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37 pages, 11631 KiB

Open AccessArticle

Determination of Accurate Dynamic Topography for the Baltic Sea Using Satellite Altimetry and a Marine Geoid Model

by Majid Mostafavi, Nicole Delpeche-Ellmann, Artu Ellmann and Vahidreza Jahanmard

Remote Sens. 2023, 15(8), 2189; https://doi.org/10.3390/rs15082189 - 20 Apr 2023

Cited by 5 | Viewed by 2157

Abstract

Accurate determination of dynamic topography (DT) is expected to quantify a realistic sea surface with respect to its vertical datum and in identifying sub-mesoscale features of ocean dynamics. This study explores a method that derives DT by using satellite altimetry (SA) in conjunction [...] Read more.

Accurate determination of dynamic topography (DT) is expected to quantify a realistic sea surface with respect to its vertical datum and in identifying sub-mesoscale features of ocean dynamics. This study explores a method that derives DT by using satellite altimetry (SA) in conjunction with a high-resolution marine geoid model. To assess the method, DT was computed using along-track SA from Sentinel- 3A (S3A), Sentinel-3B (S3B), and Jason-3 (JA3), then compared with DT derived from a tide-gauge-corrected hydrodynamic model (HDM) for the period 2017–2019 over the Baltic Sea. Comparison of SA-derived DT and corrected HDM showed average discrepancies in the range of ±20 cm, with root mean square errors of 9 cm (for S3B) and 6 cm (for S3A and JA6) and a standard deviation between 2 and 16 cm. Inter-comparisons between data sources and multi-mission SA over the Baltic Sea also potentially identified certain persistent and semi-persistent problematic areas that are either associated with deficiencies in the geoid, tide gauge, HDM, and SA or a combination of all of these. In addition, it was observed that SA data have the potential to show a more realistic (detailed) variation of DT compared to HDM, which tended to generate only a smooth (low-pass) surface and underestimate DT. Full article

(This article belongs to the Special Issue Satellite Altimetry: Technology and Application in Geodesy)

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17 pages, 1775 KiB

Open AccessArticle

Estimation of Geopotential Value W₀ for the Geoid and Local Vertical Datum Parameters

by Xinyu Liu, Shanshan Li, Jiajia Yuan, Diao Fan and Xuli Tan

Remote Sens. 2023, 15(4), 912; https://doi.org/10.3390/rs15040912 - 7 Feb 2023

Cited by 1 | Viewed by 1834

Abstract

Unification of the global vertical datum has been a key problem to be solved for geodesy over a long period, and the main challenge for unifying the global vertical datum system is to determine the geopotential value W₀ of the geoid and [...] Read more.

Unification of the global vertical datum has been a key problem to be solved for geodesy over a long period, and the main challenge for unifying the global vertical datum system is to determine the geopotential value W₀ of the geoid and to calculate the vertical offset between the local vertical datum and the global vertical datum W₀. The geopotential value W₀ can be calculated using the grid mean sea surface (GMSS) data and the global geopotential model (GGM). In this study, this GMSS data was measured with adjustment methods and 24 years of merged multi-satellite altimetry data. The data of HaiYang-2A (HY-2A) and Jason-3 were first used to calculate W₀. The geopotential value W₀ was determined to be 62,636,856.82 m²s⁻² by combining the EIGEN-6C4 (European Improved Gravity Model of the Earth by New Techniques) and the GMSS data. Then, the geopotential difference approach and geodetic boundary value problem (GBVP) approach were used to determine the vertical datum parameters in this study. To compensate for the omission error of the GGM, this study utilized the remove–compute–restore (RCR) technique and the residual terrain model (RTM)-recovered high-frequency gravity signals. Finally, as a result of the GBVP solution, the geopotential value of the Australian Height Datum (AHD) was 62,636,851.935 m²s⁻², and the vertical offset of the AHD relative to the global vertical datum W₀ was 0.4885 m. As a result of the geopotential difference approach, the geopotential value of the Chinese Height datum was 62636861.412 m²s⁻², and the vertical offset of the Chinese Height datum was −0.4592 m. Full article

(This article belongs to the Special Issue Satellite Altimetry: Technology and Application in Geodesy)

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21 pages, 8103 KiB

Open AccessArticle

Monitoring Spatial-Temporal Variations of Lake Level in Western China Using ICESat-1 and CryoSat-2 Satellite Altimetry

by Jun Chen and Zheng Duan

Remote Sens. 2022, 14(22), 5709; https://doi.org/10.3390/rs14225709 - 11 Nov 2022

Cited by 9 | Viewed by 1959

Abstract

The lakes in the arid or semi-arid regions of western China are more sensitive to climate changes, and lake levels are considered as a direct indicator of regional climate variability. In this study, we combined satellite altimetry data from ICESat-1 with a smaller [...] Read more.

The lakes in the arid or semi-arid regions of western China are more sensitive to climate changes, and lake levels are considered as a direct indicator of regional climate variability. In this study, we combined satellite altimetry data from ICESat-1 with a smaller footprint and higher accuracy (compared to radar altimetry) and CryoSat-2 with a higher resolution in the along-track direction to monitor lake levels in western China and their trends over a long time period from 2003 to 2021. Our satellite altimetry derived lake levels were well-validated by comparing them against in situ measurements for a lake and independent altimetry-derived product from the DAHITI database for the common lakes. Furthermore, the commonly used linear model was applied to our derived lake level time-series to estimate the overall change trends in 67 typical lake levels over western China. Our results showed that 55 (82%) of these lakes displayed an increasing tendency in water levels, and the remaining 12 (18%) lakes showed a decreasing trend. Overall, the mean water level changing rate in western China was +0.15 m/yr (−1.40 to +0.58 m/yr) during the studied time period. The spatial patterns of the lake level variations can be grouped into three subregions: lake level changes between 2003 and 2021 showed general rising lake levels for the central–northern TP (Tibetan Plateau) endorheic region and Xinjiang, but declining levels for the southern TP exorheic region. The seasonal characteristic of lake level changes showed a significant increase during the summer monsoon season, followed by decreases during the non-monsoon season. The precipitation variations play a leading role in the lake level changes in the context of warm and humid climate states. There were good correspondences between the monthly variations in the lake level and monthly mean precipitation. Additionally, the lake levels also showed a relationship with the air temperature change, in particular, the lake level increase showed a small degree of hysteresis behavior compared with the rising temperatures. Geographically, the precipitation increase in the westerlies regions led to widespread lake expansion in the central–northern TP and Xinjiang. Conversely, precipitation decrease in the Indian monsoon regions caused lake shrinkage in the exorheic region of the southern TP. This study helps us achieve a better understanding of the spatial-temporal patterns of lake level changes in the arid or semi-arid region of western China. Full article

(This article belongs to the Special Issue Satellite Altimetry: Technology and Application in Geodesy)

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22 pages, 9080 KiB

Open AccessArticle

Regional Seafloor Topography by Extended Kalman Filtering of Marine Gravity Data without Ship-Track Information

by Lucía Seoane, Guillaume Ramillien, Benjamin Beirens, José Darrozes, Didier Rouxel, Thierry Schmitt, Corinne Salaün and Frédéric Frappart

Remote Sens. 2022, 14(1), 169; https://doi.org/10.3390/rs14010169 - 31 Dec 2021

Cited by 9 | Viewed by 2820

Abstract

An iterative Extended Kalman Filter (EKF) approach is proposed to recover a regional set of topographic heights composing an undersea volcanic mount by the successive combination of large numbers of gravity measurements at sea surface using altimetry satellite-derived grids and taking the error [...] Read more.

An iterative Extended Kalman Filter (EKF) approach is proposed to recover a regional set of topographic heights composing an undersea volcanic mount by the successive combination of large numbers of gravity measurements at sea surface using altimetry satellite-derived grids and taking the error uncertainties into account. The integration of the non-linear Newtonian operators versus the radial and angular distances (and its first derivatives) enables the estimation process to accelerate and requires only few iterations, instead of summing Legendre polynomial series or using noise-degraded 2D-FFT decomposition. To show the effectiveness of the EKF approach, we apply it to the real case of the bathymetry around the Great Meteor seamount in the Atlantic Ocean by combining only geoid height/free-air anomaly datasets and using ship-track soundings as reference for validation. Topography of the Great Meteor seamounts structures are well-reconstructed, especially when regional compensation is considered. Best solution gives a RMS equal to 400 m with respect to the single beam depth observations and it is comparable to RMS obtained for ETOPO1 of about 365 m. Larger discrepancies are located in the seamount flanks due to missing high-resolution information for gradients. This approach can improve the knowledge of seafloor topography in regions where few echo-sounder measurements are available. Full article

(This article belongs to the Special Issue Satellite Altimetry: Technology and Application in Geodesy)

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10 pages, 4088 KiB

Open AccessCommunication

Limiting Accuracy of Height Measurement for a Precision Radar Altimeter in a Low Altitude Flying Vehicle above the Sea Surface

by Aleksandr I. Baskakov, Alexey A. Komarov, Anna V. Ruban and Min-Ho Ka

Remote Sens. 2021, 13(14), 2660; https://doi.org/10.3390/rs13142660 - 6 Jul 2021

Cited by 4 | Viewed by 2019

Abstract

This study presents mathematical analysis and numerical modeling for the estimation of measurement errors of height estimation over the sea surface for a precision radar altimeter installed in a low altitude flying vehicle. Reflective properties of the electromagnetic signals from the sea surface [...] Read more.

This study presents mathematical analysis and numerical modeling for the estimation of measurement errors of height estimation over the sea surface for a precision radar altimeter installed in a low altitude flying vehicle. Reflective properties of the electromagnetic signals from the sea surface are determined by the local backscattering patterns of the sea surface illuminated. The height estimation of the flying vehicle from the received echo signals at the output of its tracking system is the sum of three factors: the first factor is the height to the average sea level the second is the bias of the estimation of the height, which is time-varying and depends on the slope of large-scale roughness; the third is the terms related to the surface topography. For the calculation of the estimation errors of the height measurement of a low altitude precision radar altimeter, a reasonable approximation of the large roughness of the sea surface by a deterministic function is necessary. In this study, we performed the derivation of the estimation function and the analysis of the limiting accuracy of the height measurement using the calculation of the estimation errors in spectral domain method describing the large-scale sea surface roughness. The results obtained for the limiting accuracy of a flying vehicle at low altitude above the sea surface, allows to obtain reasonable system parameters minimizing height errors of the flight altitude. Full article

(This article belongs to the Special Issue Satellite Altimetry: Technology and Application in Geodesy)

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19 pages, 4559 KiB

Open AccessArticle

Increased Ice Thinning over Svalbard Measured by ICESat/ICESat-2 Laser Altimetry

by Lukas Sochor, Thorsten Seehaus and Matthias H. Braun

Remote Sens. 2021, 13(11), 2089; https://doi.org/10.3390/rs13112089 - 26 May 2021

Cited by 10 | Viewed by 4401

Abstract

A decade-long pronounced increase in temperatures in the Arctic, especially in the Barents Sea region, resulted in a global warming hotspot over Svalbard. Associated changes in the cryosphere are the consequence and lead to a demand for monitoring of the glacier changes. This [...] Read more.

A decade-long pronounced increase in temperatures in the Arctic, especially in the Barents Sea region, resulted in a global warming hotspot over Svalbard. Associated changes in the cryosphere are the consequence and lead to a demand for monitoring of the glacier changes. This study uses spaceborne laser altimetry data from the ICESat and ICESat-2 missions to obtain ice elevation and mass change rates between 2003–2008 and 2019. Elevation changes are derived at orbit crossover locations throughout the study area, and regional volume and mass changes are estimated using a hypsometric approach. A Svalbard-wide annual elevation change rate of −0.30 ± 0.15 m yr⁻¹ was found, which corresponds to a mass loss rate of −12.40 ± 4.28 Gt yr⁻¹. Compared to the ICESat period (2003–2009), thinning has increased over most regions, including the highest negative rates along the west coast and areas bordering the Barents Sea. The overall negative regime is expected to be linked to Arctic warming in the last decades and associated changes in glacier climatic mass balance. Further, observed increased thinning rates and pronounced changes at the eastern side of Svalbard since the ICESat period are found to correlate with atmospheric and oceanic warming in the respective regions. Full article

(This article belongs to the Special Issue Satellite Altimetry: Technology and Application in Geodesy)

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17 pages, 4586 KiB

Open AccessArticle

Sea Level Fusion of Satellite Altimetry and Tide Gauge Data by Deep Learning in the Mediterranean Sea

by Lianjun Yang, Taoyong Jin, Xianwen Gao, Hanjiang Wen, Tilo Schöne, Mingyu Xiao and Hailan Huang

Remote Sens. 2021, 13(5), 908; https://doi.org/10.3390/rs13050908 - 28 Feb 2021

Cited by 8 | Viewed by 3597

Abstract

Satellite altimetry and tide gauges are the two main techniques used to measure sea level. Due to the limitations of satellite altimetry, a high-quality unified sea level model from coast to open ocean has traditionally been difficult to achieve. This study proposes a [...] Read more.

Satellite altimetry and tide gauges are the two main techniques used to measure sea level. Due to the limitations of satellite altimetry, a high-quality unified sea level model from coast to open ocean has traditionally been difficult to achieve. This study proposes a fusion approach of altimetry and tide gauge data based on a deep belief network (DBN) method. Taking the Mediterranean Sea as the case study area, a progressive three-step experiment was designed to compare the fused sea level anomalies from the DBN method with those from the inverse distance weighted (IDW) method, the kriging (KRG) method and the curvature continuous splines in tension (CCS) method for different cases. The results show that the fusion precision varies with the methods and the input measurements. The precision of the DBN method is better than that of the other three methods in most schemes and is reduced by approximately 20% when the limited altimetry along-track data and in-situ tide gauge data are used. In addition, the distribution of satellite altimetry data and tide gauge data has a large effect on the other three methods but less impact on the DBN model. Furthermore, the sea level anomalies in the Mediterranean Sea with a spatial resolution of 0.25° × 0.25° generated by the DBN model contain more spatial distribution information than others, which means the DBN can be applied as a more feasible and robust way to fuse these two kinds of sea levels. Full article

(This article belongs to the Special Issue Satellite Altimetry: Technology and Application in Geodesy)

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18 pages, 3979 KiB

Open AccessTechnical Note

Annual Sea Level Amplitude Analysis over the North Pacific Ocean Coast by Ensemble Empirical Mode Decomposition Method

by Wen-Hau Lan, Chung-Yen Kuo, Li-Ching Lin and Huan-Chin Kao

Remote Sens. 2021, 13(4), 730; https://doi.org/10.3390/rs13040730 - 17 Feb 2021

Cited by 7 | Viewed by 2765

Abstract

Understanding spatial and temporal changes of seasonal sea level cycles is important because of direct influence on coastal systems. The annual sea level cycle is substantially larger than semi-annual cycle in most parts of the ocean. Ensemble empirical mode decomposition (EEMD) method has [...] Read more.

Understanding spatial and temporal changes of seasonal sea level cycles is important because of direct influence on coastal systems. The annual sea level cycle is substantially larger than semi-annual cycle in most parts of the ocean. Ensemble empirical mode decomposition (EEMD) method has been widely used to study tidal component, long-term sea level rise, and decadal sea level variation. In this work, EEMD is used to analyze the observed monthly sea level anomalies and detect annual cycle characteristics. Considering that the variations of the annual sea level variation in the Northeast Pacific Ocean are poorly studied, the trend and characteristics of annual sea level amplitudes and related mechanisms in the North Pacific Ocean are investigated using long-term tide gauge records covering 1950–2016. The average annual amplitude of coastal sea level exhibits interannual-to-decadal variability within the range of 14–220 mm. The largest value of ~174 mm is observed in the west coast of South China Sea. In the other coastal regions of North Pacific Ocean, the mean annual amplitude is relatively low between 77 and 124 mm for the western coast and 84 and 87 mm for the eastern coast. The estimated trend values for annual sea level amplitudes in the western coastal areas of South China Sea and Northeast Pacific Ocean have statistically decreased over 1952–2014 with a range of −0.77 mm·yr⁻¹ to −0.11 mm·yr⁻¹. Our results suggested that the decreasing annual amplitude in the west coast of South China Sea is in good agreement with the annual mean wind stress associated with the Pacific Decadal Oscillation (PDO). This wind phenomenon also explains the temporal variations of annual sea level amplitude in Northeast Pacific Ocean, especially the high correlations since 1980 (R = 0.61−0.72). Full article

(This article belongs to the Special Issue Satellite Altimetry: Technology and Application in Geodesy)

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