Altimeter Data Research Articles

High-performance altimeter Doppler processing for measuring sea level height under varying sea state conditions

This paper describes an innovative method for processing nadir altimeter data acquired in Synthetic Aperture Radar (SAR) mode, enhancing the system performances over open ocean. Similarly to the current SAR data processing scheme, the so-called LR-RMC (Low Resolution with Range Migration Correction) method, originally designed by Phalippou and Demeester (2011), includes Doppler beam forming, Doppler shift correction and range correction. In LR-RMC, however, an alternative and less complex averaging (stacking) operation is used so that all the Doppler beams produced in a radar cycle (4 bursts of 64 beams for the open-burst Sentinel-3-mode altimeter) are incoherently combined to form a multi-beam echo. In that manner, contrarily to the narrow-band SAR technique, the LR-RMC processing enlarges the effective footprint to average out the effects of surface waves and particularly those from small sub-mesoscale structures (<1 km) that are known to impact SAR-mode performances. On the other hand, the number of averaged beams is as high as in current SAR-mode processing, thus providing a noise reduction at least equally good. The LR-RMC method has the added benefit of reducing the incoherent integration time with respect to the SAR-mode processing (50 ms compared to 2.5 s) limiting possible surface movement effects. By processing one year of Sentinel-3A SRAL SAR-mode data using the LR-RMC method, it is shown that the swell impact on the SAR altimeter performances is totally removed and that an improvement of 10–50% is obtained in the measurement noise of the sea surface height and significant wave height with respect to SAR mode. Additionally, observational capabilities over the middle scales are enhanced potentially allowing the ocean mesoscale features to be retrieved and observations assimilated more usefully in ocean models.

The geometry of mesoscale eddies in the South China Sea: characteristics and implications

ABSTRACT The symmetrical circular shape of mesoscale eddies has been widely used in their scientific researches. Recently, an elliptical average eddy shape has been confirmed for eddies in the global ocean using multi-satellite altimeter data. As a regional extension of a previous study on the geometry of global eddies, a mean eddy shape in the South China Sea (SCS) has been derived by averaging a large number of orientational eddy boundaries. The mean shape is approximately a mathematic ellipse with a semimajor axis of 101.3 km and a semiminor axis of 61.3 km. Its size is larger than the global one. The principal eddy orientation in the SCS is 74°/254° (nearly northeast-southwest), different from that of eddies in the global ocean (171°/351°, nearly east–west). Composite analyses of chlorophyll (CHL) concentrations and sea surface temperature anomalies (SSTA) indicate a dipole structure for circular eddies in the non-rotated coordinate system. While a monopole structure for elliptical eddies in the eddy-centric coordinate system is obtained. The results demonstrate that the elliptical shape of eddies affects oceanographical variables. The findings provide a new approach for exploring the role of air–sea interactions on oceanic eddies.

Assessment of Arctic Sea Ice Classification Ability of Chinese HY-2B Dual-Band Radar Altimeter During Winter to Early Spring Conditions

Accurate identification of sea ice type from altimeter data is important for inversion of sea ice freeboard by radar altimeter, as well as for maritime navigation in polar regions. Previous sea ice classification studies have rarely involved multifrequency altimeter data. In this article, benefiting from the dual-frequency observation capability of the Chinese HY-2B altimeter, differences between waveform features of C- and Ku-band radar altimeter over different types of sea ice were investigated and analyzed for the first time. To validate and evaluate the sea ice classification capability of different feature combinations of the different bands, we extracted 14 features from the waveforms of C- and Ku-band HY-2B altimeter data acquired between December 2019 and April 2020. Our experiments demonstrated that the C- and Ku-band radar waveforms are sensitive to sea ice type, and that sea ice type classification could be improved by combining Ku- and C- band data. This work also proposed a sea ice classification methodology using C- and Ku-band altimeter waveforms. Comparison with results from previous studies indicated that the overall accuracy of the proposed method (up to 93.45%) is much higher than that of existing state-of-the-art methods.

A Simulation Experiment on In-Situ Observation of Short-Wavelength Scale Dynamic Processes and Potential Applications to Wide-Swath Interferometric Altimetry Validation

Short-wavelength (15–150 km) ocean phenomena are difficult to be observed by conventional altimeter due to the sampling method limited the resolution of the data. It is, therefore, the future wide-swath interferometric altimetry (WIA) missions like “SWOT” and “Guanlan” are designed to observe these phenomena. However, observing the short-wavelength dynamic process of the high eddy kinetic energy (EKE) region is still a challenge. The short-wavelength validation of WIA is also an urgent problem to be solved. In-situ observation platforms array can provide data set with high spatial and temporal resolution, which can capture short-wavelength dynamic process and validate the altimeter data. Simulation is a key to verify the feasibility of the observation strategy. In this article, we carried out simulated observation experiments based on high spatial-temporal resolution numerical models and introduced motion model of mooring and keep-station glider to improve the accuracy of in-situ observation simulation under the influence of the dynamic ocean currents which optimized by deep learning methods. In this experiment, the steric height, reconstructed by different observation array strategies, and the sea surface height of the measured positions are compared based on wavenumber spectrum. We tested the performance of the mooring array, station-keeping glider array, mooring and wave-energy profiling buoy (M&WPB) combination array, and multidevice array. The results show that the glider and M&WPB arrays have better performance of strong current regions. It could be helpful for the observation of dynamic processes in high EKE regions and has potential application for the validation of WIA.

In-Orbit Calibration and Validation of HY-2B Altimeter Using an Improved Transponder

HY-2B (Haiyang) is the follow-on mission to HY-2A, the Chinese first sea satellite for oceanography, launched in October 2018. The main payloads onboard are similar to HY-2A. For the altimeter, an in-orbit calibration mode was designed to achieve higher calibration precision. An improved transponder was built and deployed for the calibration campaign. The purpose of this article is to describe the in-orbit calibration and validation of HY-2B altimeter based on an improved transponder. A signal-rebuilt transponder was developed and employed in HY-2A altimeter calibration. It played an important role in the quantitative analysis of HY-2A altimeter data products. For HY-2B altimeter, a newly designed calibration mode was added, and an improved transponder was built. It has some advantages over the previous one in the rebuilt signal forms and modulation ways. Two calibration campaigns were carried out, in April 2019 and October 2020 at different sites. In the two campaigns, the precision of the altimeter range calibration is obtained less than 1 cm. For further validation and assessment of the result, a comparison between HY-2B and Jason-3 was performed in terms of sea surface height on the tens of cross track points in the open ocean. The comparison result shows high degree of accuracy and stability of HY-2B altimeter instrument. In China's next five-year plan (2021–2025), many more transponders and related calibration facilities would be installed in the ocean calibration fields, serving a variety of different satellite altimetry missions.

Reliability of Extreme Significant Wave Height Estimation from Satellite Altimetry and In Situ Measurements in the Coastal Zone

Measurements of significant wave height from satellite altimeter missions are finding increasing application in investigations of wave climate, sea state variability and trends, in particular as the means to mitigate the general sparsity of in situ measurements. However, many questions remain over the suitability of altimeter data for the representation of extreme sea states and applications in the coastal zone. In this paper, the limitations of altimeter data to estimate coastal Hs extremes (&lt;10 km from shore) are investigated using the European Space Agency Sea State Climate Change Initiative L2P altimeter data v1.1 product recently released. This Sea State CCI product provides near complete global coverage and a continuous record of 28 years. It is used here together with in situ data from moored wave buoys at six sites around the coast of the United States. The limitations of estimating extreme values based on satellite data are quantified and linked to several factors including the impact of data corruption nearshore, the influence of coastline morphology and local wave climate dynamics, and the spatio-temporal sampling achieved by altimeters. The factors combine to lead to considerable underestimation of estimated Hs 10-yr return levels. Sensitivity to these factors is evaluated at specific sites, leading to recommendations about the use of satellite data to estimate extremes and their temporal evolution in coastal environments.

Global chlorophyll distribution induced by mesoscale eddies

While mesoscale eddies can trap and transport chlorophyll (CHL) within the water columns, satellite measurements can only observe CHL at the sea surface. Here, we estimate the eddy-induced CHL distribution based on satellite observations, Argo float measurements, and global empirical models. The combination of satellite altimeter data and Argo measurements is used to detect eddy boundaries by tracking the outermost closed contours of potential vorticity (PV) at depth; then, sea surface CHL from satellite observations, together with their vertical distributions estimated from models are used to derive the CHL within eddy boundaries. We find that the CHL trapped by eddies can reach 3.2 × 1012g, which is about half of the total CHL in the ocean; the global time-mean CHL eddy-induced zonal transport adds up to 5.7 × 103 g/s westward. The results show that oceanic mesoscale eddies play an important role in the elevated CHL in the interior of the ocean.

Mean Sea Surface Model over the Sea of Japan Determined from Multi-Satellite Altimeter Data and Tide Gauge Records

Mean sea surface (MSS) is an important datum for the study of sea-level changes and charting data, and its accuracy in coastal waters has always been the focus of marine geophysics and oceanography. A new MSS model with a grid of 1′ × 1′ over the Sea of Japan and its adjacent ocean (named SJAO2020) (25° N~50° N, 125° E~150° E) was established. It ingested 12 different satellites altimeter data (including TOPEX/Poseidon, Jason-1/2/3, ERS-1/2, Envisat, GFO, HaiYang-2A, SRL/Altika, Sentinel-3A, Cryosat-2) and 24 tide gauge stations’ records and joint GNSS data. The latter were used to correct the sea surface height within 10 km from the coastline by using the Gaussian inverse distance weighting method in SJAO2020. The differences among SJAO2020, CLS15, and DTU18, as well as the differences between them and the altimeter data of HY-2A, Jason-3, and Sentinel-3A were introduced. By comparing with tide gauge records, satellite altimeter data, and other models (DTU18, DTU15, CLS15, CLS11 and WHU13), it was demonstrated that SJAO2020 produces the smallest errors, and its coastal accuracy is relatively reliable.

Surface heights and crevasse morphologies of surging and fast-moving glaciers from ICESat-2 laser altimeter data - Application of the density-dimension algorithm (DDA-ice) and evaluation using airborne altimeter and Planet SkySat data

NASA’s Ice, Cloud and land Elevation Satellite ICESat-2, launched September 15, 2018, carries the first space-borne multi-beam micro-pulse photon-counting laser altimeter system, the Advanced Topographic Laser Altimeter System (ATLAS). Observations from ATLAS are acquired in three pairs of weak and strong beams with 0.7 ​m nominal along-track spacing (under clear-sky conditions). The recording of the observations as a photon point cloud, which includes signal and background/noise events, requires a dedicated algorithm for identification of signal photons and determination of surface heights. The objectives of this paper are to demonstrate that measurements from ICESat-2 allow determination of heights over heavily crevassed ice surfaces and yield elevation profiles that present morphological characteristics that are typical of fast-moving and accelerating glaciers. Surface-height determination from the photon point cloud is facilitated by the density-dimension algorithm for ice surfaces, the DDA-ice. The DDA-ice returns surface heights at the 0.7 ​m sensor resolution for strong and weak beams, it utilizes a radial basis function for data aggregation and automatically adapts to changing environmental conditions and background characteristics, including time of day and apparent surface reflectance. In contrast, the official Land-Ice Along-Track Height Product, ATL06, provides surface heights at 40 ​m resolution with 20 ​m postings. The DDA-ice signal classification consistently identifies photons from complex reflectors in both the strong and weak ATLAS beams and hence constitutes a significant advance over the signal classification on the ATL03 Global Geolocated Photons Product. Results are evaluated using (1) airborne laser altimeter data collected during our ICESat-2 validation campaign over Negribreen, Svalbard, during surge, and (2) high-resolution (0.72 ​m or 0.86 ​m) satellite image data from Planet SkySat acquired over Ilulissat Ice Stream (Jakobshavn Isbræ), Greenland. Using DDA-ice analysis, ICESat-2 data allow discrimination of ice-surface types from surging glaciers (Negribreen) and continuously fast-moving and accelerating glaciers (Jakobshavn Isbræ) based on morphological characteristics.

Assessment of retracked sea levels from Sentinel-3A Synthetic Aperture Radar (SAR) mode altimetry over the marginal seas at Southeast Asia

ABSTRACT This paper presents the assessment of altimetric data from Sentinel-3A satellite operating in Synthetic Aperture Radar (SAR) mode for sea level research studies and applications over the largest archipelagos at Southeast Asia. Both qualitative and quantitative assessments are conducted by analysing the physical shapes of waveforms, comparing with quasi-independent geoidal height data and independent tide gauge measurements. The results identified the percentage of ocean like and non-ocean like waveforms are 91% and 9%, respectively. Off 9% of non-ocean like waveforms, the generic class is multi-peak (7%) followed by the quasi-specular waveforms (2%) observed near the coastline (< 10 km). Ocean like waveforms typically appear beyond 500 m from the coastline. When comparing with geoidal heights and tide gauge measurements, the performance of sea levels from several retrackers are assessed. The SAMOSA+ (SAR Altimetry Mode Studies and Applications) retracker outperforms other retrackers (i.e. sub-waveform and modified threshold retrackers with 30%, 20%, and 10%). That is, the standard deviation of differences against geoidal heights, and the temporal correlation against tide gauges are superior in most cases. In terms of root mean square error (RMSE), all retrackers are ranging with RMSE ≤ 20 cm in all cases. It can be concluded that in general, the SAMOSA+ retracked sea levels are accurate over the complicated regions at the Southeast Asia.

High Resolution Digital Terrain Models of Mercury

We refined our Shape from Shading (SfS) algorithm, which has previously been used to create digital terrain models (DTMs) of the Lunar and Martian surfaces, to generate high-resolution DTMs of Mercury from MESSENGER imagery. To adapt the reconstruction procedure to the specific conditions of Mercury and the available imagery, we introduced two methodic innovations. First, we extended the SfS algorithm to enable the 3D-reconstruction from image mosaics. Because most mosaic tiles were acquired at different times and under various illumination conditions, the brightness of adjacent tiles may vary. Brightness variations that are not fully captured by the reflectance model may yield discontinuities at tile borders. We found that the relaxation of the constraint for a continuous albedo map improves the topographic results of an extensive region removing discontinuities at tile borders. The second innovation enables the generation of accurate DTMs from images with substantial albedo variations, such as hollows. We employed an iterative procedure that initializes the SfS algorithm with the albedo map that was obtained by the previous iteration step. This approach converges and yields a reasonable albedo map and topography. With these approaches, we generated DTMs of several science targets such as the Rachmaninoff basin, Praxiteles crater, fault lines, and several hollows. To evaluate the results, we compared our DTMs with stereo DTMs and laser altimeter data. In contrast to coarse laser altimetry tracks and stereo algorithms, which tend to be affected by interpolation artifacts, SfS can generate DTMs almost at image resolution. The root mean squared errors (RMSE) at our target sites are below the size of the horizontal image resolution. For some targets, we could achieve an effective resolution of less than 10 m/pixel, which is the best resolution of Mercury to date. We critically discuss the limitations of the evaluation methodology.

Assessment of DUACS Sentinel-3A Altimetry Data in the Coastal Band of the European Seas: Comparison with Tide Gauge Measurements

The quality of the Data Unification and Altimeter Combination System (DUACS) Sentinel-3A altimeter data in the coastal area of the European seas is investigated through a comparison with in situ tide gauge measurements. The comparison was also conducted using altimetry data from Jason-3 for inter-comparison purposes. We found that Sentinel-3A improved the root mean square differences (RMSD) by 13% with respect to the Jason-3 mission. In addition, the variance in the differences between the two datasets was reduced by 25%. To explain the improved capture of Sea Level Anomaly by Sentinel-3A in the coastal band, the impact of the measurement noise on the synthetic aperture radar altimeter, the distance to the coast, and Long Wave Error correction applied on altimetry data were checked. The results confirmed that the synthetic aperture radar altimeter instrument onboard the Sentinel-3A mission better solves the signal in the coastal band. Moreover, the Long Wave Error processing contributes to reduce the errors in altimetry, enhancing the consistency between the altimeter and in situ datasets.

Seasonal variability of mesoscale eddies in the Banda Sea inferred from altimeter data

Using the mesoscale eddy trajectory atlas product derived from satellite altimeter data from 1993 to 2016, this study analyzes statistical characteristics and seasonal variability of mesoscale eddies in the Banda Sea of the Indonesian seas. The results show that there were 147 mesoscale eddies that occurred in the Banda Sea, of which 137 eddies were locally generated and 10 originated from outside. The total numbers of cyclonic eddies (CEs, clockwise) and anticyclonic eddies (AEs, anticlockwise) are 76 and 71, respectively. Seasonally, the number of CEs (AEs) is twice larger than the number of AEs (CEs) in winter (summer). In winter, CEs are distributed in the southern and AEs in the northern basins, respectively, but the opposite thing occurs in summer, i.e., the polarities of mesoscale eddies observed at the same location reverse seasonally. The mechanisms of polarity distribution reversal (PDR) of mesoscale eddies are examined with reanalysis data of ocean currents and winds. The results indicate that the basin-scale vorticity, wind stress curl, and the meridional shear of zonal current reverse seasonally, which are favorable to the PDR of mesoscale eddies. The possible generation mechanisms of mesoscale eddies include direct wind forcing, barotropic and baroclinic instabilities, of which the direct wind forcing should play the dominant role.

Accuracy assessment of global ocean tide models in the South China Sea using satellite altimeter and tide gauge data

In this study, to meet the need for accurate tidal prediction, the accuracy of global ocean tide models was assessed in the South China Sea (0°–26°N, 99°–121°E). Seven tide models, namely, DTU10, EOT11a, FES2014, GOT4.8, HAMTIDE12, OSU12 and TPXO8, were considered. The accuracy of eight major tidal constituents (i.e., Q1, O1, P1, K1, N2, M2, S2 and K2) were assessed for the shallow water and coastal areas based on the tidal constants derived from multi-mission satellite altimetry (TOPEX and Jason series) and tide gauge observations. The root mean square values of each constituent between satellite-derived tidal constants and tide models were found in the range of 0.72–1.90 cm in the deep ocean (depth>200 m) and 1.18–5.63 cm in shallow water area (depth<200 m). Large inter-model discrepancies were noted in the Strait of Malacca and the Taiwan Strait, which could be attributable to the complicated hydrodynamic systems and the paucity of high-quality satellite altimetry data. In coastal regions, an accuracy performance was investigated using tidal results from 37 tide gauge stations. The root sum square values were in the range of 9.35–19.11 cm, with the FES2014 model exhibiting slightly superior performance.

Ejecta Thickness Distribution of the Schrödinger Basin on the Moon

AbstractThickness distribution of impact ejecta gives insights into the processes of impact cratering, the stratigraphy of planetary crust, future sampling site selection, and the source of lunar samples. In this study, we conduct detailed measurements of ejecta deposit thicknesses for the Schrödinger basin, the second youngest and the best‐preserved peak‐ring impact basin on the Moon. Based on Lunar Reconnaissance Orbiter Camera Wide Angle Camera, Kaguya Terrain Camera, and the Lunar Orbiter Laser Altimeter data, the thicknesses of ejecta deposits are estimated from the measurements of partially filled pre‐Schrödinger craters by using crater morphometry, and the effect of crater degradation is taken into considerations. The thickness distribution of ejecta is derived from the measured thicknesses of ejecta deposits using an updated ballistic sedimentation model. Our measurements yield an ejecta thickness of ∼704 m at the current Schrödinger basin rim under the decay with a power‐law index of −3.0. Based on these measurements, we find that the model of Pike (1974, https://doi.org/10.1016/0012-821x(74)90114-9) provides the best estimates of transient cavity rim radius of 95.5 km and excavation depth of 19.6 km for the Schrödinger basin, whereas the widely used model of McGetchin et al. (1973, https://doi.org/10.1016/0012-821x(73)90162-3) significantly underestimate the ejecta thickness of Schrödinger basin. In addition, our results indicate that the Schrödinger basin‐forming impact does not penetrate through the lunar crust.

Ballistic deployment of the Hayabusa2 artificial landmarks in the microgravity environment of Ryugu

Hayabusa2 deployed two artificial landmarks called “target markers (TMs)” on the asteroid Ryugu for autonomous landing control. To achieve precise deployment on target landing sites, the TMs were designed to dissipate kinetic energy and released near the asteroid surface (with an altitude of less than 40 m). This study evaluates the performance of the ballistic deployment in the actual microgravity environment by reconstructing the trajectories of the TMs from optical, altimetric, and radiometric data. In addition, based on the reconstructed trajectories, low-velocity impacts of the TMs on the surface of Ryugu are characterized with dynamical parameters, such as dissipated energy and a coefficient of restitution. The physical implications of the impact analysis are discussed in comparison with on-ground experimental data. Furthermore, the gravitational environment is investigated using the reconstructed trajectory data and a shape model of Ryugu, providing information on the local gravity anomaly. Consequently, this paper demonstrates the usefulness of deployable artificial landmarks for small-body landings and further offers insight on surface conditions and internal structures near the Hayabusa2 landing sites where samples of Ryugu were collected.

Global Ocean Mass Change From GRACE and GRACE Follow‐On and Altimeter and Argo Measurements

AbstractSatellite altimeter sea surface heights, in combination with Argo ocean temperature and salinity observations, provide an independent measure of global mean ocean mass (GMOM) change. Over the period January 2005 to April 2020, GMOM rates observed by the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow‐On (GFO) range from 1.88 ± 0.10 to 2.63 ± 0.10 mm/year, compared to 2.85 ± 0.37 mm/year from Altimeter‐Argo. Over much of the GRACE era, GRACE GMOM estimates agree well with Altimeter‐Argo over a broadband of frequencies. However, during the late stage of the GRACE mission (after August 2016) and into the GFO era, differences between GRACE/GFO and Altimeter‐Argo GMOM estimates become substantially larger and systematic, which may be related to the single accelerometer mode of operation during late‐stage GRACE and GFO missions and/or errors of Argo and altimeter data.

A three-dimensional gravest empirical mode determined from hydrographic observations in the western equatorial Pacific Ocean

A spatially varying non-stationary gravest empirical mode (GEM) field is constructed by incorporating the calendar month as an extra parameter to infer the subsurface thermohaline structure from the Pressure-sensor equipped Inverted Echo Sounder (PIES) observations in the western equatorial Pacific Ocean. This monthly GEM (MGEM) overcomes the application limitation of traditional GEM in the equatorial ocean due to the poor vertical coherence thermohaline structure there. The MGEM can capture 77% of the thermohaline variance for the upper 500 dbar, which is only 62% by the traditional GEM method. The most significant improvement of MGEM is for the deep ocean between 500 and 1900 dbar, where only 12% of the thermohaline variance can be captured by the GEM, but improved to 44% by the MGEM method. Based on the MGEM, a 24-month volume transport time series is estimated from observations by three PIESs deployed across the nascent North Equatorial Countercurrent (NECC) at 4°N, 6°N, and 8°N along the 130°E transect during 2014–2016. By establishing an empirical relationship between the 24 month-long volume transport and the synchronous altimeter data, a 24 year-long time series of NECC volume transport during 1993–2016 are constructed, and the seasonal and interannual variations of the NECC are investigated. The nascent NECC between 4°N-6°N was found to have quite strong seasonal variations with the strongest transport of 23 Sv occurring in February and the weakest transport of 11 Sv occurring in September. It is strengthened during the developing phase of El Niños, and is weakened during La Niñas. The 2015/16 El Niño exerts a significant impact on the nascent NECC with the amplitude comparable to that of the 1997/98 event, which even obscures the NECC's seasonality. • Pressure-sensor equipped Inverted Echo Sounder (PIES) observations in the western equatorial Pacific Ocean for the first time. • Development of a 3-dimensional GEM field with the third axis being time-of-year (MGEM field). • Significant improvement of MGEM in capturing thermohaline variance of deep ocean between 500 and 1900 dbar. • The nascent NECC has strong seasonal variations with its transport being doubled in winter compared with that in fall.

Advances in NE-Atlantic coastal sea level change monitoring by Delay Doppler altimetry

Abstract A significant part of the World population lives in the coastal zone, which is affected by sea level rise and extreme events. Consistent and precise new measurements are needed to assess and predict these changes. New altimeter missions equipped with Synthetic Aperture Radar (SAR) mode provide more accurate sea level heights. In this work we analyse their impacts on the estimation of the coastal sea level variability in the last 10 km. First, by analysing various standard and improved SAR altimetric products it is found that SAR altimetry will reduce the minimum usable distance from five to three kilometres. The best performance is achieved with the SAR coastal retrackers SAMOSA+ and SAMOSA++. A comparable performance is obtained in low resolution Reduced SAR (RDSAR) mode with the Spatio-Temporal Altimeter sub-waveform Retracker (STAR). In both cases, sea level heights are recovered with 4 cm accuracy up to 3 km from the coast at the Helgoland station, and the accuracy does not decrease with the distance to coast. In estuaries and coastal zones with hight tidal regimes, the discrepancy between altimetry and in-situ observations remains large (40 cm standard deviation difference (stdd) with SAR-SAM+). CryoSat-2 and Sentinel-3A have similar accuracy, despite their different repeat cycle and sampling, which require a different post-processing. The monthly coastal variability from the SAR-SAM+ product agrees most favourably with the NEMO-WAM model, with stdd 3.9 and correlation 0.90 for Sentinel-3A. Instead, the maximum departure between altimeter data products is between SAR-SAM+ and RDSAR-TALES CryoSat-2 data (stdd 2.3 and correlation 0.96). An average trend of about 3 ± 1.3 mm/yr has been detected using low resolution altimetry at ninetheen in-situ stations along the German coast over time intervals longer than 15 years. The difference of the trends of altimetry and tide gauge co-located time-series (al-tg) agrees with the GNSS rates within 1.5 mm/yr at half of the co-located locations, with error of al-tg larger than the error of the GPS rate by a factor bigger than two. The larger departure between monthly tide gauge and LRM altimetric time-series as between tide gauge and SAR altimetric time-series confirms the higher accuracy of SAR data compared to LRM. Finally, the sea level trends of the merged LRM and SAR altimetry time-series are consistent with the LRM trends.

Mapping the grounding zone of Larsen C Ice Shelf, Antarctica, from ICESat-2 laser altimetry

Abstract. We present a new, fully automated method of mapping the Antarctic Ice Sheet's grounding zone using a repeat-track analysis and crossover analysis of newly acquired ICESat-2 laser altimeter data. We map the position of the landward limit of tidal flexure and the inshore limit of hydrostatic equilibrium, as demonstrated over the mountainous and hitherto difficult to survey grounding zone of Larsen C Ice Shelf. Since the start of data acquisition in 2018, our method has already achieved a near 9-fold increase in the number of grounding zone observations compared with ICESat, which operated between 2003 and 2009. We have improved coverage in particular over the previously poorly mapped the Bawden and Gipps ice rises and Hearst Island. Acting as a reliable proxy for the grounding line, which cannot be directly imaged by satellites, our ICESat-2-derived landward limit of tidal flexure locations agrees well with independently obtained measurements, with a mean absolute difference and standard deviation of 0.39 and 0.32 km, respectively, compared to interferometric synthetic-aperture-radar-based observations. Our results demonstrate the efficiency, density, and high spatial accuracy with which ICESat-2 can image complex grounding zones and its clear potential for future mapping of the pan-ice sheet grounding zone.