Coastal Altimetry Research Articles

Validation of Sea levels from coastal altimetry waveform retracking expert system: a case study around the Prince William Sound in Alaska

This paper presents the validation of Coastal Altimetry Waveform Retracking Expert System (CAWRES), a novel method to optimize the Jason satellite altimetric sea levels from multiple retracking solutions. The validation is conducted over the region of Prince William Sound in Alaska, USA, where altimetric waveforms are perturbed by emerged land and sea states. Validation is performed in twofold. First, comparison with existing retrackers (i.e. MLE4 and Ice) from the Sensor Geophysical Data Records (SGDR), and second, comparison with in-situ tide gauge data. From the first validation assessment, in general, CAWRES outperforms the MLE4 and Ice retrackers. In 4 out of 6 cases, the value of improvement percentage (standard deviation of difference) is higher (lower) than those of the SGDR retrackers. CAWRES also presents the best performance in producing valid observations, and has the lowest noise when compared to the SGDR retrackers. From the second assessment with tide gauge, CAWRES retracked sea level anomalies (SLAs) are consistent with those of the tide gauge. The accuracy of CAWRES retracked SLAs is slightly better than those of the MLE4. However, the performance of Ice retracker is better than those of CAWRES and MLE4, suggesting the empirical-based retracker is more effective. The results demonstrate that the CAWRES would have potential to be applied to coastal regions elsewhere.

Comparison of retracked coastal altimetry sea levels against high frequency radar on the continental shelf of the Great Barrier Reef, Australia

Comparison of Jason-1 altimetry retracked sea levels and high frequency (HF) radar velocity is examined within the region of the Great Barrier Reef, Australia. The comparison between both datasets is not direct because the altimetry derives only the geostrophic component, while the HF radar velocity includes information on both geostrophic and ageostrophic components, such as tides and winds. The comparison of altimetry and HF radar data is performed based on the parameter of surface velocity inferred from both datasets. The results show that 48% (10 out of 21 cases) of data have high (≥0.5) spatial correlation. The mean of spatial correlation for all 21 cases is 0.43. This value is within the range (0.42 to 0.5) observed by other studies. Low correlation is observed due to disagreement in the trend of velocity signals in which sometimes they have contradictions in the signal direction and the position of the peak is shifted. In terms of standard deviation of difference and root mean square error, both datasets show reasonable agreement with ≤2.5 cm s−1.

Poleward currents from coastal altimetry: The west coast of Southern Baja California, Mexico

The west coast of Southern Baja California is subject to intense seasonal variability, presenting lowest temperatures from February to April partly due to the upwelling season but also to cold water advection associated to the California Current. The summer advance of a poleward current is responsible for the coastal temperature maxima. In this work we use a time series of currents from a moored Acoustic Doppler Profiler (ADP) to validate coastal altimetry data, in order to study seasonal and interannual coastal current variability. Almost eleven years of coastal altimetry data (2002–2012) from X-TRACK, 20–40km from the coast, reveal a persistent seasonal poleward flow from July to October and equatorward flow, modulated by mesoscale processes the rest of the year. Near the coast of the peninsula sea level raises towards the coast while the poleward current carries a warm water mass against the climatological wind. It is present from July to October in a coastal band 100km wide from the surface to 80m depth with speeds 0.2–0.3ms−1. The interannual variability observed in this period is unusually weak, compared to the previous decade.

Assessment of Altimetric Range and Geophysical Corrections and Mean Sea Surface Models—Impacts on Sea Level Variability around the Indonesian Seas

The focus of this study is the assessment of the main range and geophysical corrections needed to derive accurate sea level time series from satellite altimetry in the Indonesia seas, the ultimate aim being the determination of sea level trend for this region. Due to its island nature, this is an area of large complexity for altimetric studies, a true laboratory for coastal altimetry. For this reason, the selection of the best corrections for sea level anomaly estimation from satellite altimetry is of particular relevance in the Indonesian seas. The same happens with the mean sea surface adopted in the sea level anomaly computation due to the large gradients of the mean sea surface in this part of the ocean. This study has been performed using altimetric data from the three reference missions, TOPEX/Poseidon, Jason-1 and Jason-2, extracted from the Radar Altimeter Database System. Analyses of sea level anomaly variance differences, function of distance from the coast and at altimeter crossovers were used to assess the quality of the various corrections and mean sea surface models. The selected set of corrections and mean sea surface have been used to estimate the sea level anomaly time series. The rate of sea level rise for the Indonesian seas was found to be 4.2 ± 0.2 mm/year over the 23-year period (1993–2015).

Fully Focused SAR Altimetry: Theory and Applications

In this paper, we introduce the concept, develop the theory, and demonstrate the advantages of fully focused coherent processing of pulse echoes from a nadir-looking pulse-limited radar altimeter. This process, similar to synthetic aperture radar (SAR) imaging systems, reduces the along-track resolution down to the theoretical limit equal to half the antenna length. We call this the fully focused SAR (FF-SAR) altimetry processing. The technique is directly applicable to SAR altimetry missions such as CryoSat-2, Sentinel-3, or Sentinel-6/Jason-CS. The footprint of an FF-SAR altimeter measurement is a narrow strip on the surface, which is pulse limited across track and SAR focused along track. Despite the asymmetry of the altimeter footprint, the fully focused technique may be useful for applications in which one needs to separate specific targets within highly heterogeneous scenes, such as in the case of sea ice lead detection, hydrology, and coastal altimetry applications. In addition, over rough homogeneous surfaces, such as the ocean or ice sheets, the improved multilooking capability of FF-SAR leads to a significant increase in the effective number of looks with respect to the delay/Doppler processing, resulting in better geophysical parameter estimation. The proposed processing technique was verified by processing full bit rate CryoSat-2 SAR mode data over the Svalbard transponder. Hydrology, sea ice, and open ocean applications are also demonstrated in this paper, showing the improvement of this technique with respect to conventional and delay/Doppler altimetry.

Coastal Altimetry Products in the Strait of Gibraltar

This paper analyzes the availability and accuracy of coastal altimetry sea level products in the Strait of Gibraltar. All possible repeats of two sections of the Envisat and AltiKa ground-tracks were used in the eastern and western portions of the strait. For Envisat, along-track sea level anomalies (SLAs) at 18-Hz posting rate were computed using ranges from two sources, namely, the official Sensor Geophysical Data Records (SGDRs) and the outputs of a coastal waveform retracker, the Adaptive Leading Edge Subwaveform (ALES) retracker; in addition, SLAs at 1 Hz were obtained from the Centre for Topographic studies of the Ocean and Hydrosphere (CTOH). For AltiKa, along-track SLA at 40 Hz was also computed both from SGDR and ALES ranges. The sea state bias correction was recomputed for the ALES-retracked Envisat SLA. The quality of these altimeter products was validated using two tide gauges located on the southern coast of Spain. For Envisat, the availability of data close to the coast depends crucially on the strategy followed for data screening. Most of the rejected data were due to the radar instrument operating in a low-precision nonocean mode. We observed an improvement of about 20% in the accuracy of the Envisat SLAs from ALES compared to the standard (SGDR) and the reprocessed CTOH data sets. AltiKa shows higher accuracy, with no significant differences between SGDR and ALES. The use of products from both missions allows longer times series, leading to a better understanding of the hydrodynamic processes in the study area.

Exploiting coastal altimetry to improve the surface circulation scheme over the central Mediterranean Sea

AbstractThis work is the first study exploiting along track altimetry data to observe and monitor coastal ocean features over the transition area between the western and eastern Mediterranean Basins. The relative performances of both the AVISO and the X‐TRACK research regional altimetric data sets are compared using in situ observations. Both products are cross validated with tide gauge records. The altimeter‐derived geostrophic velocities are also compared with observations from a moored Acoustic Doppler Current Profiler. Results indicate the good potential of satellite altimetry to retrieve dynamic features over the area. However, X‐TRACK shows a more homogenous data coverage than AVISO, with longer time series in the 50 km coastal band. The seasonal evolution of the surface circulation is therefore analyzed by conjointly using X‐TRACK data and remotely sensed sea surface temperature observations. This combined data set clearly depicts different current regimes and bifurcations, which allows us to propose a new seasonal circulation scheme for the central Mediterranean. The analysis shows variations of the path and temporal behavior of the main circulation features: the Atlantic Tunisian Current, the Atlantic Ionian Stream, the Atlantic Libyan Current, and the Sidra Gyre. The resulting bifurcating veins of these currents are also discussed, and a new current branch is observed for the first time.

Coastal Altimetry

Coastal Altimetry

A comparative assessment of coastal mean dynamic topography in Norway by geodetic and ocean approaches

AbstractThe ocean's mean dynamic topography (MDT) is the surface representation of ocean circulation. It may be determined by the ocean approach, using numerical ocean circulation models, or by the geodetic approach, where MDT is the height of the mean sea surface (MSS), or mean sea level (MSL), above the geoid. Using new geoid models, geodetic MDT profiles based on tide gauges, dedicated coastal altimetry products, and conventional altimetry are compared with six ocean MDT estimates independent of geodetic data. Emphasis is put on the determination of high‐resolution geoid models, combining ESA's fifth release (R5) of GOCE satellite‐only global gravity models (GGMs) with a regional geoid model for Norway by a filtering technique. Differences between MDT profiles along the Norwegian coast together with Taylor diagrams confirm that geodetic and ocean MDTs agree on the ∼3–7 cm level at the tide gauges, and on the ∼5–11 cm level at the altimetry sites. Some geodetic MDTs correlate more with the best‐performing ocean MDT than do other ocean MDTs, suggesting a convergence of the methods. While the GOCE R5 geoids are shown to be more accurate over land, they do not necessarily show the best agreement over the ocean. Pointwise monomission altimetry products give results comparable with the multimission DTU13MSS grid on the ∼5 cm level. However, dedicated coastal altimetry products generally do not offer an improvement over conventional altimetry along the Norwegian coast.

Validation and Intercomparison of SARAL/AltiKa and PISTACH-Derived Coastal Wave Heights Using In-Situ Measurements

SARAL/AltiKa, the first Ka-band altimeter, now provides an opportunity to study wave characteristics in the world’s coastal ocean with improved accuracy. In the present work, AltiKa-derived significant wave heights (Hs) in the coastal ocean and inland water bodies have been analyzed using in-situ measurements. Analysis shows that AltiKa measured Hs agree well with the in-situ measurements with high correlation (0.98), low bias (6 cm), and low RMSE (19 cm) in the coastal ocean, and the performance is highly consistent across different coastal zones in the three tropical oceans. AltiKa performance is found to be very good ( $\textbf{RMSE} = \textbf{24}\;\textbf{cm}$ and $\textbf{correlation} = \textbf{0.94}$ ) near to the coast ( ${ ). In addition to the evaluation of AltiKa Hs, another coastal altimetry product, Innovative Processing System Prototype for Coastal and Hydrology Applications (PISTACH)-derived Hs using Jason-2 altimeter, has also been validated with in-situ measurements. The OCE3 retracking algorithm provided in PISTACH is able to improve the Jason-2 Hs in the 100–25 km coastal zone. None of the retracking algorithms showed significant improvement of Hs in the 0–10 km coastal zone.

Empirical Results of a Surface-Level GNSS-R Experiment in a Wave Channel

The scattering of GNSS signals over a water surface is studied when the receiver is at a low height, as in GNSS-R coastal altimetry. The precise determination of the local sea level and wave state from the coast will provide useful altimetry and wave information as “dry” tide and wave gauges. An experiment has been conducted at the Canal d'Investigació i Experimentació Marítima (CIEM) wave channel for two simulated “sea” states. The GNSS-reflectometer used is the P(Y) and C/A ReflectOmeter (PYCARO) instrument, a closed-loop receiver with delay and Doppler tracking loops that uses the conventional GNSS-R technique for the GPS C/A code. After retracking of the scattered GPS signals, the coherent and incoherent components have been studied. To reproduce the transmitted GPS signals indoors, a Rohde and Schwarz signal generator is used. It is found that, despite the ratio of the coherent and incoherent components being ~1, the coherent component is strong enough that it can be tracked. The coherent component comes from clusters of points on the surface that approximately satisfy the specular reflection conditions (“roughed facet”). The Pearson’s linear correlation coefficients of the derived “sea” surface height with the wave gauge data are: 0.78, 0.85 and 0.81 for a SWH = 36 cm and 0.34, 0.74, and 0.72 for a SWH = 64 cm, respectively, for transmitter elevation angles of = 60°, 75° and 86°, respectively. Finally, the rms phase of the received signal before the retracking processing is used to estimate the effective rms surface height of the ‘facets’, where the waves get scattered. It is found to be between 2.5- and 4.1-times smaller than the theoretical values corresponding to the half of the coherent reflectivity decaying factor.

Annual sea level variability of the coastal ocean: The Baltic Sea‐North Sea transition zone

AbstractThe annual cycle is a major contribution to the nontidal variability in sea level. Its characteristics can vary substantially even at a regional scale, particularly in an area of high variability such as the coastal ocean. This study uses previously validated coastal altimetry solutions (from ALES data set) and the reference ESA Sea Level Climate Change Initiative data set to improve the understanding of the annual cycle during the Envisat years (2002–2010) in the North Sea‐Baltic Sea transition area. This area of study is chosen because of the complex coastal morphology and the availability of in situ measurements. To our knowledge, this is the first time that the improvements brought by coastal satellite altimetry to the description of the annual variability of the sea level have been evaluated and discussed. The findings are interpreted with the help of a local climatology and wind stress from a reanalysis model. The coastal amplitude of the annual cycle estimated from ALES altimetry data is in better agreement with estimations derived from in situ data than the one from the reference data set. Wind stress is found to be the main driver of annual cycle variability throughout the domain, while different steric contributions are responsible for the differences within and among the subbasins. We conclude that the ALES coastal altimetry product is a reliable data set to study the annual cycle of the sea level at a regional scale, and the strategy described in this research can be applied to other areas of the coastal ocean where the coverage from the tide gauges is not sufficient.

Coastal Altimetry Challenges Our Understanding of Short Scales in the Ocean

8th Coastal Altimetry Workshop; Konstanz, Germany, 23–24 October 2014

An investigation of ensemble-based assimilation of satellite altimetry and tide gauge data in storm surge prediction

Cyclogenesis and long-fetched winds along the southeastern coast of South America may lead to floods in populated areas, as the Buenos Aires Province, with important economic and social impacts. A numerical model (SMARA) has already been implemented in the region to forecast storm surges. The propagation time of the surge in such extensive and shallow area allows the detection of anomalies based on observations from several hours up to the order of a day prior to the event. Here, we investigate the impact and potential benefit of storm surge level data assimilation into the SMARA model, with the objective of improving the forecast. In the experiments, the surface wind stress from an ensemble prediction system drives a storm surge model ensemble, based on the operational 2-D depth-averaged SMARA model. A 4-D Local Ensemble Transform Kalman Filter (4D-LETKF) initializes the ensemble in a 6-h cycle, assimilating the very few tide gauge observations available along the northern coast and satellite altimeter data. The sparse coverage of the altimeters is a challenge to data assimilation; however, the 4D-LETKF evolving covariance of the ensemble perturbations provides realistic cross-track analysis increments. Improvements on the forecast ensemble mean show the potential of an effective use of the sparse satellite altimeter and tidal gauges observations in the data assimilation prototype. Furthermore, the effects of the localization scale and of the observational errors of coastal altimetry and tidal gauges in the data assimilation approach are assessed.

The importance of coastal altimetry retracking and detiding: a case study around the Great Barrier Reef, Australia

A new approach for improving the accuracy of altimetry-derived sea level anomalies (SLAs) near the coast is presented. Estimation of SLAs is optimized using optimal waveform retracking through a fuzzy multiple retracking system and the most appropriate detiding method. With the retracking system, fuzzy-retracked SLAs become available within 5 km of the coast; meanwhile it becomes more important to use pointwise tide modelling rather than state-of-the-art global tidal models, as the latter leave residual ocean tide signals in retracked SLAs. These improvements are demonstrated for Jason-2 waveforms in the area of the Great Barrier Reef, Australia. Comparing the retrieved SLAs with in situ tide gauge data from Townsville and Bundaberg stations showed that the SLAs from this study generally outperform those from conventional methods, demonstrating that adequate waveform retracking and detiding are equally important in bringing altimetry SLAs closer to the coast.

Coastal Altimetry Benefits From CryoSat‐2 Synthetic Aperture Measurements

The scientific community involved in the research and development of applications of satellite altimetry in the coastal zone met for the 6th Coastal Altimetry Workshop. Research in coastal altimetry is going through exciting times: Experimental data sets for the coastal zone are now available, such as those from the French PISTACH project for the Jason‐2 satellite and from the European Space Agency (ESA)–funded COASTALT project for the Envisat satellite, and many diverse applications were shown at the workshop. These range from observing coastal currents to monitoring river and lake levels, ice margins, and storm surges to gravity mapping in coastal zones.

Estimates of vertical land motion along the southwestern coasts of Turkey from coastal altimetry and tide gauge data

The differences between coastal altimetry and sea level time series of tide gauges in between March 1993 and December 2009 are used to estimate the rates of vertical land motion at three tide gauge locations along the southwestern coasts of Turkey. The CTOH/LEGOS along-track coastal altimetry retrieves altimetric sea level anomalies closer to the coast than the standard along-track altimetry products. However, the use of altimetry very close to the coast is not found to improve the results. On the contrary, the gridded and interpolated AVISO merged product exhibits the best agreement with tide gauge data as it provides the smoothest variability both in space and time compared with along track altimetry data. The Antalya gauge to the south (in the Mediterranean Sea) and the Mentes/Izmir gauge to the west (in the Aegean Sea) both show subsidence while the Bodrum tide gauge to the south (in the Aegean Sea) shows no significant vertical land motion. The results are compared and assessed with three independent geophysical vertical land motion estimates like from GPS. The GIA effect in the region is negligible. The VLM estimates from altimetry and tide gauge data are in good agreement both with GPS derived vertical velocity estimates and those inferred from geological and archaeological investigations.

Evaluating the feasibility of GPS measurements of SSH on board a ship along the Portuguese West Coast

Aiming to validate coastal altimetry data, an exploratory experiment was carried out, using a survey ship with onboard GPS and motion compensation systems. The ship navigation plan followed a track as coincident as possible with the passage of two altimetry satellites, Envisat and Jason-1, along the Portuguese coast, in June 2007.GPS data were processed in order to achieve high accuracy epoch-by-epoch three dimensional coordinates of the onboard antennas. GPS ellipsoidal heights were corrected from ocean tides, solid Earth tides, vessel motion effects and vessel load variation. Satellite altimetry data were corrected using new models for ocean tide and wet tropospheric delay.The results for the comparison, without data smoothing, between the satellite altimetry and the SSH measurements, with more than 500 points compared for each satellite, shows 9cm standard deviation for the Envisat pass and 13cm for the Jason-1 pass. These results are within the expected values for SSH variability, which is captured in different ways by the two measurement systems. This approach is a simple and flexible data collection system, in a well-defined terrestrial reference frame that can be implemented, with some improvements, to obtain precise SSH measurements for coastal altimetry calibration or sea level monitoring.

New technologies for marine research: five years of glider activities at IMEDEA

This paper summarizes the glider activities carried out in the last 5 years by the IMEDEA Department of Marine Technologies, Operational Oceanography and Sustainability (TMOOS). TMOOS has been operating gliders in the Western Mediterranean Sea since 2006 and has set-up electronic and maintenance laboratories in order to establish a key glider port in the area. Twenty-two glider missions have been performed to date and over 17000 hydrographic and biogeochemical profiles collected. TMOOS is using gliders for operational, technological and scientific objectives. Studies of path planning analysis and adaptive sampling for gliders in combination with other platforms have been undertaken and new methodologies have been developed to process data from gliders. Thus far, IMEDEA gliders have contributed to the better understanding of mesoscale processes in the upper ocean, including the coupling between the physical and biogeochemical process of the marine ecosystem and, in combination with remote sensing observations, high-resolution glider data has enabled advances in new methodologies to improve coastal altimetry. Gliders have also proved to be important platforms for the development of operational oceanography tools and useful vehicles on which to test and implement new sensors for ocean monitoring.

A coastal altimetry retracking strategy based on waveform classification and sub-waveform extraction

Satellite radar altimetry is successful in the open ocean. However, the application of altimetry is limited in coastal regions, in part because the complex waveforms returned from the coastal sea are contaminated by land reflections. We performed a systematic analysis in which we compared a large number of waveform and retracking algorithms. Based on this analysis, we developed a retracking strategy consisting of waveform classification and sub-waveform extraction (OceanCS). The altimetry waveforms are classified into three types (ocean-like, coastal-like and land-like), according to a set of indicators based on waveform shapes. The sub-waveform of coastal-like type that corresponds to the ocean return signal is extracted. The retracking processes are carried out for ocean-like and for sub-waveforms of coastal-like types by using the Ocean retracker. Thirty-one cycles (from March 2006 to February 2007, cycle 155 to cycle 188) of 20 Hz waveform data from Jason1 were reprocessed using this retracking strategy to obtain not only the improved sea surface height (SSH) but also the significant wave height (SWH) estimates. The sea-level measurements from a tide gauge station and SWH from a buoy were used to validate the new retracking strategy. Comparisons show that the OceanCS retracking strategy was more suitable than the other five algorithms tested (Ocean, OCOG, Ice-2, Beta5 and Threshold) and has uniform performance in both the open ocean and coastal regions. The waveform retracking algorithm proposed in this article is helpful for solving the problem of coastal altimeter waveform processing based on explicit physical description and without using a geoid model or other external data source. Furthermore, the OceanCS retracking strategy furnishes not only the SSH but also the SWH in the coastal region.