Coastal Altimetry Research Articles

A RIP-based SAR retracker and its application in North East Atlantic with Sentinel-3

Just as CryoSat-2, Sentinel-3 embarks on board a radar altimeter (SRAL) with the novel Synthetic Aperture Radar (SAR) mode that enables higher resolution and more accurate altimeter-derived parameters in the coastal zone, thanks to the reduced along-track footprint. Exploiting the SAR data in the recent years, many researchers have already proven that the performance of SAR altimetry with specific coastal retrackers is superior to collocated Pseudo-Low Resolution Mode (PLRM) coastal altimetry algorithms but they also pointed out that residual errors due to land contamination are still present in the very proximity of the land (0–3 km).

Spectral Analysis of Satellite Altimeter and Tide Gauge Data around the Northern Australian Coast

The north of Australia is known for its complex tidal system, where the highest astronomical tides (HATs) reach 12 m. This paper investigates the tidal behaviour in this region by developing spectral climatology for tide gauge and altimetry data. Power spectral density analysis is applied to detect the magnitude of ocean tides in 20 years of sea-level data from multimission satellite altimeters and tide gauges. The spectra of altimetry sea level anomaly (SLA) time series have their strongest peaks centred at approximately 2.11, 5.88, and 7.99 cycles per year (cpy), corresponding to the diurnal and semidiurnal tidal constituents K1, M2, and O1, respectively. Closer to the coastline, the spectra peak at high-frequency overtide and shallow-water constituents such as M4, MK4, and MK3. There have been many large, high-frequency spectral peaks near the coastline, indicating the difficulty of predicting tidal signals by coastal altimetry. Similar to altimetry observations, there are dominant semidiurnal and diurnal tidal peaks in tide gauge SLA time series accompanying a number of overtides. The semidiurnal and diurnal peaks are mostly higher on the northwest coast of Australia compared with the north and northeast coast. The results from both altimetry and tide gauges indicate that tidal range increases with increasing continental shelf.

Modelling the Altitude Dependence of the Wet Path Delay for Coastal Altimetry Using 3-D Fields from ERA5

Wet path delay (WPD) for satellite altimetry has been provided from external sources, raising the need of converting this value between different altitudes. The only expression available for this purpose considers the same altitude reduction, irrespective of geographic location and time. The focus of this study is the modelling of the WPD altitude dependence, aiming at developing improved expressions. Using ERA5 pressure level fields (2010–2013), WPD vertical profiles were computed globally. At each location and for each vertical profile, an exponential function was fitted using least squares, determining the corresponding decay coefficient. The time evolution of these coefficients reveals regions where they are highly variable, making this modelling more difficult, and regions where an annual signal exists. The output of this modelling consists of a set of so-called University of Porto (UP) coefficients, dependent on geographic location and time. An assessment with ERA5 data (2014) shows that for the location where the Kouba coefficient results in a maximum Root Mean Square (RMS) error of 3.2 cm, using UP coefficients this value is 1.2 cm. Independent comparisons with WPD derived from Global Navigation Satellite Systems and radiosondes show that the use of UP coefficients instead of Kouba’s leads to a decrease in the RMS error larger than 1 cm.

Regional validation of the Coastal Altimetry Waveform Retracking Expert System (CAWRES) over the largest archipelago in Southeast Asian seas

ABSTRACT This paper presents the latest finding on the validation of the novel Coastal Altimetry Waveform Retracking Expert System (CAWRES) that was previously developed for optimizing the sea level anomaly (SLA) over coastal oceans. The work is meant for the continuation of our previous works that have been successfully tested at two coastal sites in Australia and the United States. To further examine the potential of CAWRES of being applied to coastal region elsewhere, we extend the works to the largest archipelago at the Southeast Asia regions where complicated coastal topographies are experienced. The Jason-2 altimetric waveforms have been reprocessed using CAWRES to retrieve the retracked SLAs. Results in this study indicate that over coastal oceans, CAWRES records much higher (>80%) percentage of data than the standard Maximum Likelihood Estimator-4 (MLE4), <20%) retracker. The SLAs minimum distance of CAWRES is found to be similar to those of the Ice-1 ranging from 0.6 to 9 km. Comparison with tide gauge indicates that in six out of nine cases, the correlation of CAWRES is superior to those of the other retrackers with correlation ranging from 0.48 to 0.90. However, the Root Mean Square Error (RMSE) indicates no significant difference among all retrackers. Based on the results, it can be concluded that the performance of CAWRES is nearly similar to those of the Ice-1 retracker, except that CAWRES has a better correlation with tide gauge data. This remarks the needs of embedding the Ice-1 retracker in the CAWRES system.

Satellite Altimetry Measurements of Sea Level in the Coastal Zone

Satellite radar altimetry provides a unique sea level data set that extends over more than 25 years back in time and that has an almost global coverage. However, when approaching the coasts, the extraction of correct sea level estimates is challenging due to corrupted waveforms and to errors in most of the corrections and in some auxiliary information used in the data processing. The development of methods dedicated to the improvement of altimeter data in the coastal zone dates back to the 1990s, but the major progress happened during the last decade thanks to progress in radar technology [e.g., synthetic aperture radar (SAR) mode and Ka-band frequency], improved waveform retracking algorithms, the availability of new/improved corrections (e.g., wet troposphere and tidal models) and processing workflows oriented to the coastal zone. Today, a set of techniques exists for the processing of coastal altimetry data, generally called “coastal altimetry.” They have been used to generate coastal altimetry products. Altimetry is now recognized as part of the integrated observing system devoted to coastal sea level monitoring. In this article, we review the recent technical advances in processing and the new technological capabilities of satellite radar altimetry in the coastal zone. We also illustrate the fast-growing use of coastal altimetry data sets in coastal sea level research and applications, as high-frequency (tides and storm surge) and long-term sea level change studies.

Design and regional assessment of an empirical tidal model based on FES2014 and coastal altimetry

The updated Empirical Ocean Tide model (EOT19p) currently available on limited regions is presented in this paper. Its implementation is focused on improving the accuracy of tidal estimation at the coast. EOT19p is derived using circa 27 years of coast-dedicated altimetric data and the FES2014 tide model, and it is based on a multi-mission, weighted least-squares approach. In this work the performance of EOT19p is assessed with respect to state-of-the-art tide models against in situ observations. The study is performed on two critical regions: the North Sea and the Malay Archipelago. Comparisons for eight major tidal constituents show that EOT19p improves coastal results with respect to its former version, EOT11a, of 35.39% in the North Sea and 20.07% in the Malay Archipelago, with large error reduction in the English Channel and the Irish Sea. The median absolute differences (MAD) against in situ data show that EOT19p is in line with the latest models, with lowest error occurring for single tidal constituents, such as M2 (with a MAD of 1.63 cm in the Malay Archipelago). Finally, the tidal height differences of three models (EOT19p, FES2014, and DTU16) compared with those obtained of their former versions (EOT11a, FES2012, and DTU10), show an increased agreement at coastal areas with the latest versions, and the regional average difference for M2 constituent drops from 3.75 cm to 0.70 cm in the North Sea, and from 3.02 to 1.44 cm in the Malay Archipelago.

Wave energy resource assessment with improved satellite altimetry data over the Malaysian coastal sea

This paper presents the assessment of wave energy resources over Malaysian seas using the improved satellite altimetry data. Instead of the standard altimetry data, the coastal altimetry products from Jason-2/PISTACH and AltiKa/PEACHI are considered to offer better estimation of significant wave height (SWH) over coastal oceans. In selecting the appropriate SWH, Jason-2/PISTACH from MLE4, Oce3 and Red3 retracking algorithms are examined, with respect to the limited data from Acoustic Doppler Current Profiler. The results in this study indicate that the Oce3 algorithm is the appropriate retracker. Unfortunately, it is only available for a Jason-2/PISTACH product, not for AltiKa/PEACHI. Therefore, this study uses the improved SWH from Red3 retracker, recorded as the second appropriate retracker, in the study of wave energy resource. The Malaysian sea is dominated by low SWH (0.5–1 m) and wave period (4–5.5 s). High wave energy during the strong monsoon season can be harvested, with wave energy during a northeast monsoon ranging from 8 to 20 kW/m, and between 4 and 5 kW/m during a southwest monsoon. Ten out of 14 zones are recorded as high energy zones, producing the energy storage more than 40 MW h/m. The Malaysian sea has a potential of continuous wave energy throughout the year from the variability of SWH exceeding the minimum requirement of SWH for several wave energy converters (e.g. C5 Wave Star) to be operated.

Wideband Ocean Altimetry Using Ku-Band and K-Band Satellite Signals of Opportunity: Proof of Concept

A proof-of-concept experiment has demonstrated that wideband (400 MHz) signals of opportunity (SoOp) transmitted in K- and Ku-bands from geostationary satellites can be used for coastal altimetry. An essential finding from this experiment is that the full broadcast spectrum consisting of multiple digital channels can be processed as a single wideband signal source. An established error model for Global Navigation Satellite System interferometric altimetry was shown to accurately represent the sea surface height (SSH) retrievals when evaluated using the full bandwidth. This experiment was conducted over a 72-h period at Platform Harvest off the Pacific Coast. Colocated tide gauge and LiDAR measurements were used as in situ data. Two anomalies were observed in the experiment: 1) multiple peaks in the cross correlation waveform from one polarization of Ku-band frequency and 2) decrease in signal-to-noise ratio from loss of a data channel. When the instances of multiple peaks were eliminated and the equivalent bandwidth recomputed using only the active channels, SSH error from these cases agreed well with the model prediction. Application of SoOp wideband altimetry will, therefore, require a monitoring capability to identify changes in the transmission spectrum, total power, and waveform shape, for quality control and setting an appropriate observation error covariance. Measurement precision from a satellite receiver is predicted to be between 4 and 6 cm using the error model. SoOp altimetry with these signals may improve coastal measurements and increase the sampling and revisit rate through the use of a constellation of small satellites.

Toward science-oriented validations of coastal altimetry: Application to the Ligurian Sea

This study is a preliminary contribution to the European Space Agency's efforts aimed at establishing reference in situ networks specifically targeted to validate coastal altimetry. For this purpose, we processed and cross-compared conjointly improved altimetry data and in situ measurements acquired over the Ligurian Sea – a coastal region of the Mediterranean characterised by complex, fine-scale and rapidly evolving oceanic features. We made use of several kinds of multi-sensor in situ observations located along SARAL and Jason-2 tracks. The main objectives of the study were to assess improved coastal oriented validation strategies, including the usage of a new in situ platform (Moving Vessel Profiler), while better understanding potential differences owing to physical content inconsistency and instrumental or data processing limitations. The results show remarkable agreements over spatial scales of few tens of kilometres, paving the way for the deployment of future in situ networks and the definition of science-oriented diagnostics targeted to assess the capability of present and future high-resolution altimetric missions in resolving small-scale physical features.

Coastal Sea Level and Related Fields from Existing Observing Systems

We review the status of current sea-level observing systems with a focus on the coastal zone. Tide gauges are the major source of coastal sea-level observations monitoring most of the world coastlines, although with limited extent in Africa and part of South America. The longest tide gauge records, however, are unevenly distributed and mostly concentrated along the European and North American coasts. Tide gauges measure relative sea level but the monitoring of vertical land motion through high-precision GNSS, despite being essential to disentangle land and ocean contributions in tide gauge records, is only available in a limited number of stations. (25% of tide gauges have a GNSS station at less than 10 km.) Other data sources are new in situ observing systems fostered by recent progress in GNSS data processing (e.g., GPS reflectometry, GNSS-towed platforms) and coastal altimetry currently measuring sea level as close as 5 km from the coastline. Understanding observed coastal sea level also requires information on various contributing processes, and we provide an overview of some other relevant observing systems, including those on (offshore and coastal) wind waves and water density and mass changes.

Wind-induced cross-strait sea level variability in the Strait of Gibraltar from coastal altimetry and in-situ measurements

Coastal altimetry products are available and are being extensively validated. Their accuracy has been assessed in many coastal zones around the world and they are ready for exploitation near the shore. This opens a variety of applications of the sea level data obtained from the specific reprocessing of radar altimeter signals in the coastal strip. In this work, we retracked altimeter waveforms of the European Space Agency satellites: ERS-2 RA and Envisat RA-2 from descending track (#0360) over the eastern side of the Strait of Gibraltar using the Adaptive Leading Edge Sub-waveform (ALES) retracker. We estimated along-track Sea Level Anomaly (AT_SLA) profiles (RA-2) at high posting rate (18 Hz) using improved range and geophysical corrections. Tides were removed with a global model (DTU10) that displays a good performance in the study area: the mean root square sum (RSS) of the main constituents obtained with DTU10 and 11 tide gauge stations was 4.3 cm in agreement with the RSS using a high-resolution local hydrodynamic model (UCA2.5D) (4.2 cm). We also estimated a local mean sea surface by reprocessing ERS-2/Envisat waveforms (track #0360) with ALES. The use of this local model gave more realistic AT_SLA than the values obtained with the global model DTU15MSS. Finally, the along-track Absolute Dynamic Topography (AT_ADT) was estimated using a local Mean Dynamic Topography obtained with the local hydrodynamic model UCA2.5D. We analysed the cross-strait variability of the sea level difference between the African/Spanish coasts along the selected track segment. This was compared to the sea level cross-strait difference from the records of two tide gauges located in the African (Ceuta) and Spanish (Tarifa) coasts. The sea level differences from altimetry and tide gauges were linked to the zonal component of the wind. We found a positive and significant (>95% c.l.) correlation between easterlies/westerlies and positive/negative cross-strait sea level differences between the southern and northern coasts of the Strait in both datasets (altimetry: r = 0.54 and in-situ: r = 0.82).

Joint analysis of coastal altimetry and high-frequency (HF) radar data: observability of seasonal and mesoscale ocean dynamics in the Bay of Biscay

Abstract. Land-based coastal high-frequency (HF) radar systems provide operational measurements of coastal surface currents (within 1–3 m depth) with high spatial (300 m–10 km) and temporal (≤1 h) sampling resolutions, while the near-continuous altimetry missions provide information, from 1993 until today, on geostrophic currents in the global ocean with typical along-track and temporal sampling resolutions of &gt;7 km and &gt;9 days, respectively. During the last years, the altimetry community has made a step forward in improving these data in the coastal area, where the data present lower quality than in the open ocean. The combination of HF radar and altimetry measurements arises as a promising strategy to improve the continuous monitoring of the coastal area (e.g. by expanding the measurements made by HF radars to adjacent areas covered by the altimetry or by validating/confirming improvements brought by specific coastal algorithms or new altimeter missions). A first step towards this combination is the comparison of both data sets in overlapping areas. In this study, a HF radar system and two Jason-2 satellite altimetry products with different processing are compared over the period from 1 January 2009 to 24 July 2015. The results provide an evaluation of the performance of different coastal altimetry data sets within the study area and a better understanding of the ocean variability contained in the HF radar and altimetry data sets. Both observing systems detect the main mesoscale processes within the study area (the Iberian Poleward Current and mesoscale eddies), and the highest correlations between radar and altimetry (up to 0.64) occur in the slope where the Iberian Poleward Current represents a significant part of the variability in the circulation. Besides, the use of an Ekman model, to add the wind-induced current component to the altimetry-derived geostrophic currents, increases the agreement between both data sets (increasing the correlation by around 10 %).

New Achievements in Coastal Altimetry

11th Coastal Altimetry Workshop; Frascati, Italy, 12–15 June 2018

An assessment of a coastal altimetry data product in the Indonesian Waters

We analyzed the percentage of valid coastal altimetry Jason-2 X-TRACK-SLA data by Center for Topographic Studies in Sea and Hydrosphere (CTOH) and the waveforms from the Sensor Geophysical Data Record (SDGR) that are distributed by NOAA National Ocean Data Center around the Indonesian waters. In general, the percentage of valid data after the first point of the shoreline is greater than 90%. The percentage of valid data in steeper waters (86%) is higher than sloping waters (34%). The waveform types formed in coastal waters are peaky and Brown. However, spatially there is a difference where in a steep coast at a distance of > 5 km from the coastline the type of waveform is identified Brown, while on the sloping coast the brown type is found at a distance > 10 km. The SLA time series indicate seasonal variations in which the SLA is negative during the Southeast Monsoon (May-October) and positive during Northwest Monsoon (November-April).

Coastal currents in the Eastern Gulf of Tehuantepec from coastal altimetry

We study the seasonal and interannual variability of the coastal currents along the eastern shelf of the Gulf of Tehuantepec. We use sea level from coastal altimetry distributed by the Centre de Topographie des Océans et de l’Hydrosphère (Toulouse, France). Local validation using tide gauge and hydrographic data indicates that this coastal product measures well the seasonal and interannual scales along this tropical coastline. Long (12 years) time series of altimetry-derived currents, 39–57 km from the coast, reveal new details of the seasonal cycle: currents flow poleward from October to February and equatorward from April to August. Spectral analysis reveals that mesoscale processes are at least as energetic as the seasonal mean. The combination of both determines to a large extent the seasonal and interannual variability of the coastal currents.

SARin mode, and a window delay approach, for coastal altimetry

The CryoSat-2 (CS-2) mission is an altimetric mission designed to meet the measurement requirements for ice-sheet elevation and sea-ice ‘freeboard’. In addition to Cryosphere applications, this mission is of great interest to the Hydrosphere and Oceanographic communities since it allows for quantitative assessment of expected enhanced altimetric capabilities in coastal monitoring, ocean floor topography, gravity field and inland water monitoring. The CS-2 main payload is a Synthetic aperture Interferometric Radar ALtimeter (SIRAL) operating in three different modes, including the SAR interferometric mode (SARin). In the work described in this paper, dedicated algorithms are developed for exploiting SARin mode for enhancing the sea level mapping at coastal regions. When operating in SARin mode the altimeter measurements allow for the derivation of the across-track Angle of Arrival (AoA), and so the reflection points at any range bin can be geolocated. In coastal altimetry, land contamination highly degrades the backscattered echo. This is sometimes, overcome with the SARM when tracks are perpendicular to the coast, since this mode provides an along-track resolution enhancement with respect to LRM mode due to the different footprints shapes (LRM is circular, while SARM is a stripe of the same wide across track but only around 300 m along track). However, across-track interferences still remain an issue in coastal altimetry and SARin mode can provide a solution for their mitigation. This paper proposes two different solutions for the improvement of SSH retrievals in coastal zones starting from L1b SARin products, developing post-L1b algorithms. The first solution is based in the use of the AoA. The second solution goes beyond SARin mode, and provides an algorithm for all operational modes, based in the historical window delay within a coastal track section. For both options, a sub-waveform approach is considered for the waveforms cleaning and retracking. In addition, a SAR L2 retracker inherited from the SAMOSA model is adapted to SARin. The results are analyzed and the potential improvements are assessed with respect to SARin data retracker by the CS-2 ground processor. A dedicated CS-2 SARin area (mask) covering the Cuban archipelago has been created for this investigation. A land-sea mask has been created from the Open Street Map high resolution catalogue for this study. A dataset has been produced including both ESA CS2 IPF Baseline B SSH and the SSH series coming from this study.

SARAL/AltiKa Mission: Applications Using Ka-band Altimetry

Observations from AltiKa (nadir altimeter in Ka-band) on board the Satellite with ARGOS and AltiKa (SARAL) mission have been used for exploring new, unique and challenging areas. These application areas are coastal altimetry, inland hydrology and cryosphere (continental and sea ice). High rate (40-Hz) data from AltiKa has been exploited for retrieving very useful oceanic parameters (sea surface height anomaly, significant wave height and ocean surface wind) in the coastal waters (up to ~ 3 km off coast) of Indian landmass region. Validation carried out using buoys and tide gauge suggest exceedingly high correlation (~ 0.7–0.9) in the region of 3–10 km from the coast for each of these parameters. AltiKa is able to provide reasonably good quality ocean parameters much nearer to the coast (up to 3–10 km) as compared to Jason-2 (up to 8–10 km). Under the land hydrology, some of the extreme events such as flood wave (Brahmaputra river) and hydrological drought (Ukai reservoir) over India have been captured extremely well by AltiKa data. Brahmaputra river flood plain experiences flood waves almost every year. Flood wave of Brahmaputra river was captured using the AltiKa data on 9th June 2015. Two important aspects of cryospheric applications of AltiKa data has revealed interesting findings. During 2013–2015, East Antarctica is gaining ice with maximum change of 0.6 m in many regions. While in the west near Thwaites glacier and Pine Island glacier, more than 2 m reduction in the elevation has been observed. Winter sea ice freeboard thickness values were found to be higher than those estimated during summer period in both Arctic and Antarctic regions.

Interannual Variations of Surface Currents and Transports in the Sicily Channel Derived From Coastal Altimetry

AbstractA 20 year coastal altimetry data set (X‐TRACK) is used, for the first time, to gain insight into the long‐term interannual variations of the surface circulation in the Sicily Channel. First, a spectral along with a time/space diagram analysis are applied to the monthly means. They reveal a regionally coherent current patterns from track to track with a marked interannual variability that is unequally shared between the Atlantic Tunisian Current and Atlantic Ionian Stream inflows in the Sicily Channel and the Bifurcation Tyrrhenian Current outflow northeast of Sicily. Second, an empirical altimetry‐based transport‐like technique is proposed to quantify volume budgets inside the closed boxes formed by the crossing of the altimetry tracks and coastlines over the study area. A set of hydrographic measurements is used to validate the method. The inferred altimetry transports give a well‐balanced mean eastward Atlantic Waters baroclinic flow of 0.4 Sv and standard deviations of 0.2 Sv on a yearly basis throughout the Sicily Channel and toward the Ionian Sea, which is fairly coherent with those found in the literature. Furthermore, the analysis allows to quantify the intrusions of Atlantic Waters over the Tunisian Shelf (0.12 ± 0.1 Sv) and highlights two main modes of variability of the main surface waters path over the Sicily Channel through the Bifurcation Atlantic Tunisian Current and Atlantic Ionian Stream systems. Some physical mechanisms are finally discussed with regards to changes in the observed currents and transports.

Challenges and Opportunities for Coastal Altimetry

10th Coastal Altimetry Workshop; Florence, Italy, 21–24 February 2017

CAWRES: A Waveform Retracking Fuzzy Expert System for Optimizing Coastal Sea Levels from Jason-1 and Jason-2 Satellite Altimetry Data

This paper presents the Coastal Altimetry Waveform Retracking Expert System (CAWRES), a novel method to optimise the Jason satellite altimetric sea levels from multiple retracking solutions. CAWRES’ aim is to achieve the highest possible accuracy of coastal sea levels, thus bringing measurement of radar altimetry data closer to the coast. The principles of CAWRES are twofold. The first is to reprocess altimeter waveforms using the optimal retracker, which is sought based on the analysis from a fuzzy expert system. The second is to minimise the relative offset in the retrieved sea levels caused by switching from one retracker to another using a neural network. The innovative system is validated against geoid height and tide gauges in the Great Barrier Reef, Australia for Jason-1 and Jason-2 satellite missions. The regional investigations have demonstrated that the CAWRES can effectively enhance the quality of 20 Hz sea level data and recover up to 16% more data than the standard MLE4 retracker over the tested region. Comparison against tide gauge indicates that the CAWRES sea levels are more reliable than those of Sensor Geophysical Data Records (SGDR) products, because the former has a higher (≥0.77) temporal correlation and smaller (≤19 cm) root mean square errors. The results demonstrate that the CAWRES can be applied to coastal regions elsewhere as well as other satellite altimeter missions.