Coastal ocean tides from SWOT wide-swath satellite altimetry
Ocean tides are a key component of ocean dynamics, and their precise knowledge is relevant for a variety of applications. Over the last decades, models have continued to improve, but limitations remain in coastal environments, where tidal dynamics are complex and satellite observations less reliable. A new study, led by DGFI-TUM and performed in cooperation with several international partners, now takes advantage of the wide-swath altimetry data from the Surface Water and Ocean Topography (SWOT) satellite.
The study concentrated on two complex coastal regions with varying tidal characteristics, the Bristol Channel (UK) and the Long Island Sound (USA). Here global models and along-track altimetry have traditionally struggled. Nearly three months of the 1-day Calibration and Validation (Cal/Val) phase of SWOT were used to derive the major M2 tide. Comparisons with global models and in-situ measurements demonstrated unprecedented spatial variability captured in these coastal regions by SWOT, which provided high levels of accuracy compared to in-situ measurements. Additionally, a fully-focused SAR (FFSAR) pass was evaluated in the Bristol Channel, which in itself is a novel result, demonstrated an advanced retrieval of tidal characteristics in the near coastal regions and supported the estimations made from SWOT data.
These insights strongly motivate the incorporation of the wide-swath data into future tide models, allowing for continued developments in coastal altimetry. The study is published in the article Tides in complex coastal regions: Early case studies from wide-swath SWOT measurements (Geophysical Research Letters, 2024, DOI: 10.1029/2024GL109983).
Satellite Monitoring of River Discharge
The majority of the world's population lives close to rivers and uses them as freshwater resources. For sustainable water supply and flood protection, water management decisions depend on accurate, timely and distributed discharge data. River discharge is also one of the Essential Climate Variables (ECVs) defined by the UN Global Climate Observing System (GCOS) to help identify and adapt to changes in the global water cycle and determine the flow of freshwater into the oceans. In-situ discharge measurements, however, are costly and steadily declining, especially in remote areas.
A novel optimization approach has now been developed at DGFI-TUM to increase the global coverage of discharge observations. It combines 20 years of satellite altimetry data with high-resolution satellite imagery and does not require existing in-situ stations for calibration. Small rivers and their width changes can be detected using deep learning classification to reconstruct the river bathymetry. The river slope is determined from precise ICESat-2 lidar measurements. Unknown flow parameters, such as the roughness of the river bed, are optimized according to the principle of mass conservation by minimizing the discharge difference over multiple consecutive cross-sections. Realistic uncertainties, crucial for data assimilation into hydrological models, are provided by taking into account errors and uncertainties in the various input quantities.
The approach is validated globally for 27 river sections with a median error of 12%. On average, the 90% uncertainty range includes 91% of the in-situ measurements. Details can be found in the article Monitoring river discharge from space: An optimization approach with uncertainty quantification for small ungauged rivers (Remote Sensing of Environment, 2024, DOI: 10.1016/j.rse.2024.114434).
Regional changes in relative sea level are strongly influenced by non-linear vertical land motion
The sea level relative to a coast is influenced on the one hand by climate-driven absolute sea level changes and on the other hand by vertical land motion (VLM). Regionally, the relative sea level change can deviate from the absolute sea level change by several cm/year. Previous sea level projections, such as those in the latest IPCC Assessment Report (AR6), have generally modelled VLM as a linear process, although there is evidence of non-linear movements. These are related to tectonic activity, changes in surface loading or groundwater extraction.
DGFI-TUM has developed a probabilistic global VLM reconstruction for the period from 1995 to 2020 to determine the impact of regional and non-linear VLM on relative sea level projections up to the year 2150. The study shows that the regional differences in projected coastal sea level changes are equally influenced by VLM and climate-related processes, with VLM causing relative sea level changes of up to 50 cm by 2150. The consideration of non-linear VLM increases the uncertainty of the projections by up to 1 m on a regional scale. This emphasizes the uncertainty of future coastal impacts and the importance of including non-linear VLM in sea level projections. Details can be found in the article Regional variations in relative sea level changes influenced by non-linear vertical land motion (Nature Geoscience, 2024, DOI: 10.1038/s41561-023-01357-2).
The VLM reconstruction can be used in research of past, present and future sea level changes, as well as for a variety of other applications, including satellite altimetry, impact assessment studies, and process understanding. Data is available from Zenodo (DOI: 10.5281/zenodo.8308346).
DTRF2020: New realization of the International Terrestrial Reference System (ITRS)
DGFI-TUM has been one of the three ITRS Combination Centers of the International Earth Rotation and Reference Systems Service (IERS) since 2001 and in this role is responsible for the realization of the ITRS alongside the IGN (France) and JPL/NASA (USA). As the fundamental coordinate system of the Earth, the ITRS provides the framework for referencing geodetic and astronomical observations. A highly accurate realization of the coordinate system, the so-called Reference Frame, is of paramount importance for various socially relevant applications including navigation and positioning, and forms the backbone for scientific exploration of the Earth system by providing the reference for monitoring dynamic processes and effects of climate change, such as ice melt or sea level rise.
The DTRF2020 is DGFI-TUM’s latest ITRS realization. It was calculated from the combination of up to 41 years of observation data from the four geodetic space techniques VLBI (Very Long Baseline Interferometry), SLR (Satellite Laser Ranging), GNSS (Global Navigation Satellite Systems), and DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite), pre-processed and analyzed in a major international effort coordinated by the Scientific Services of the International Association of Geodesy (IAG).
For a global network of 1829 stations, the DTRF2020 contains high-precision coordinates and coordinate changes. It is the only ITRS realization that accounts for station displacements caused by atmospheric, hydrologic, and oceanic mass redistributions (so-called Non-Tidal Loading, NTL). Furthermore, for the first time, the DTRF2020 also considers post-seismic deformation of stations affected by earthquakes. The DTRF2020 release is available via Zenodo (DOI: 10.5281/zenodo.8220523). More information is provided on the website of DGFI-TUM’s ITRS Combination Center at https://dtrf.dgfi.tum.de.
Regional tide model improves water level predictions in the Northwest European Continental Shelf
Ocean tides play an important role in coastal protection. When high tides coincide with episodic rises in sea level, such as storm surges, they exacerbate water levels and can cause coastal flooding. This is a worldwide problem, but is crucial in low-lying regions with high tidal ranges. In the Netherlands, for example, preventive measures such as flood barriers have been put in place to address such events. But accurate ocean model predictions are required to determine when to use these measures.
A new regional empirical ocean tide model for the North European continental shelf and the North Sea (EOT-NECS) based on 30 years of altimetry data was developed by DGFI-TUM. It shows consistent improvement over the global tide models EOT20 and FES2014b throughout the area.
A recent study used EOT-NECS as a boundary forcing for the DCSM-FM ocean model of the Dutch Rijkswaterstaat to optimize the model's predictions of total water levels along the Northwestern European Continental Shelf (domain in the Figure). An overall improvement of 0.29 cm and 0.25 cm was found in the model’s tidal height and total water level predictions, respectively. The positive results of the predictions in this study have resulted in Rijkswaterstaat operational forecasts now using the EOT-NECS tidal model as boundary forcing. The two-day forecast can be accessed here: waterinfo.rws.nl/#!/kaart/waterhoogte/
The study is published in the article Altimetry-derived tide model for improved tide and water level forecasting along the European continental shelf (Ocean Dynamics, 2023, DOI: 10.1007/s10236-023-01560-0). The EOT-NECS model, which can also be used in regional altimetry applications, is available from SEANOE (DOI: 10.17882/94705).
IRIS: Global dataset of River Surface Slopes
The new ICESat-2 River Surface Slope (IRIS) dataset provides global-scale average and extreme water surface slopes (WSS) determined from ICESat-2 observations. IRIS enables insights into the complex spatio-temporal dynamics of runoff in global river systems. One of the key advantages of IRIS is its reference to the widely used SWOT River Database (SWORD), allowing IRIS to be easily combined with observations from the new SWOT mission.
Accurate WSS data are critical for determining river discharge, one of the Essential Climate Variables (ECV) of the UN Global Climate Observing System (GCOS). In addition, WSS as a correction for the ground track variability of altimetry satellites significantly reduces the error of water level time series.
Global determination of WSS and its spatio-temporal variability is challenging. Global field measurements of WSS are not feasible, long-range interferometric SAR measurements are relatively inaccurate, and gauge stations or virtual stations from radar altimetry lack simultaneous observations over short distances. Using ICESat-2's unique measurement geometry of six parallel lidar beams, WSS of IRIS are calculated over beam pairs or along individual beams, depending on the intersection angle between satellite ground track and river centerline, to ensure maximum spatial and temporal coverage.
IRIS covers WSS between 2018 and 2022 referenced to SWORD(v2). Data is available from Zenodo (DOI: 10.5281/zenodo.7098113). IRIS will be updated progressively by adding future ICESat-2 cycles and new SWORD versions. Data are described in the article ICESat-2 river surface slope (IRIS): A global reach-scale water surface slope dataset (Nature Scientific Data, 2023, DOI: 10.1038/s41597-023-02215-x). More details on methodology and validation are provided in ICESat-2 based River Surface Slope and its Impact on Water Level Time Series from Satellite Altimetry (Water Resources Research, 2022, DOI: 10.1029/2022WR032842). An interactive map of IRIS is available in the Database of Hydrological Time Series of Inland Waters (DAHITI).
Monitoring thin ice and open water in the Arctic Ocean with CryoSat-2 SAR altimetry
Dwindling sea ice and an increasing number of open water areas have a significant impact on sea ice dynamics in the Arctic Ocean and the energy exchange between ocean and atmosphere. Areas of water (polynyas) and fractures in the sea ice (leads) are not permanently open, but partially frozen and covered by a thin layer of ice, up to about 25 cm thick. The surface temperature of the so-called thin ice is between that of the open water and the thicker sea ice surfaces, affecting the heat flux between ocean and atmosphere. This must be taken into account in climate models and predictions.
DGFI-TUM and the Alfred Wegener Institute (AWI) have jointly developed an unsupervised classification of altimeter radar echoes from ESA's CryoSat-2 for thin ice layer detection. The classification results were compared and validated with thin ice thickness derived from MODIS thermal imagery and with radar images from ESA's Sentinel-1 Copernicus mission.
The results demonstrate how monitoring of the polar oceans can be improved. They contribute to the knowledge of the time-varying Arctic ice cover, particularly by monitoring the overall sea ice thickness distribution. Moreover, the CryoSat-2 classification supports the development of improved waveform retracking algorithms that provide more reliable estimates of sea ice freeboard or sea level in the polar oceans. Details are provided in the article Monitoring Arctic thin ice: a comparison between CryoSat-2 SAR altimetry data and MODIS thermal-infrared imagery (The Cryosphere, 2023, DOI: 10.5194/tc-17-809-2023, [PDF]).
Reached a milestone: DAHITI provides hydrological data for 10,000 targets worldwide
The Database for Hydrological Time Series of Inland Waters (DAHITI) of DGFI-TUM provides free hydrological information for meanwhile 10,000 inland waters distributed worldwide. For rivers, lakes, reservoirs and wetlands, it provides water levels, area and volume changes based on satellite data in near-real-time.
With several thousand registered users and more than two hundred thousand downloads in recent years, DAHITI is a widely used data source for numerous applications in science and practice. The Global Climate Observing System (GCOS) lists the data base as an openly accessible data source for the Essential Climate Variable (ECV) "Lakes". To date, DAHITI provides water level and water extent for more than 1400 lakes and reservoirs, continuously and automatically updated with the latest satellite information.
Station-dependent satellite laser ranging (SLR) correction improves the orbit of the TOPEX/Poseidon altimetry satellite
TOPEX/Poseidon (T/P) was one of the first major altimetry satellite missions. It was operational between 1992 and 2006 and serves as the reference mission for its successors in the Jason and Sentinel-6 series. As such, it is of great importance for studies of global and regional sea level change, ocean circulation, and climate phenomena such as El Niño. With a diameter of over 160 cm, its Laser Retroreflector Array (LRA), the on-board target for Satellite Laser Ranging (SLR) measurements, is not ideally designed for centimeter orbit accuracies. The resulting large phase center fluctuations are a major limiting factor for precise orbit determination of T/P and thus for ocean surface surveys.
Scientists of DGFI-TUM have developed a correction function for SLR observations that resolves LRA-related phase center variations as well as effects that depend on satellite and observing station, such as range biases. The function uses the viewing angles of the observation to determine a correction value that is added to the SLR distance measurement. Since the function is continuous, interpolation between tabulated values is not required and interpolation errors are avoided. The correction reduces the root mean square (RMS) of the SLR observation residuals (observed - computed) for the entire T/P mission from 33.78 cm to 1.97 cm (1.59 cm for SLR core stations). Details can be found in the article Station-dependent satellite laser ranging measurement corrections for TOPEX/Poseidon (Advances in Space Research, 2023, DOI: 10.1016/j.asr.2022.09.002). The station-dependent correction parameters are published as a supplement to the article, so that the measurement correction can be implemented in any precise orbit determination software.
Novel multi-resolution representation scheme for regional gravity field refinement
The optimal combination of different observation types is a key to obtaining high-resolution and high-precision regional gravity field models. Existing literature suggests that the single-level model based on spherical radial basis function (SRBF) may be biased toward high-resolution measurements. To avoid this effect, a spectral combination by a multiresolution representation (MRR) allows to decompose the gravity signal into an expansion in the form of spherical harmonics for the global long-wavelength part and a number of detail signals in the form of wavelet functions for the regional mid- and high-frequency parts.
The article Combination of different observation types through a multi-resolution representation of the regional gravity field using the pyramid algorithm and parameter estimation (Journal of Geodesy, 2022, DOI: 10.1007/s00190-022-01670-5, [PDF]) presents an innovative MRR method based on the pyramid algorithm and sequential parameter estimation. Instead of estimating the coefficients of the detail signals at each resolution level independently, the pyramid algorithm connects the different levels and estimates the coefficients sequentially. Numerical investigations based on simulated and real gravity data demonstrate the ability of the MRR scheme based on the pyramid algorithm to adequately capture gravity information not only from high-resolution terrestrial data, but also from medium- to low-resolution measurements. It actually provides a model accuracy improvement of more than 30% in terms of RMS error over validation data, compared to the single-level approach.
River slopes determined from spaceborne lidar onboard ICESat-2
Knowledge of the water surface slope (WSS) is essential for estimating the discharge and flow velocity of rivers. These parameters are among the Essential Climate Variables (ECVs) as defined by the Global Climate Observing System (GCOS). They critically contribute to the characterization of Earth’s climate, and their determination on a global scale is thus of great scientific relevance.
Field surveys of WSS are, however, very costly, while remote sensing approaches are limited by several factors. While, for example, wide swath SAR interferometric measurements are relatively inaccurate, precise point measurements of radar altimetry lack simultaneous observations over short distances. This obstructs the observation of WSS that vary greatly in space and time. In contrast, the unique measurement geometry of ICESat-2 with six parallel laser beams enables instant, highly accurate WSS observations. A new approach uses two different methods, depending on the intersection angle between the satellite orbit and the river: If multiple beams cross a river reach nearly perpendicularly, the WSS between the crossings can be calculated with the across-track approach. Otherwise, if satellite orbit and river are nearly parallel, the along-track approach derives the WSS directly from the continuous water level observations along a single intersecting crossing beam. The method can be applied globally, and the long-repeat orbit pattern of ICESat-2 allows continuous WSS monitoring, revealing details of the highly varying WSS with little effort.
WSS is not only relevant for the derivation of river discharge, but also as a correction for the ground track variability of altimetry satellites. In this way, errors at regularly observed sites (so-called virtual stations) can be reduced by up to 30 cm or 66 %. Details of the study are presented in the article ICESat‐2 based River Surface Slope and its Impact on Water Level Time Series from Satellite Altimetry (Water Resources Research, 2022, DOI: 10.1029/2022WR032842).
Machine learning techniques for Vertical Total Electron Content forecasting
Space weather is considered the greatest risk to global navigation satellite systems (GNSS). High-precision GNSS applications, such as positioning and navigation, require advanced forecast methods for the effects of space weather and the ionosphere on GNSS. Impacts are difficult to model adequately using conventional mathematical approaches since the relationships are often non-linear.
DGFI-TUM and ETH Zurich jointly developed a novel forecast model for ionospheric vertical total electron content (VTEC) using decision tree-based machine learning techniques of Random Forest, Adaptive Boosting (AdaBoost), and eXtreme Gradient Boosting (XGBoost). A novelty of the approach is that an ensemble meta-estimator, i.e., a Voting Regressor (VR) model, combines predictions of several powerful machine learning techniques to create a final VTEC forecast with higher accuracy and better adaptation to data outside the training set. The VR model forecasts VTEC for 1 hour and 24 hours for both calm and stormy conditions. In addition, the relative importance of individual input features to VTEC forecasting was estimated to provide information on which input data the model is particularly good at learning from. In this way, the influence of a predictor on the target variable can be quantified and physical understanding improved. Details of the study are given in the article Ensemble Machine Learning of Random Forest, AdaBoost and XGBoost for Vertical Total Electron Content Forecasting (Remote Sensing, 2022, DOI: 10.3390/rs14153547, [PDF]).
DiscoTimeS: New approach to detect change points in GNSS, satellite altimetry, tide gauge and other geophysical time series
Precise knowledge of coastal vertical land motion (VLM) is essential for assessing the impact of sea level rise on coastal regions and its consequences for local populations. It links relative sea level, measured by tide gauges with a fixed connection to land, with absolute sea level, measured by satellite altimetry. VLM can be determined pointwise using Global Navigation Satellite Systems (GNSS) or from the combination of tide gauge measurements with absolute sea level changes measured by satellite altimetry.
Among the largest sources of uncertainty in the determination of VLM are discontinuities and trend changes in the observed time series. Discontinuities are often caused by instrumental changes, especially in GNSS. Trend changes often have seismic causes, but can also be due to long-term changes in surface loading or to local effects. Although these issues have been addressed extensively for GNSS data analysis, there is limited knowledge of how to directly detect and mitigate such events when determining VLM from the difference of altimetry and tide gauge (SATTG).
The novel Bayesian approach DiscoTimeS automatically and simultaneously detects discontinuities and trend changes, for the first time not only for GNSS time series but also for VLM derived from SATTG. We show that accounting for time-varying VLM significantly increases the agreement of SATTG with GNSS measurements (by 0.36 mm/year on average for 339 globally distributed station pairs). Bayesian change point detection is applied to 606 SATTG and 381 GNSS time series. One of the main results of this work is that the determination of time-varying VLM now makes it possible to avoid extrapolation errors of coastal VLM and sea level change projections. Details of the study are presented in the article Bayesian modeling of piecewise trends and discontinuities to improve the estimation of coastal vertical land motion (Journal of Geodesy, 2022, DOI: 10.1007/s00190-022-01645-6, [PDF]).
Assessment of non-tidal loading data for DGFI-TUM’s upcoming ITRS 2020 realization DTRF2020
Site displacements caused by non-tidal loading (NTL) are among the major limiting factors for the accuracy of today's reference frames. On the occasion of the current 2020 realization of the International Terrestrial Reference System (ITRS), we investigated the NTL models of two providers with respect to their suitability for the determination of long-term stable reference frames.
As one of the three ITRS Combination Centers of the International Earth Rotation and Reference Systems Service (IERS), DGFI-TUM is in charge of realizing the ITRS in regular intervals. Our upcoming solution, the DTRF2020, will correct for site displacements caused by mass load changes of atmosphere, ocean and hydrology.
Since there is no conventional model for the application of NTL yet, the best approach and data for the DTRF2020 needs to be investigated. The appropriate modeling of NTL is crucial to our ITRS realization as accounting for NTL site displacements will affect the estimated coordinates of reference points (i.e., geodetic observatories), but should not affect the realized geocenter. We analyzed two sets of site displacements based on different geophysical models. One is the Global Geophysical Fluid Center contribution (labelled GCTI20) to the ITRS 2020 realization, the other is the operational NTL data from the Earth System Modelling group of the Deutsches GeoForschungsZentrum (ESMGFZ; already applied at DGFI-TUM e.g. for VLBI analysis). Among other things, we compared the displacements with time series of GNSS station residuals and calculated the contributions to geocenter motion (see Figure).
While the correlations are satisfactory, neither data set could be identified as having the better agreement with the residual GNSS station positions. The main differences between GCTI20 and ESMGFZ are the hydrological loading components and the presence of artificial trend changes in ESMGFZ site displacements (and hence geocenter motion contributions). The latter is a hindrance to realizing a secular reference frame. As a result, GCTI20 will be applied for the DTRF2020. The study is published in the article Comparison of non‐tidal loading data for application in a secular terrestrial reference frame (Earth, Planets and Space, 2022, DOI: 10.1186/s40623-022-01634-1, [PDF]).
Improved modeling of atmospheric drag in precise orbit determination
A major problem in the precise orbit determination (POD) of satellites at altitudes below 1,000 km is the modelling of atmospheric drag, which depends mainly on thermospheric density and causes the largest non-gravitational acceleration. Normally, thermospheric densities at satellite positions are determined by empirical models, which have limited accuracy. But conversely, satellites orbiting the Earth within the thermosphere can be used to derive thermospheric density information because of their sensitivity to perturbing accelerations.
Scientists from DGFI-TUM and the Institute of Geodesy and Geoinformation at the University of Bonn (IGG Bonn) have for the first time compared thermospheric density corrections in the form of scale factors for the NRLMSISE-00 model with a temporal resolution of 12 hours. It was shown that time-averaged scale factors from in-situ acceleration measurements on board CHAMP and GRACE fit well to arc-wise scale factors from the Satellite Laser Ranging (SLR) technique applied to the spherical satellites Larets, Stella, WESTPAC and Starlette. The estimated scale factors vary by up to 30% around the value of 1 at low solar activity and by up to 70% at high solar activity. This shows the extent to which the NRLMSISE-00 model values of thermospheric density deviate from the observed values. On average, at low solar activity the model overestimates the thermospheric density and has to be scaled down with the estimated scale factors, while at high solar activity the model underestimates the density values and has to be scaled up.
Depending on the altitude, there are correlations of up to 0.8 between the scale factors derived from accelerometer data and those estimated from SLR. To check the reliability of the latter, the POD results from two different software packages were compared, namely DOGS-OC at DGFI-TUM and GROOPS at IGG Bonn. Above 680 km altitude, a linear decrease in the estimated thermospheric density scale factors of about -5% per decade was observed, possibly related to climate change. The results of this study are published in the article Scale Factors of the Thermospheric Density: A Comparison of Satellite Laser Ranging and Accelerometer Solutions (Journal of Geophysical Research: Space Physics, 2021, DOI: 10.1029/2021JA029708, [PDF]).
New global ocean tide model EOT20 from multi-mission satellite altimetry
DGFI-TUM recently published the latest in a series of empirical ocean tide (EOT) models. The new model, named EOT20, shows improved results compared to other global tide models (including our earlier model EOT11a), especially in the coastal region.
Ocean tides play a vital role in various practical applications, especially in the coastal environment. In addition, tides are of importance in geodetic data analysis, for example in improving the observation of sea surface processes from along-track satellite altimetry and in determining high-resolution gravity fields from missions such as GRACE. Although in recent years tide models have made significant progress in the estimation of tides using satellite altimetry, the coastal region remains a challenge due to the complexity of shorelines, poorly resolved bathymetry and land contamination of altimetry radar echoes.
EOT20 benefits from advances in coastal altimetry, particularly in the use of the ALES retracker. The EOT20 approach relies on residual tidal analysis with respect to a reference tide model (FES2014) to estimate residual signals of the ocean tides. Further developments include the incorporation of more altimetry data, improved coastline representation and triangular gridding.
The model's accuracy was evauated using in-situ tide gauge data from DGFI-TUM's TICON dataset. Error reduction was found for the eight major tidal constituents in EOT20 compared to other global ocean tide models in the coastal region, with an error reduction of ~0.2 cm compared to the next best model (FES2014). EOT20 is on par with the best tide models in shelf regions and the open ocean, with improvement over EOT11a throughout. When used as a tidal correction for satellite altimetry, EOT20 reduced the sea level variance compared to both EOT11a and FES2014. These improvements, particularly in the coastal region, encourage the use of EOT20 as a tidal correction for satellite altimetry in sea-level research.
The ocean tide and load tide datasets of EOT20 are available in our Science Data Products section. Methodology and results are described in the publication EOT20: a global ocean tide model from multi-mission satellite altimetry (Earth System Science Data, 2021, doi: 10.5194/essd-13-3869-2021, [PDF]).
First comprehensive measurements of sea level changes in the Baltic Sea and the North Sea
Precise data for improved coastline protection: Led by DGFI-TUM, an international team of researchers has created the first comprehensive data sets of regional sea level rise in the North Sea and the Baltic Sea, including coastal areas and regions covered by sea ice. The data sets provide new insights into long-term and seasonal seal level changes over the past quarter century. This information is of vital importance for planning protective measures and for understanding dynamic processes in the oceans and the climate system.
Especially near coastlines, where many cities and industry facilities are located, the quality and quantity of data collected by the satellites are compromised by strong perturbances of the radar signal. Another problem is sea ice, which covers parts of the oceans in winter, and is impenetrable to radar. In the ESA Baltic Sea Level project (Baltic SEAL) the researchers developed algorithms to process the measurement data from radar satellites to permit precise and high-resolution measurements of sea level changes even in coastal areas and beneath sea ice. In this effort, the Baltic Sea can be seen as a model region since the complex shape of the coastline and sea ice make the data analysis particularly difficult. Analytical methods that work here can be easily adapted to other regions. Hundreds of millions of radar measurements taken between 1995 and 2019 were processed in a newly developed multi-stage process, comprising the identification of signals from the ice-covered sea water in the radar reflections produced along cracks and fissures, the development of new computational methods to achieve better quality of sea level data close to land, and finally the calibration and combination of measurements from the various satellite missions.
The analysis of these data for the Baltic Sea shows that the sea level has risen at an annual rate of 2-3 millimeters in the south, on the German and Danish coasts, as compared to 6 millimeters in the north-east, in the Bay of Bothnia. The cause of this above-average rise: Strong south-westerly winds connected to the NAO drive the waters to the north-east. The developed method has also been applied to the North Sea, where the sea level is rising by 2.6 millimeters per year, and by 3.2 millimeters in the German Bight.
The data sets Baltic SEAL and North SEAL of sea level changes are available for download in our Science Data Products section. Methods and results are described in the respective publications Absolute Baltic Sea Level Trends in the Satellite Altimetry Era: A Revisit (Frontiers in Marine Science, 2021, doi: 10.3389/fmars.2021.647607, [PDF]) and North SEAL: A new Dataset of Sea Level Changes in the North Sea from Satellite Altimetry (Earth System Science Data, 2021, doi: 10.5194/essd-13-3733-2021, [PDF]).
The study is also subject of a current TUM press release (English, German).
Global coastal attenuation of wind-waves observed with radar altimetry
Knowledge of ocean wave heights at the coast is essential for several operational applications, ranging from coastal protection to energy exploitation. In this context, the Significant Wave Height (SWH) is one of the most general quantitative parameters that describe the sea state at a particular location. SWH, representing the average height of the highest waves, can be measured from satellites using radar altimeters. Over the open ocean, such measurements are routinely used, for example, for ocean weather predictions. In the coastal zone however, the radar measurements were not considered reliable. As an alternative, in-situ buoys or high-resolution ocean models are employed. While the network of in-situ buoys is very sparse and can only provide data at specific locations, appropriate ocean models are computationally very expensive and not globally available, besides requiring constant validation.
Led by DGFI-TUM, an international team has now analyzed reprocessed data from radar altimetry, specifically tailored to improve the quality and quantity of coastal measurements. The results, published in the article Global coastal attenuation of wind-waves observed with radar altimetry (Nature Communications, 2021, doi: 10.1038/s41467-021-23982-4, [PDF]), provide a global picture of the average wave climate when going from offshore (about 30 km) to the coast (up to 3 km from land). The typical attenuation of the waves when approaching the coast, for example due to the shading effect from the land, is quantified to be about 20% of the wave height reached offshore. As a consequence, the energy flux transported by the waves is calculated to decline by about 40% on a global average. This result is paramount for coastal assessments, which until now are often based on models with validation relative to offshore satellite altimetry data.
Long-term measurements document sea level rise in the Arctic
Tracking down climate change with radar eyes: Over the past 22 years, the sea level in the Arctic Ocean has risen an average of 2.2 millimeters per year. This is the conclusion of an investigation performed jointly by DTU Space and DGFI-TUM as part of ESA's Sea Level Climate Change Initiative (CCI) project.
The most complete and precise overview of the sea level changes in the Arctic Ocean to date was obtained after evaluating 1.5 billion radar measurements of various altimetry satellites. A major challenge for a comprehensive analysis is the presence of sea ice which covers vast areas of the Arctic Ocean and obscures the ocean surface underneath. Applying DGFI-TUM's dedicated retracking algorithm ALES+ to ENVISAT and ERS-2 original measurements, radar echoes reflected even from small water openings in the ice could be identified and analysed. After harmonizing observation data from ice-covered and open water areas, maps of monthly sea level elevations were computed for 1996-2018.
Analysis of the long-term measurements revealed significant regional differences of sea level trends: Within the Beaufort Gyre north of Greenland, Canada and Alaska, the water stage rose twice as fast as on average. Low-salinity meltwater collects here, while a steady east wind produces currents that prevent the meltwater from mixing with other ocean currents. Along the coast of Greenland, on the other hand, the sea level is falling, on the west coast by more than 5 mm per year. Here, the melting glaciers weaken the gravity attraction. More information about the study can be found in the open access article Arctic Ocean Sea Level Record from the Complete Radar Altimetry Era: 1991–2018 (Remote Sensing, 2019, DOI: 10.3390/rs11141672, [PDF]). The results are also subject of a current TUM press release (English, German).
DGFI-TUM contributes to the implementation of an UN Resolution for a Global Geodetic Reference Frame
In February 2015, the UN General Assembly adopted its first geospatial resolution „A Global Geodetic Reference Frame for Sustainable Development“. This resolution recognizes the importance of geodesy for many societal and economic benefit areas, including navigation and transport, construction and monitoring of infrastructure, process control, surveying and mapping, and the growing demand for precisely observing our planet's changes in space and time. The resolution stresses the significance of the global reference frame for accomplishing these tasks, for natural disaster management, and to provide reliable information for decision-makers.
The United Nations Global Geospatial Information Management (UN-GGIM) Working Group on the Global Geodetic Reference Frame (GGRF) has the task for drafting a roadmap for the enhancement of the GGRF under UN mandate.
Based on its competence in the realization of reference frames DGFI-TUM is involved in this activity by contributing to the compilation of a concept paper in the frame of the International Association of Geodesy (IAG). The main purpose of this paper is to provide a common understanding for the definition of the GGRF and the scientific basis for the preparation of the roadmap to be accomplished by the UN-GGIM Working Group on the GGRF. [more]
News Archive
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Improved parameters of geodetic VLBI by correcting for all types of non-tidal loading (2020)
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New coastal sea level record from reprocessed Jason satellite altimetry (2020)
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Adaptive modeling of the global ionosphere Vertical Total Electron Content (2020)
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Satellite-based time series of volume variations of small inland water bodies (2020)
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Combining heterogeneous observations in regional gravity field modeling (2020)
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Wave heights in the ocean: A round robin assessment of satellite altimetry retracking algorithms (2020)
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Regional high-resolution ionosphere maps for continental regions (2020)
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Observation-based attitude model improves the orbits of Jason altimetry satellites (2020)
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High-resolution ionosphere maps for precise GNSS applications (2019)
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Enhancement of the Global Geodetic Observing System: Where to put the next SLR station? (2019)
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Time-variable surface areas of lakes and reservoirs monitored from space (2019)
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Reference Systems – the Backbone for Positioning, Navigation and Earth System Research (2018)
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The Alps in motion (2018)
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Coastal altimetry reveals wind-induced cross-strait sea level variability in the Strait of Gibraltar (2018)
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New approach to estimate thermospheric density from SLR observations of LEO satellites (2018)
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Resolving water level changes of small rivers with CryoSat-2 SAR altimetry (2017)
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Next-generation global geodetic reference frames: The potential of Satellite Laser Ranging (2017)
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Tracking openings in sea ice of the Arctic and Antarctic Ocean (2017)
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Monitoring the Arctic Seas: Satellite altimetry traces open water in sea-ice regions (2017)
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Near real-time modelling of the electron content in the ionosphere (2017)
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Sea level change in the Mediterranean and North Sea with improved coastal performances (2017)
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High-resolution river water levels based on multi-mission altimetry data (2016)
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DGFI-TUM releases a new realization of the International Terrestrial Reference System: DTRF2014 (2016)
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IAG adopts a new conventional value for the reference gravity potential W0 of the geoid (2016)
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Innovative processing of altimetry data allows for monitoring water levels in wetlands (2016)
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