mainly on satellite nadir altimetry data, which were limited to observe scales greater than approximately 60 km. However, the recent launch of the wide-swath SWOT mission in December 2022 marks a significant advancement, enabling the two-dimensional global observation of finer oceanic scales (~15 km). While the direct analysis of the two-dimensional content of these swaths can provide valuable insights into ocean surface dynamics, integrating such data into mapping systems presents several challenges. This study focuses on integrating the SWOT mission into multi-mission mapping systems. Specifically, it examines the contribution of the SWOT mission to both the current nadir altimetry constellation (six/seven nadirs) and a reduced nadir altimetry constellation (three nadirs). Our study indicates that within the current nadir altimetry constellation, SWOT's impact is moderate, as existing nadir altimeters effectively constrain surface dynamics. However, in a hypothetical scenario where a reduced nadir altimetry constellation is envisioned to be operational by 2030, the significance of wide-swath data in mapping becomes more pronounced. Alternatively, we found that data-driven and dynamical mapping systems can significantly participate in refining the resolution of the multi-mission gridded products. Consequently, integrating high-resolution ocean surface topography observations with advanced mapping techniques can enhance the resolution of satellite-derived products, providing promising solutions for studying and monitoring sea-level variability at finer scales.
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Status: open (until 29 Oct 2024)
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RC1: 'Comment on egusphere-2024-2345', Jinbo Wang, 25 Sep 2024
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This work focuses on integrating high-spatial-resolution sea surface height (SSH) measurements from the SWOT mission into high-level gridded SSH maps derived from a constellation of nadir altimeters. The integration method is based on the Multiscale Inversion of Ocean Surface Topography (MIOST) technique (Ubelmann et al., 2021, 2022). In addition to data integration, the paper explores hypothetical scenarios for future wide-swath mission development and assesses the trade-offs between nadir altimeters and SWOT-type wide-swath altimeters. The study concludes that while SWOT's contribution is moderate with six nadir altimeters, it becomes substantial if the number of nadir altimeters is reduced to three, suggesting potential future configurations. The study also highlights the promise of several alternative data-driven methods for gridding.
The research is well-designed and thoroughly executed, with clear and well-structured writing. Here I have a major comment which only involves adding some discussion in the abstract and a minor suggestion in using SWOT data.
Major comment:
SWOT has demonstrated exceptional performance, with extremely low noise levels, capturing SSH signals beyond the geostrophic flow. However, this study focused on signals larger than 65 km, meaning the full potential of SWOT was not fully realized. This limitation is understandable, as neither the methodology nor the validation data is designed to capture signals below 65 km. As a result, the conclusion regarding SWOT's contribution is only partially realized, as the existing gridding framework and validation data impose constraints. It could be a missed opportunity if we focus solely on gridding geostrophic information, which represents only a small portion of SWOT's advancements. I recommend the authors include this discussion in the abstract and highlight that new innovations are needed to fully harness SWOT's potential beyond geostrophic surface flow, which include nonlinear eddy dynamics as well as linear and nonlinear internal waves. Those small-scale phenomena are non-retrievable in the nadir altimeter regardless of the number of conventional satellites. This is important factor to consider in discussing advantages and disadvantages of including SWOT in the future constellation. I do not believe that we have yet setup the data gridding or assimilation framework to address this question in an OSE environment.
Minor suggestion:
Since the validation relies on withheld nadir altimeter data and lacks reliable small-scale (<65 km) ground truth, I suggest applying a low-pass filter to SWOT data before integrating it into the MIOST for gridding. This would effectively remove high-amplitude internal tides, solitary waves, and small-scale residual mean sea surface variations from SWOT, thereby improving the accuracy of the MIOST gridding, especially in low latitudes where internal solitons are prevalent. In these regions, the inclusion of unfiltered SWOT data tends to degrade the mapping accuracy as shown in the manuscript.
Citation: https://doi.org/10.5194/egusphere-2024-2345-RC1
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