Altimeter Data Research Articles

CONTROLE ESTRUTURAL SOBRE A DRENAGEM DE SUB-BACIAS DA BACIA HIDROGRÁFICA DO RIACHO GRANDE, ALAGOAS

O objetivo principal dessa pesquisa se constitui em analisar aspectos da dinâmica geomorfológica em sub-bacias de quarta ordem na bacia hidrográfica do Riacho Grande, localizada na porção oeste do estado de Alagoas. Assim, se utilizou de dados altimétricos de um modelo digital de elevação TOPODATA do projeto SRTM (Shuttle Radar Topography Mission) com resolução de 30 metros. O MDE forneceu uma base de dados para a extração de dados de elevação que foram aplicados na obtenção dos índices morfométricos. Os índices utilizados em relação à forma da bacia foram: Fator de forma, índice de circularidade e índice de compacidade. Também foi utilizado o índice de densidade de drenagem e feita análise da extração de lineamentos. Os resultados mostraram que a maior parte das sub-bacias apresentam uma dinâmica homogênea com formatos circulares com baixa presença de lineamentos, onde a drenagem provavelmente evoluiu impondo sua dinâmica. A sub-bacia H mostrou os resultados mais discrepantes, com um formato alongado e concentração de lineamentos que indicam que essa sub-bacia possa ser controlada principalmente pelas estruturas.

Extraction and analysis of elevation changes in Antarctic ice sheet from CryoSat-2 and Sentinel-3 radar altimeters

New radar altimeters have supported elevation modeling with greater accuracy and extended periods in recent years. Sentinel-3 has been rarely used in previous studies for monitoring the Antarctic ice sheet. The CryoSat-2 and Sentinel-3 satellite radar altimeter data were combined to extract the elevation changes in the Antarctic ice sheet. First, a subregional data filtering method based on a clustering algorithm was proposed to improve the accuracy of the elevation change results for the outliers in the original measurements. Next, an improved model-fitting model that incorporated multiple parameters was used to extract elevation changes, and the results were validated using airborne laser altimetry data and ICESat-2 data. The average changes in the Antarctic ice sheet elevation from 2016 to 2019 were −4.3 ± 0.9 cm / y. The elevation changes of the ice shelf edges with large slopes and complex topography were much larger than the interior. Some specific areas showed substantial elevation changes. The most significant surface decreases occurred at the Pine Island Glacier and the Totten Glacier, while a pronounced surface heightening existed at the Kamb Ice Stream. We reveal an effective method for combining Antarctic ice sheet data from new radar altimeters. This method supports the long-term monitoring of the Antarctic ice sheet and global climate change in the future.

Ocean Tides near Hawaii from Satellite Altimeter Data. Part II

Abstract In Part I, the Chebyshev polynomial fitting (CPF) method has been proved to be effective to construct reliable cotidal charts for the eight major tidal constituents (M2, S2, K1, O1, N2, K2, P1, and Q1) near Hawaii and yields accurate results which are consistent with the Finite Element Solutions 2014 (FES2014), National Astronomical Observatory 99b (NAO.99b), and TPXO9 models. In this paper, the method is extended to estimate the harmonic constants of some minor tidal constituents. The mesoscale variation correction is applied to tidal elevations from satellite altimeters to eliminate the potential influence of background mesoscale ocean noise when estimating minor tidal constituents. This correction is necessary and makes the amplitude ratio between P1 and K1 constituents more consistent with the equilibrium tidal theory. Compared with the harmonic constants directly extracted from satellite altimeter data, FES2014 and NAO.99b yield mean root-mean-square (RMS) errors of 0.238 and 0.226 cm, respectively, while CPF method yields a mean RMS error of 0.210 cm, causing a 7%–12% decrease in the RMS error. At the crossover points between ascending and descending tracks, the decrease of RMS errors becomes 15%–18%. The accuracy of this method is also validated by comparing the estimated harmonic constants with those derived from tidal gauges and bottom-pressure recorders. These results indicate that the CPF method is also effective for estimating harmonic constants of minor tidal constituents. More importantly, the CPF method can obtain the harmonic constants of minor tidal constituents directly from satellite altimeter data, instead of being inferred via admittance theory. Significance Statement Ocean tides originate from the gravitational attraction of the sun and moon. Among the large number of tidal constituents, the major tidal constituents have been extensively studied. We extend the method proposed in Part I to estimate the harmonic constants of some minor tidal constituents with low amplitudes. The method relies on actual observations of water level variations from satellite altimeters without considering the hydrodynamic equations. We compared the results of this method with those of other models, using the data from satellite altimeters, tidal gauges, and bottom-pressure recorders. We find that the method performs well in estimating harmonic constants for some minor tidal constituents and causes a decrease of 7%–18% in RMS errors compared to other models.

Vertical Structure and Seasonal Variability of Shear on the Southwestern Continental Slope of the East China Sea

The vertical structure and seasonal variability of shear were examined using nearly three years of mooring ADCP (acoustic Doppler current profiler) data on the southwestern continental slope of the East China Sea (ECS). Shear spectra suggest that the sub-inertial currents (SICs); near-inertial waves (NIWs); and diurnal (D1), semidiurnal (D2), and tridiurnal (D3) internal tides (ITs) dominate the local shear field. The shear exhibits a remarkable surface-intensified pattern with high values occurring mostly in the upper 200 m. Significant seasonal variations can be found in the shear, but with differences between the upper (50–200 m averaged) and lower layers (210–570 m averaged). Satellite altimeter data indicate that the meander of the Kuroshio mainstream and the Kuroshio intrusion affect the seasonal variation of total shear by mainly influencing the shear caused by SICs. In addition, the shear efficiency (SE) of D2 ITs is obviously less than that of NIWs and that of D1 and D3 ITs via analyzing the kinetic energy (KE) densities and shear caused by these motions, since the predominant mode of the former is the first baroclinic mode, while the latter is dominated by higher baroclinic modes with large vertical wavenumbers. Moreover, the SE of incoherent ITs is relatively stronger than that of coherent ITs as a result of a larger proportion of high baroclinic modes in the incoherent component compared to the coherent component, based on modal decomposition.

Wind‐Wave Attenuation in Arctic Sea Ice: A Discussion of Remote Sensing Capabilities

AbstractWind‐generated waves strongly interact with sea ice and impact air‐sea exchanges, operations at sea, and marine life. Unfortunately, the dissipation of wave energy is not well quantified and its possible effect on upper ocean mixing and ice drift is still mysterious. As the Arctic is opening up and wave energy increases, the limited amount of in situ observations is a clear limitation to our scientific understanding. Both radar and optical remote sensing has revealed the frequent presence of waves in ice, and could be used more systematically to investigate wave‐ice interactions. Here we show that, in cloud‐free conditions, Sentinel‐2 images exhibit brightness modulations in ice‐covered water, consistent with the presence of waves measured a few hours later by the ICESat‐2 laser altimeter. We show that a full‐focus SAR processing of Sentinel‐3 radar altimeter data also reveals the presence and wavelengths of waves in sea ice, within minutes of Sentinel‐2 imagery. The SWIM instrument on CFOSAT is another source of quantitative evidence for the direction and wavelengths of waves in ice, when ice conditions are spatially homogeneous. In the presence of sea ice, a quantitative wave height measurement method is not yet available for all‐weather near‐nadir radar instruments such as altimeters and SWIM. However, their systematic colocation with optical instruments on Sentinel‐2 and ICESat‐2, which are less frequently able to observe waves in sea ice, may provide the empirical transfer functions needed to interpret and calibrate the radar data, greatly expanding the available data on wave‐ice interactions.

The 2015 exceptional swell in the Southern Pacific: Generation, advection, forecast and implied extremes

A very severe storm in the Antarctic belt is analysed that sent a very large swell throughout the South-Pacific Ocean. The reasons for the storm were a deep depression passing over an anomalous warm sea area, with consequent increased intensity, more active wind input, gustiness, with also dynamical generation. Wind and wave model results are verified with scatterometer and altimeter data. We follow the swell evolution during the five days required to reach the Galapagos Islands and a buoy off the Peruvian coast. The first forerunners peaked at 0.032 Hz at these locations, well represented in the model thanks to a purposely extended frequency range used in the WAM model. A nonlinear combined analysis is carried out to estimate the overall maximum single wave heights that may have impinged on the Galapagos coasts. Single wave heights up to 6 m have been estimated. Once generated, the swell conditions at Galapagos and the buoy are perfectly anticipated. Including generation, useful forecasts extend till at least eight days before the event. The lack of any local communication is discussed. An analysis using ERA5 winds, but a respectively higher resolution long-term wave hindcast, shows that a similar, actually stronger, event happened in 2006. A simple, but sound method, based on physical principles and elementary geometry, is proposed to estimate, firsthand and after any time, the maximum height of a once generated swell. The results for the 2015 storm are correct within 5% of the model values.

Potential and limitations of LiDAR altimetry in archaeological survey. Copper Age and Bronze Age settlements in southern Iberia

Potential and limitations of LiDAR altimetry in archaeological survey. Copper Age and Bronze Age settlements in southern Iberia

An improved algorithm for extracting crossovers of satellite ground tracks

A crossover refers to the intersection of two satellite ground tracks. Crossovers are required for performing crossover differences adjustment to remove the orbit error and for establishing the time series of elevation in surfaces, which is why crossovers are important for satellite altimetry measurements of ice sheet or ice shelf elevation changes. How to extract more crossovers precisely is the objective of this paper. On the basis of the traditional method of solving crossovers and computer graphics, this paper proposes an improved algorithm called improved rapid rejection and straddle test (IRST) for computing the position of the crossover point. This algorithm efficiently and accurately searches for two points in ascending pass and two points in descending pass that can form a crossover, and then computes crossovers’ geolocation. By using CryoSat-2 satellite altimeter data, we conducted our study on the Antarctic Ross ice shelf and Filchner–Ronne ice shelf using the fixed iteration (FI) algorithm, rapid rejection and straddle test (RST) algorithm, and the IRST algorithm. Results show that IRST is superior to the two other algorithms in terms of the number of crossovers, geolocation accuracy and computational efficiency. These advantages are more noticeable in the border areas between the ice shelf and the mountain with large terrain slope, which solves the problem that crossovers are less in these areas with poor-quality data coverage.

Distribution and Morphology of Lava Tube Systems on the Western Flank of Alba Mons, Mars

AbstractIntegrated analyses of Thermal Emission Imaging System (THEMIS) IR, Context Camera (CTX), and Mars Orbiter Laser Altimeter (MOLA) data sets have been used to characterize the western flank of the Martian volcano Alba Mons, which hosts a prominent population of lava tube systems. Identification and mapping of lava tube systems is based on both morphologic and topographic analyses, including the presence of chains of collapse depressions and elongate, sinuous ridges. Lava tubes and adjacent tabular lava flows with lengths of 100+ km form an extensive lava flow field. Analyses of topographic data sets, including slope maps, suggest continuity of the radial flow field pattern across the full western flank. Concurrent surface activity across the western flank is consistent with age constraints from geologic mapping and crater size‐frequency distributions that indicate Early Amazonian ages. The mapped population of 331 lava tube systems in the western flank geologic map quadrangle has a mean length of 36.2 km and a total length of ∼12,000 km. Individual lava tube systems extend up to ∼400 km. Orientation and slope data for lava tube systems show small deviations compared to regional values in 50‐km grid cells defined by the MOLA Digital Elevation Model, suggesting a strong coupling of lava tubes to the current slopes of Alba Mons. Mapping of lava tube systems documents segments both showing collapse and with no collapse, indicating the potential for extensive subsurface cavities that would be important astrobiological targets.

PHOTON-COUNTING LASER ALTIMETER DATA FILTERING BASED ON HIERARCHICAL ADAPTIVE FILTER FOR FOREST SCENARIO

Abstract. Photon-counting laser altimeter data has widely used in forest surveying and mapping with the successfully launch of ICESat-2. However, it is a challenge work that extract signal information from a large amount of noise photons, especially in complex scenario like forest. In this paper, we proposed the hierarchical adaptive filter method to extract signal photons and used this method to filtering ATL03 data in forest scenario. The hierarchical adaptive filter method follows a coarse-to-fine strategy to identify noise photons step by step. The most of noise are removed by filter based on feature of local distance, firstly. Then, the modified adaptive direction filter method is used to remove the noise photons near ground surface and vegetation. Finally, the filter based on continuity of topographic is used to remove the residual noise in the atmosphere. The airborne lidar data in experiment region was used to validate the effectiveness of method. The filter result overall recall is 97.51%, the overall precision is 98.58%, and the F-score is 98.04%. The result show that the hierarchical adaptive method we proposed can extract signal photons in background information effectively and preserve vegetation photons well.

Distinct Variability between Semidiurnal and Diurnal Internal Tides at the East China Sea Shelf

Breaking internal tides and induced mixing are critical to shelf dynamics, including heat and mass exchanges. Spatiotemporal variability of internal tides and modulation factors for the southern East China Sea shelf were examined based on a combination of a three-month mooring velocity and satellite altimeter data. Semidiurnal and diurnal internal tides exhibited distinct temporal trends, with the semidiurnal internal tides enhanced by an order of magnitude during the latter half of the record, while the diurnal internal tides followed quasi spring-neap cycles with a generally stable intensity except for two specific periods of strengthening. These internal tides probably originated remotely over the shelf-slope area northeast of Taiwan. Time-varying stratification was the most important factor for the internal tidal magnitude. In addition, varying background currents influenced the diurnal critical latitude band, which explains the slightly enhanced diurnal internal tides during the two periods. Although both semidiurnal and diurnal internal tides were mode-1 dominated, the semidiurnal internal tides were surface intensified while the diurnal tides were bottom intensified. The proportion of higher mode internal tides increased during robust eddy activities. Stronger background vertical shear corresponded to high-frequency events and energy transfers from tidal frequencies to high frequencies associated with turbulent mixing.

Kara and Barents sea ice thickness estimation based on CryoSat-2 radar altimeter and Sentinel-1 dual-polarized synthetic aperture radar

Abstract. We present a method to combine CryoSat-2 (CS2) radar altimeter and Sentinel-1 synthetic aperture radar (SAR) data to obtain sea ice thickness (SIT) estimates for the Barents and Kara seas. From the viewpoint of tactical navigation, along-track altimeter SIT estimates are sparse, and the goal of our study is to develop a method to interpolate altimeter SIT measurements between CS2 ground tracks. The SIT estimation method developed here is based on the interpolation of CS2 SIT utilizing SAR segmentation and segmentwise SAR texture features. The SIT results are compared to SIT data derived from the AARI ice charts; to ORAS5, PIOMAS and TOPAZ4 ocean–sea ice data assimilation system reanalyses; to combined CS2 and Soil Moisture and Ocean Salinity (SMOS) radiometer weekly SIT (CS2SMOS SIT) charts; and to the daily MODIS (Moderate Resolution Imaging Spectroradiometer) SIT chart. We studied two approaches: CS2 directly interpolated to SAR segments and CS2 SIT interpolated to SAR segments with mapping of the CS2 SIT distributions to correspond to SIT distribution of the PIOMAS ice model. Our approaches yield larger spatial coverage and better accuracy compared to SIT estimates based on either CS2 or SAR data alone. The agreement with modelled SIT is better than with the CS2SMOS SIT. The average differences when compared to ice models and the AARI ice chart SIT were typically tens of centimetres, and there was a significant positive bias when compared to the AARI SIT (on average 27 cm) and a similar bias (24 cm) when compared to the CS2SMOS SIT. Our results are directly applicable to the future CRISTAL mission and Copernicus programme SAR missions.

Performance Analysis of Ku/Ka Dual-Band SAR Altimeter from an Airborne Experiment over South China Sea

Satellite radar altimeters have been successfully used for sea surface height (SSH) measurement for decades, gaining great insight in oceanography, meteorology, marine geology, etc. To further improve the observation precision and spatial resolution, radar altimeters have evolved from real aperture to synthetic aperture, from the Ku-band to Ka-band. Future synthetic aperture radar (SAR) altimeter of the Ka-band is expected to achieve better performance than its predecessors. To verify the SAR altimeter data processing method and explore the system advantage of the Ka-band, a Ku/Ka dual-band SAR altimeter airborne experiment was carried out over South China Sea on 6 November 2021. Through dedicated hardware design, this campaign has acquired the Ku and Ka dual-band echo data simultaneously. The airborne data are processed to estimate the SSH retrieval precision after a series of procedures (including height compensation, range migration correction, multi-look processing, waveform re-tracking). To accustom to the airborne experiment design, a SAR echo model that fully considers both the attitude variation of the aircraft and the elliptical footprint of radar beam is established. The retrieved SSH data are compared with the public SSH data along the flight path at the experiment day, showing good consistence for both bands. By calculating the theoretical precision of waveform re-tracking and re-processing the dual-band airborne data into different bandwidths, it is demonstrated that the Ku/Ka precision ratio is possible to achieve 1.4 within the 27 km offshore area, which indicates that Ka-band has better performance.

Evaluation of Nearshore QuikSCAT 4.1 and ERA-5 Wind Stress and Wind Stress Curl Fields over Eastern Boundary Currents

Fields of coastal wind stress and wind stress curl in the 10–100 km next to the land control the processes of upwelling and downwelling of nutrients and water properties that are vital to highly productive coastal marine ecosystems. Here we ask the question: Do the present surface wind stress products from a satellite-borne scatterometer (QuikSCAT) and an atmospheric reanalysis model (ERA-5) systematically overestimate the magnitude of wind speed and stress in the 10–50 km next to the coast? We compare QuikSCAT wind speed retrievals to the relatively unused wind speed retrievals from satellite altimeters, which are able to approach closer to the coast than scatterometers without land reflections, due to their smaller radar footprints. Altimeter data on tracks approaching and crossing the coast indicate that the increases in coastal QuikSCAT wind speed values and ERA-5 coastal wind stress values are unrealistic. For analyses of wind speed and stress requiring high accuracy, especially those involving wind stress curl, we suggest considering individual Level 2B scatterometer wind retrievals as suspect at distances of 10 km and less from the coast, along with use of the Poor Coastal Processing flag. We found that similar increases in wind stress values next to the coast in gridded ERA-5 fields are not due to errors in the model physics or wind speeds. They are created during the interpolation of wind stress from the original model grid to a regular rectangular grid. We recommend that researchers who are analyzing wind stress and wind stress curl should calculate wind stress themselves from the gridded ERA-5 vector wind speed fields, rather than using the interpolated model wind stress or curl fields.

Direct conversion of latitude and height from one ellipsoid to another

I suggest a method for converting geodetic height and latitude from one oblate ellipsoid of revolution to another having the same center and symmetry axis. Unlike other approaches, the method here does not obtain height and latitude from Earth-centered, Earth-fixed (ECEF) Cartesian coordinates; this feature allows height conversion with high accuracy even in cases where the data format limits the precision of the latitude data. Height and latitude conversions can be expressed as a Fourier series in even multiples of latitude, with height changes having only cosines and latitude changes having only sines. The absolute difference of the flattenings of the two ellipsoids furnishes a simple upper bound on the maximum absolute latitude change. Conversions between the TOPEX and WGS84 ellipsoids, a practical necessity for the inter-mission comparison of satellite laser and radar altimeter data, illustrate the findings. Because the differences in the flattenings and semi-major axes of these ellipsoids are small, truncating the Fourier series after the term in twice the latitude gives an approximate conversion with an error less than 9 times 10–12 radians of latitude and about 6 times 10–6 m of height.

Stage estimation of Himalayan rivers using passive microwave radiometer and altimeter

ABSTRACT Hydrological variables such as surface water extent and soil moisture are continuously being monitored from satellites with good accuracy. However, orbital measurement of river stage and discharge remains a major challenge even today. Although altimeters provide valuable information on water level variations with a precision of few centimetres, they suffer from poor revisit time and limited gauging sites. Microwave radiometers that provide brightness temperature (BT) can be used to fill the gaps between altimeter passes with a revisit time of 1–2 days, although with limited accuracy. This paper presents an approach for estimation of river stage based on passive microwave radiometer using BT of river/measurement target (M) and upstream calibration target (C). We have used altimeter data from Jason-2/3 and BT data from Advanced Microwave Scanning Radiometer (AMSR2) at 36.5 GHz (descending orbit and horizontal polarization) for the period January 2013 to August 2020 to estimate river stage at 12 virtual gauging sites along Indus, Ganga, and Brahmaputra rivers. A new approach for selection of calibration targets is presented in this study which results in higher correlation between altimeter observed river stage and AMSR2 derived C/M ratio when compared to existing methods. Coefficient of determination (R 2) for C/M ratio with water level for the period 2013–2017 ranged from 0.45 to 0.89. Model functions were generated between C/M ratio and altimeter observed water level for all the gauging sites. Estimated water levels were validated against altimeter and ground observed values for the period January 2018–August 2020 with RMSE ranging from 0.35 to 1.45 m, which is 7–15% of total stage variability.

Consistency analysis of GRACE and GRACE-FO data in the study of global mean sea level change

Global mean sea level (GMSL) change is one of the important indicators of global climate change and is a crucial scientific issue of continuing interest. As satellite altimeter data, the Gravity Recovery and Climate Experiment (GRACE) and Argo continue to be updated, especially with the release of GRACE Follow-On (GRACE-FO) data, making it necessary to combine these latest data to estimate sea level change. Determinations on whether the GRACE and GRACE-FO observation systems provide unbiased global observation data have not been effectively evaluated. Therefore, this research mainly investigated the consistency of GRACE and GRACE-FO observation data in studying GMSL change. By comparing the sum of the GMSL calculated by the two gravity satellites and Argo data with the GMSL calculated by satellite altimeters, the discrepancy between GRACE-FO + Argo and satellite altimeter data is about 7.9 ± 2.3 mm, which is likely derived from the inconsistency between GRACE and GRACE-FO data.

Reply to comment by Harald Böhnel and Alejandro Rodríguez-Trejo on García et al. (2021) ‘Semicontinuous paleomagnetic record of the last 1 Ma from radiometrically dated igneous rocks (Trans-Mexican Volcanic Belt and surrounding areas)’

This work presents a few comments on the paper “Relative sea-level curve during the Holocene in Rio de Janeiro, Southeastern Brazil: A review of the indicators - RSL, altimetric and geochronological data”. Surprisingly the authors omitted in this review a former discussion on the interpretation of paleo-sea level indicator and paleo-sea level reconstructions on the Rio de Janeiro coast that questions their results. Therefore, at this present work we resume the previous discussion. We conclude that although at the commented paper the altitudes of the paleo-sea level indicators were determined precisely, they did not consider the vertical distance between a given a paleo-sea level indicator and its current homologous one, and thus do not indicate paleo-sea levels.

Development of the Yearly Mode-1 M2 Internal Tide Model in 2019

Abstract The yearly mode-1 M2 internal tide model in 2019 is constructed using sea surface height measurements made by six concurrent satellite altimetry missions: Jason-3, Sentinel-3A, Sentinel-3B, CryoSat-2, Haiyang-2A, and SARAL/AltiKa. The model is developed following a three-step procedure consisting of two rounds of plane wave analysis with a spatial bandpass filter in between. Prior mesoscale correction is made on the altimeter data using AVISO gridded mesoscale fields. The model is labeled Y2019, because it represents the 1-yr-coherent internal tide field in 2019. In contrast, the model developed using altimeter data from 1992 to 2017 is labeled MY25, because it represents the multiyear-coherent internal tide field in 25 years. Thanks to the new mapping technique, model errors in Y2019 are as low as those in MY25. Evaluation using independent altimeter data confirms that Y2019 reduces slightly less variance (∼6%) than MY25. Further analysis reveals that the altimeter data from five missions (without Jason-3) can yield an internal tide model of almost the same quality. Comparing Y2019 and MY25 shows that mode-1 M2 internal tides are subject to significant interannual variability in both amplitude and phase, and their interannual variations are a function of location. Along southward internal tides from Amukta Pass, the energy flux in Y2019 is 2 times larger and the phase speed is about 1.1% faster. This mapping technique has been applied successfully to 2017 and 2018. This work demonstrates that yearly internal tides can be observed by concurrent altimetry missions and their interannual variations can be determined. Significance Statement This work is motivated to study the interannual variations of internal tides using observation-based yearly internal tide models from satellite altimetry. Previous satellite observations of internal tides are usually based on 25 years of altimeter data from 1993 to 2017. The yearly subsetted altimeter data are short, so that the resultant yearly models are overwhelmed by noise. A new mapping technique is developed and demonstrated in this paper. It paves a path to study the interannual and decadal variations of internal tides on a global scale and monitor the global ocean changes by tracking long-range internal tides.

Analysis of vertical deflections determined from one cycle of simulated SWOT wide-swath altimeter data

Analysis of vertical deflections determined from one cycle of simulated SWOT wide-swath altimeter data