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Applications of Satellite Altimetry in Ocean Observation

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A special issue of Remote Sensing (ISSN 2072-4292). This special issue belongs to the section "Ocean Remote Sensing".

Deadline for manuscript submissions: 15 February 2025 | Viewed by 13540

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Special Issue Editors

Dr. Antonio Sánchez-Román

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Guest Editor

Mediterranean Institute for Advanced Studies (IMEDEA), C/ Miquel Marques 21, 07190 Esporles, Spain
Interests: ocean remote sensing; in situ observations; physical oceanography; mesoscale ocean dynamics

Dr. Bàrbara Barceló-Llull

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Guest Editor

Mediterranean Institute for Advanced Studies (IMEDEA), C/ Miquel Marques 21, 07190 Esporles, Spain
Interests: mesoscale and submesoscale; ocean dynamics; in situ and remote sensing observations; biophysical interactions; machine-learning techniques; Lagrangian analysis
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Dr. Juan M. Sayol

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Guest Editor

Department of Applied Mathematics, Polytechnic School, University of Alicante, 03690 Sant Vicent del Raspeig, Spain
Interests: ocean dynamics;air–sea interaction; extreme events; ocean state indicators; machine learning; Lagrangian dynamics
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Over the last 30 years, satellite altimetry has provided global high-accuracy sea surface height measurements, enhancing our understanding of the upper ocean circulation from the (sub-) mesoscale to larger scales. The development of novel technologies, such as SAR Doppler altimetry, interferometric altimetry and swath instruments, together with new methods of reprocessing historical data, offers unique opportunities to study ocean dynamics from satellite altimetry data.

Satellite altimetry was initially designed to monitor the open ocean. However, new missions and efforts have aimed at extending the capabilities of current altimeters towards high latitudes and coastal zones. A common objective of recent altimetric missions, such as the Sentinel-3 series or the wide-swath SWOT mission, has been to resolve finer scales than previous altimeters. Altimetric observations are used to study the ocean circulation at the mesoscale and submesoscale (spatial scales > 15 km). These small scales drive the vertical transport of properties, and the two-dimensional data provided by altimeters are key to improving our understanding of the dynamics at play. In addition, synergies between satellite altimetry and in situ observations (e.g. the Argo system) allow for the assessment of the three-dimensional dynamics associated with these structures.

In this Special Issue, we invite high-quality scientific papers that use satellite altimetry observations to study the dynamics of the ocean. We welcome studies dealing with: (i) the assessment of global and regional sea level, (ii) surface currents and sea state at different spatiotemporal scales, (iii) multi-platform observations, (iv) the interaction between the open ocean and the coastal seas and (v) the evaluation of uncertainties related to altimetry data.

Dr. Antonio Sánchez-Román
Dr. Bàrbara Barceló-Llull
Dr. Juan M. Sayol
Guest Editors

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Keywords

radar altimetry
ocean circulation
sea level variability
mesoscale and sub-mesoscale features
large-scale processes
sea surface currents
coastal ocean monitoring
in situ observations
ocean waves
new altimeters

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Published Papers (9 papers)

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Research

17 pages, 8493 KiB

Open AccessArticle

Fine-Scale Eddies Detected by SWOT in the Kuroshio Extension

by Tianshi Du and Zhao Jing

Remote Sens. 2024, 16(18), 3488; https://doi.org/10.3390/rs16183488 - 20 Sep 2024

Viewed by 780

Abstract

Conventional altimetry has greatly advanced our understanding of mesoscale eddies but falls short in studying fine-scale eddies (<150 km). The newly launched Surface Water and Ocean Topography (SWOT) altimeter, however, with its unprecedented high-resolution capabilities, offers new opportunities to observe these fine-scale eddies. In this study, we use SWOT data to explore these previously elusive fine-scale eddies in the Kuroshio Extension. During SWOT’s fast sampling phase from 29 May 2023 to 10 July 2023, we identified an average of 4.5 fine-scale eddies within each 120 km wide swath. Cyclonic eddies, which are slightly more frequent than the anticyclonic ones (ratio of 1.16), have a similar mean radius of 23.4 km. However, cyclonic eddies exhibit higher amplitudes, averaging 3.5 cm compared to 2.8 cm for anticyclonic eddies. In contrast to the mesoscale eddies detected by conventional altimeters, the fine-scale eddies revealed by SWOT are characterized by smaller sizes and weaker amplitudes. This study offers a preliminary view of fine-scale eddy characteristics from space, highlighting SWOT’s potential to advance our understanding of these dynamic processes. Nonetheless, it also emphasizes the necessity for comprehensive analysis to fully exploit the satellite’s capabilities in monitoring and interpreting complex eddy behaviors. Full article

(This article belongs to the Special Issue Applications of Satellite Altimetry in Ocean Observation)

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20 pages, 8907 KiB

Open AccessArticle

Reconstruction of Baltic Gridded Sea Levels from Tide Gauge and Altimetry Observations Using Spatiotemporal Statistics from Reanalysis

by Jüri Elken, Amirhossein Barzandeh, Ilja Maljutenko and Sander Rikka

Remote Sens. 2024, 16(15), 2702; https://doi.org/10.3390/rs16152702 - 24 Jul 2024

Viewed by 631

Abstract

A method for reconstructing weekly Baltic gridded sea levels was developed and tested. This method uses input data from tide gauge and altimetry observations. The reconstruction is based on sea level empirical orthogonal function (EOF) modes, calculated as spatiotemporal statistics from daily model reanalysis results available from the Copernicus Marine Service for the 1993–2021 period. In the semi-enclosed, tideless Baltic Sea, the four leading EOF modes cover 99% of the sea level variance. Several experiments with different combinations of input data were carried out. This method was validated against coastal tide gauges and altimetry data. The best reconstruction was obtained when both the tide gauges and altimetry data were used as inputs. An assessment of the centered root-mean-square difference (cRMSD) of the reconstruction versus the tide gauges revealed a value of 0.05 m, and a result of 0.10 m was revealed versus altimetry. The average coefficient of determination (R²) was determined to be 0.93 for the tide gauges and 0.82 for the altimetry data. In the cases where only one type of input data was used, the reconstruction worsened with respect to other data sources. The reconstruction method demonstrated its usefulness for the reconstruction of coastal sea levels in unsampled locations and the calculation of changes in sea volume. Full article

(This article belongs to the Special Issue Applications of Satellite Altimetry in Ocean Observation)

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16 pages, 2230 KiB

Open AccessArticle

Block-Circulant Approximation of the Precision Matrix for Assimilating SWOT Altimetry Data

by Max Yaremchuk, Christopher Beattie, Gleb Panteleev and Joseph D’Addezio

Remote Sens. 2024, 16(11), 1954; https://doi.org/10.3390/rs16111954 - 29 May 2024

Viewed by 640

Abstract

The recently deployed Surface Water and Ocean Topography (SWOT) mission for the first time has observed the ocean surface at a spatial resolution of 1 km, thus giving an opportunity to directly monitor submesoscale sea surface height (SSH) variations that have a typical magnitude of a few centimeters. This progress comes at the expense of the necessity to take into account numerous uncertainties in calibration of the quality-controlled altimeter data. Of particular importance is the proper filtering of spatially correlated errors caused by the uncertainties in geometry and orientation of the on-board interferometer. These “systematic” errors dominate the SWOT error budget and are likely to have a notable signature in the SSH products available to the oceanographic community. In this study, we explore the utility of the block-circulant (BC) approximation of the SWOT precision matrix developed by the Jet Propulsion Laboratory for assessment of a mission’s accuracy, including the possible impact of the systematic errors on the assimilation of the wide-swath altimeter data into numerical models. It is found that BC approximation of the precision matrix has sufficient (90–99%) accuracy for a wide range of significant wave heights of the ocean surface, and, therefore, could potentially serve as an efficient preconditioner for data assimilation problems involving altimetry observations by space-borne interferometers. An extensive set of variational data assimilation (DA) experiments demonstrates that BC approximation provides more accurate SSH retrievals compared to approximations, assuming a spatially uncorrelated observation error field as is currently adopted in operational DA systems. Full article

(This article belongs to the Special Issue Applications of Satellite Altimetry in Ocean Observation)

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13 pages, 2331 KiB

Open AccessCommunication

Sea Surface Height Wavenumber Spectrum from Airborne Interferometric Radar Altimeter

by Jinchao He, Yongsheng Xu, Hanwei Sun, Qiufu Jiang, Lei Yang, Weiya Kong and Yalong Liu

Remote Sens. 2024, 16(8), 1359; https://doi.org/10.3390/rs16081359 - 12 Apr 2024

Cited by 1 | Viewed by 1415

Abstract

The proposed “Guanlan” ocean science satellite, led by China’s Laoshan Laboratory, includes an interferometric radar altimeter (IRA) as a key payload. As an integral part of its development, an airborne IRA experiment was conducted on 6 November 2021, with a flight path of approximately 90 km in the South China Sea. This study investigates the IRA’s ability to observe ocean sea surface height (SSH) across scales ranging from meters to mesoscale. The sea surface height anomaly (SSHA) of the IRA is aligned with the SSHA of the AVISO at scales greater than 30 km, but also demonstrates the ability to capture small-scale SSHA changes in two dimensions. We analyzed wavenumber spectra of SSHA obtained from the airborne IRA, ICESat-2, and SARAL/AltiKa satellite for this region. The results show a good agreement in power spectral density (PSD) levels between ICESat-2, SARAL/AltiKa and IRA at scales larger than 30 km. Within the submesoscale range of 1–10 km, the IRA SSHA spectrum exhibits a distinctly negative slope and the lowest energy level. The minimum PSD level of the IRA fell in the range of 10⁻⁴–10⁻³ m²/cycle/km, at scales around 1 km, which is more than an order of magnitude lower than that of ICESat-2, forming a spectral gap that is in agreement with the theoretical expectation. Furthermore, IRA-derived wave direction and significant wave height matched well with the MFWAM wave data. The results of this study underscore the considerable potential of airborne IRA in capturing SSHA across a range of scales, from oceanic waves to submesoscale. Full article

(This article belongs to the Special Issue Applications of Satellite Altimetry in Ocean Observation)

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20 pages, 5247 KiB

Open AccessArticle

Evolution of Different Types of Eddies Originating from Different Baroclinic Instability Types

by Jiaxin Liu, Jue Ning and Xu Chen

Remote Sens. 2023, 15(24), 5730; https://doi.org/10.3390/rs15245730 - 14 Dec 2023

Viewed by 1034

Abstract

This paper investigates the evolution of global eddies and various types of eddies originating from baroclinic instability (BCI) by utilizing satellite altimetry, Argo profiles, and climatology datasets. The structure of global eddies with regard to potential temperature anomalies experiences downward propagation and spreading from the periods of eddy growth to stabilization. However, from the eddy’s stabilization to the decay period, the process of spreading primarily occurs horizontally, and this process is usually accompanied by weakening. By comparing the evolution of eddies in three typical regions dominated by distinct types of BCI, we found that the basic properties of eddies related to different BCI types evolve similarly; however, there are notable differences in their vertical structures and evolution. Eddies associated with Phillips + Charney_s-type, Charney_s-type, and Eady-type BCIs exhibit dual-core, single-core, and dual-core structures, respectively. In particular, the intrusion of the Okhotsk cold water mass into the Northwest Pacific region forms cold-core anticyclonic eddies, resulting in AEs that are significantly distinct from the rest of the ocean. The evolution of surface-layer cores closely resembles that of the global eddies, while the decay of subsurface and bottom-layer cores is comparably sluggish. Additionally, we examine the impact of local oceanic stratification conditions on eddy decay and determine that stronger vertical gradients result in more vigorous eddy decay, accounting for the concentration of eddies at depths where vertical gradients are weaker during their evolution. Full article

(This article belongs to the Special Issue Applications of Satellite Altimetry in Ocean Observation)

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14 pages, 3321 KiB

Open AccessArticle

The Effect of Spatially Correlated Errors on Sea Surface Height Retrieval from SWOT Altimetry

by Max Yaremchuk, Christopher Beattie, Gleb Panteleev, Joseph M. D’Addezio and Scott Smith

Remote Sens. 2023, 15(17), 4277; https://doi.org/10.3390/rs15174277 - 31 Aug 2023

Cited by 2 | Viewed by 1363

Abstract

The upcoming technology of wide-swath altimetry from space will enable monitoring the ocean surface at 4–5 times better spatial resolution and 2–3 times better accuracy than traditional nadir altimeters. This development will provide a chance to directly observe submesoscale sea surface height (SSH) variations that have a typical magnitude of a few centimeters. Taking full advantage of this opportunity requires correct treatment of the correlated SSH errors caused by uncertainties in environmental conditions beneath the satellite and in the geometry and orientation of the on-board interferometer. These observation errors are highly correlated both along and across the surface swath scanned by the satellite, and this presents a significant challenge for accurate processing. In particular, the SWOT precision matrix has off-diagonal elements that are too numerous to allow standard approaches to remain tractable. In this study, we explore the utility of a block-diagonal approximation to the SWOT precision matrix in order to reconstruct SSH variability in the region east of Greenland. An extensive set of 2dVar assimilation experiments demonstrates that the sparse approximation proposed for the precision matrix provides accurate SSH retrievals when the background-to-observation error ratio ν does not exceed 3 and significant wave height is below 2.5 m. We also quantify the range of ν and significant wave heights over which the retrieval accuracy of the exact spatially correlated SWOT error model will outperform the uncorrelated model. In particular, the estimated range is found to be substantially wider (

ν < 10

with significant wave heights below 8–10 m), indicating the potential benefits of further improving the accuracy of approximations for the SWOT precision matrix. Full article

(This article belongs to the Special Issue Applications of Satellite Altimetry in Ocean Observation)

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15 pages, 2922 KiB

Open AccessArticle

Implementation of the Optical Flow to Estimate the Propagation of Eddies in the South Atlantic Ocean

by Denis L. Volkov and Shahriar Negahdaripour

Remote Sens. 2023, 15(15), 3894; https://doi.org/10.3390/rs15153894 - 7 Aug 2023

Cited by 1 | Viewed by 1635

Abstract

The ocean is filled with mesoscale eddies that account for most of the oceanic kinetic energy. The importance of eddies in transporting properties and energy across the ocean basins has led to numerous efforts to track their motion. Here, we implement a computer vision technique—the optical flow—to map the pathways of mesoscale eddies in the South Atlantic Ocean. The optical flow is applied to the pairs of consecutive sea surface height maps produced from a nearly 30-year-long satellite altimetry record. In contrast to other methods to estimate the eddy propagation velocity, the optical flow can reveal the temporal evolution of eddy motion, which is particularly useful in the regions of strong currents. We present the time-dependent estimates of the speed and direction of eddy propagation in the Eulerian frame of reference. In an excellent agreement with earlier studies, the obtained pattern of eddy propagation reveals the interaction of eddies with the background flow and the bottom topography. We show that in the Antarctic Circumpolar Current, the variability of the eddy propagation velocity is correlated with the variability of the surface geostrophic velocity, demonstrating the robustness of the optical flow to detect the time-variable part of eddy motion. Full article

(This article belongs to the Special Issue Applications of Satellite Altimetry in Ocean Observation)

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24 pages, 16057 KiB

Open AccessEditor’s ChoiceArticle

KaRIn Noise Reduction Using a Convolutional Neural Network for the SWOT Ocean Products

by Anaëlle Tréboutte, Elisa Carli, Maxime Ballarotta, Benjamin Carpentier, Yannice Faugère and Gérald Dibarboure

Remote Sens. 2023, 15(8), 2183; https://doi.org/10.3390/rs15082183 - 20 Apr 2023

Cited by 9 | Viewed by 2622

Abstract

The SWOT (Surface Water Ocean Topography) mission will provide high-resolution and two-dimensional measurements of sea surface height (SSH). However, despite its unprecedented precision, SWOT’s Ka-band Radar Interferometer (KaRIn) still exhibits a substantial amount of random noise. In turn, the random noise limits the ability of SWOT to capture the smallest scales of the ocean’s topography and its derivatives. In that context, this paper explores the feasibility, strengths and limits of a noise-reduction algorithm based on a convolutional neural network. The model is based on a U-Net architecture and is trained and tested with simulated data from the North Atlantic. Our results are compared to classical smoothing methods: a median filter, a Lanczos kernel smoother and the SWOT de-noising algorithm developed by Gomez-Navarro et al. Our U-Net model yields better results for all the evaluation metrics: 2 mm root mean square error, sub-millimetric bias, variance reduction by factor of 44 (16 dB) and an accurate power spectral density down to 10–20 km wavelengths. We also tested various scenarios to infer the robustness and the stability of the U-Net. The U-Net always exhibits good performance and can be further improved with retraining if necessary. This robustness in simulation is very encouraging: our findings show that the U-Net architecture is likely one of the best candidates to reduce the noise of flight data from KaRIn. Full article

(This article belongs to the Special Issue Applications of Satellite Altimetry in Ocean Observation)

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20 pages, 10677 KiB

Open AccessArticle

A Robust Algorithm for Photon Denoising and Bathymetric Estimation Based on ICESat-2 Data

by Junsheng Zhong, Xiuguo Liu, Xiang Shen and Liming Jiang

Remote Sens. 2023, 15(8), 2051; https://doi.org/10.3390/rs15082051 - 13 Apr 2023

Cited by 7 | Viewed by 2321

Abstract

The Ice, Cloud, and Land Elevation Satellite 2 (ICESat-2) is equipped with an Advanced Terrain Laser Altimeter System (ATLAS) with the capability of penetrating water bodies, making it a widely utilized tool for the bathymetry of various aquatic environments. However, the laser sensor often encounters a significant number of noise photons due to various factors such as sunlight, water quality, and after-pulse effect. These noise photons significantly compromise the accuracy of bathymetry measurements. In an effort to address this issue, this study proposes a two-step method for photon denoising by utilizing a method combining the DBSCAN algorithm and a two-dimensional window filter, achieving an F1 score of 0.94. A robust M-estimation method was employed to estimate the water depth of the denoised and refraction-corrected bathymetric photons, achieving an RMSE of 0.30 m. The method proposed in this paper preserves as much information as possible about signal photons, increases the number of bathymetric points, enhances the resistance to gross error, and guarantees the accuracy of bathymetry measurements while outlining the underwater topography. While the method is not fully automated and requires setting parameters, the fixed parameter values allow for efficient batch denoising of underwater photon points in different environments. Full article

(This article belongs to the Special Issue Applications of Satellite Altimetry in Ocean Observation)

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