The southern Africa block (SAB) is one of the 4 blocks of the Africa plate (Mukandila, 2020). Using the east African seismicity as defined by Bird (2002), the Zululand boundary between the Nubia and Somalia plates separates this block into eastern (SABE) and western (SABW) sub-blocks. In addition, the level of seismicity in this region is remarkable (e.g., the 2006 Mozamique Mw 7 and 2017 Botswana Mw 6.5 earthquakes). However, there is no dense geodetic studies that establish correlation between the seismic activity and active deformation in this intraplate tectonic domain. In this study, we use position timeseries (longer than 20 years) from about 65 GNSS stations in southern Africa. The rigorous inversion of the GNSS velocities using the least square method and Newton-Raphson methods. The combined two methods make it possible to minimize the uncertainties in the location coordinates and angular velocities of the Euler poles (0.5e**-5° and 0.5e**-5 °/Ma, respectively). This approach made it possible to determine three Euler poles with the aforementioned precision, namely the pole of the SAB and those of its respective sub-blocks. The relative velocities between the southern and northern sub-blocks of (~0.157) mm/yr. at south and (0.185) mm/yr., respectively, with respect to the Zululand bourdary line describe a predominantly extensional deformation regime that correlates with seismic activity in the region.

How to cite: Mukandila Ngalula, R., Meghraoui, M., Masson, F., and Tondozi Keto, F.: The controversial rigidity of southern Africa block: Insights from geodetic results and seismicity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8977, https://doi.org/10.5194/egusphere-egu23-8977, 2023.

A cycle of about 6 years has long been observed in the Earth’s magnetic field, length of day, dynamic oblateness, polar motions and surface displacements and attributed to dynamical processes occurring in the core and at the core mantle boundary. Recently, a 6-year cycle has also been detected in the rate of change of the global mean sea level and the ice-mass contributions from Greenland and continental glaciers. In this study, we report new observations of a 6-year cycle in the terrestrial water storage estimates based on the satellite gravity missions GRACE and GRACE-FO, consistent with precipitation and global hydrological models. The causes for such oscillations in the climate system are still unexplained, but raise the question of the respective contributions of the Earth’s deep interior and external surface fluid envelopes to the 6-year cycles reported in many geodetic variables. Indeed, while some of these 6-year fluctuations are convincingly attributed to Earth’s deep interior processes, for some other variables, climate-related processes occurring in the surface fluid envelopes or at the Earth’s surface may be more likely. This issue is exacerbated by an opposition of phase discovered between the angular momentum of the atmosphere and the length of day at around 6 years, suggesting that dynamical processes occurring in the Earth’s core induce a rotation of the solid Earth and the atmosphere as a single system. An overview of the 6-yr cycle observed in different variables of the Earth System may therefore help to better understand potential links between the solid Earth and climate.

How to cite: Pfeffer, J., Cazenave, A., Rosat, S., Mandea, M., Dehant, V., Moreira, L., and Barnoud, A.: Detections of a 6-year cycle in the Earth system, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7746, https://doi.org/10.5194/egusphere-egu23-7746, 2023.

The Copernicus Sentinel-6 mission was planned to keep studying the sea-surface height and ocean state measurements and since April 2022 its first satellite, the Sentinel-6 Michael Freilich (S6MF), has become the current reference altimetry mission.

One of the key design aspects of the S6MF altimeter is the “interleaved” chronogram pattern, which increases the number of received pulses, reducing the ambiguities in the along-track dimension, and increasing the energy obtained from the surface. Thanks to the continuous surface illumination, the echoes from the same target can be coherently integrated making a significant improvement in the along-track resolution leading to more detailed understanding of the ocean, the polar zones and inland and coastal waters dynamics. Measurement parameters such as swell state, lead and iceberg detection or inland water level changes can take advantage of these advances.

Nowadays, unfocused azimuth steering methods, such as Delay Doppler, provide along-track resolutions around 300 meters. However Fully-Focused SAR (FF-SAR) algorithms can improve it to the order of sub-meter.  In 2017, Alejandro Egido and Walter H.F. Smith published the FF-SAR method description, based on the backprojection approach. Compared to the unfocused steering, it requires more computational time, making it difficult to be implemented operationally, keeping the data generation rate in the same order of magnitude as the unfocused chain products. In 2018, Pietro Guccione et. al. published the FF-SAR 2D Frequency Domain algorithm, based on the Omega-K (WK) algorithm from SAR radar. In this paper, it is shown that the two dimensional frequency domain can be used to decrease the number of operations needed to focus the data, under some circumstances that depend on the type of orbit and the emitted signal. FF-SAR WK algorithm achieves similar results in terms of along-track resolution for the CryoSat-2 mission, but notably improving the computational efficiency. Although Omega-K algorithm intrinsic assumptions can impact negatively in the accuracy estimation of parameters such as the sea surface height, there are applications like sea ice related activities that could benefit of faster execution times. In this presentation, an adapted and redesigned Omega-K algorithm for Sentinel-6 is presented. FF-SAR Omega-K and Backprojection have been used to process Sentinel-6 data over the Crete transponder, evidencing that both FF-SAR methods are capable to achieve the expected theoretical along-track resolution. Moreover, open ocean data from Sentinel-6 has been processed for both algorithms and results have been compared.

How to cite: Hernández, S., Gibert, F., Broquetas, A., Garcia-Mondéjar, A., Kleinherenbrink, M., and Roca, M.: ¿Why should we process Fully-Focused Radar Altimetry data in near real time? Improving the computational efficiency of FF-SAR using Omega-K based algorithm., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-701, https://doi.org/10.5194/egusphere-egu23-701, 2023.

Sea level rise is among the most pressing environmental, social and economic challenges facing humanity, and requires timely and reliable information for adaptation and mitigation. The ice sheets of Greenland and Antarctica currently contribute approximately one third of global sea level rise, yet monitoring their coastal regions, which are often populated by numerous, highly dynamic outlet glaciers remains a challenge. One of the principal methods for monitoring ice sheet change is that of satellite radar altimetry, which provides a near continuous 30-year record of surface elevation and volume change. However, this technique can suffer from incomplete measurements and larger uncertainties over rugged coastal topography, where the instrument may fail to track the ice surface, or may record multiple distinct reflections within the illuminated ground footprint. In these situations, current Level 2 processing approaches can be sub-optimal, leading to inaccuracies being introduced into the resulting elevation measurements. Therefore, this study aims to explore new approaches to retrieving elevation measurements, comprising (1) multipeak waveform retracking and (2) a refined slope correction approach over complex regions. The developed approaches offer the potential for multiple elevation retrievals from a single waveform, and in turn the opportunity to increase both the reliability and quantity of elevation measurements.

Within the study, these processing techniques were developed and evaluated across Russell Glacier and the whole Greenland as two typical test cases, based upon Sentinel-3 SAR altimeter acquisitions over ice sheet regions that exhibit complex topography. Laser altimeters including Airborne Topographic Mapper (ATM) and Ice, Cloud, and Land Elevation Satellite-2 (IceSat-2) data were used as independent validation sources. Ice sheet wide analysis showed that the developed approaches were capable of delivering equally high accuracy for multiple elevation retrievals with comparable dispersion (~ 1 m) but much lower bias (~ 0.5 m) and outliers (~ 4%) compared to standard Level-2 products (~ 4 m bias and ~ 20% outliers). The developed approaches have the potential to further extend the capability of satellite radar altimetry over complex glaciological targets, and to improve the accuracy and coverage of altimeter measurements across these regions.

How to cite: Huang, Q., McMillan, M., Muir, A., Phillips, J., and Slater, T.: New approaches to processing radar altimetry waveforms over complex ice sheet topography, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3689, https://doi.org/10.5194/egusphere-egu23-3689, 2023.

The sea surface slope can be determined from satellite altimetry and used to determine geostrophic surface currents. If these currents change over time, heat transport in the oceans will change as well, with potential impacts on continental temperatures. In Europe these temperatures are partly
influenced by geostrophic currents in the Arctic Ocean and it is therefore of great interest to know whether these currents have changed in the past 10-20 years. 

One of the primary altimetry missions used for observing the Arctic region is CryoSat-2. Its advantages include a high inclination angle and the use of altimetric interferometry. The CryoSat-2 SARin acquisition mode has the highest spatial resolution and mostly covers coastal areas. 

Altimetric sea surface measurements are sparse in the Arctic, due to the presence of sea ice, reduced data quality near the coast, and limited satellite coverage near the pole. The detection of leads (openings in the sea ice) allows for measurements of the sea surface, even in the presence
of sea ice. Reliably detecting locations of leads is therefore the first step in determining the sea surface slope in the Arctic Ocean. This study aims to increase the number of accurately detected leads, by designing and implementing an unsupervised machine learning algorithm for CryoSat-2 SARin data. 

Sea ice and leads have different scattering properties, resulting in different altimetry waveform shapes. By defining a set of quantitative features to describe the waveform shape, the waveforms can be clustered based on similarities within this feature space. The features are chosen to provide
a clear distinction between sea ice and leads. A great advantage of the unsupervised classification is that no pre-labelled data are required. When new data are made available, waveforms can be assigned to an existing cluster by the K-nearest-neighbour method. Therefore, the creation of the
clusters has to be done only once. 

In order to validate the algorithm, the classification results are compared with the outputs of lead detection algorithms based on other data sources. Due to the limited number of SARin observations, results from both optical imagery and SAR imagery are used for statistical robustness.

How to cite: Veng, T. and Müller, F.: SARin lead dectection algorithm for Cryosat-2 using unsupervised classification, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12885, https://doi.org/10.5194/egusphere-egu23-12885, 2023.

Recent research has demonstrated how Greenland Ice Sheet (GrIS) near-surface density and wavelength-scale surface roughness can be estimated from satellite radar altimetry surface echo powers by way of the Radar Statistical Reconnaissance (RSR) technique, the adoption of a radar backscattering model (i.e., the small perturbation model, SPM) and calibration using in situ density profiles. Patterns in the estimated density results 1) highlight an inter-annual variability that covaries with known climatic drivers (e.g., extremely warm 2012 Greenland summer temperatures) and 2) suggest that density estimates derived from different frequency radar echoes (Ku-band from ESA CryoSat-2 and Ka-band from CNES/ISRO SARAL) correspond to different depths in the near-surface (with Ku-band densities being deeper than those from Ka). When expressed as fractions of their respective signal wavelengths, the CryoSat-2 and SARAL surface roughness estimates 1) exhibit good agreement, 2) recover anticipated surface roughness conditions (i.e., a smooth GrIS interior and rougher margin) and 3) demonstrate minimal temporal variability.

Here we present on-going work directed at validating both the use of the small perturbation model as well as the implied density-depth sensitivity in these new remote sensing observations. First, the suitability of the SPM is evaluated using the in situ density cores as well as airborne radar/laser altimetry measurements acquired along the EGIG line during the 2017 and 2019 ESA CryoVEx campaigns. Second, to quantify the depth range to which the satellite radar altimetry Ku- and Ka-band density estimates apply, we compare them against more than 400 contemporaneous (2011-2019) measured in situ density cores across the GrIS. These validation exercises are crucial to understanding the nature of these new satellite-based observations of the near-surface properties of the GrIS that, in turn, will facilitate more accurate estimations of the current and future GrIS mass balance.

How to cite: Scanlan, K. M., Simonsen, S. B., Rutishauser, A., and Vandecrux, B.: Validating Satellite Radar Altimetry-Derived Greenland Near-Surface Density and Surface Roughness using in situ and Airborne Datasets, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11155, https://doi.org/10.5194/egusphere-egu23-11155, 2023.

Water surface slope (WSS) of rivers is a key parameter in hydrological modelling, which allows for estimation of the transport and erosion capacity of a river, its flow velocity and discharge. On a local scale, WSS can be measured with a GNSS receiver installed on a boat, using remote sensing techniques (e.g. airborne lidar) or from a Digital Elevation Model (DEM). The most accurate method to measure WSS avoiding high-cost field campaigns is based on Water Surface Elevations (WSE) measured at in-situ stations. However, in poorly gauged rivers the neighboring gauges can be up to hundreds of kilometers apart, which inhibits a proper river profile observation. The gap in decreasing number of gauge readings is partially filled with satellite altimetry over rivers. Altimetry based WSE can be used to estimate WSS between neighboring measurements. Here, we present an innovative approach for estimating high-resolution WSS derived from multi-mission satellite altimetry for the largest Polish rivers.

In this study, we used measurements from 9 altimetry missions: CryoSat-2, Envisat, ICESat-2, Jason-2/-3, SARAL, Sentinel-3A/-B, and Sentinel-6A. These observations cover the years from 2002 to 2022. We extracted the river centerlines from the global “SWOT Mission River Database” (SWORD). In order to validate the obtained results, we used WSE from 81 gauges, which are maintained by the Institute of Meteorology and Water Management – National Research Institute (Instytut Meteorologii i Gospodarki Wodnej – Państwowy Instytut Badawczy, IMGW-PIB). These measurements are referenced to the Kronsztadt’86 vertical datum and they range from 01.2016 to 05.2022. Additionally, we used the reach-scale “ICESat-2 River Surface Slope” (IRIS) and the DEM-derived WSS values from SWORD.

To obtain WSS, we first determined WSE at each satellite pass crossing the studied river. Next, we split rivers into sections without dams and reservoirs. The Support Vector Regression (SVR) has been applied to reject outliers. Then, water levels were assigned to a given river kilometer (bin). For each of them a median WSE has been calculated. Finally, WSS were calculated at river sections between bins, excluding those disrupted by hydraulic structures. Finally, we weighted the section-wise WSS inversely proportional to the length of each section and applied a Least Square Adjustment with an additional Laplace condition to obtain bin-wise WSS for each river kilometer.

To assess the accuracy of the proposed approach, we compared the obtained WSS with the slopes between IMGW-PIB gauges. For large rivers (Vistula, Odra, Warta), the multi-mission approach revealed high accuracy with preliminary Root Mean Squared Error (RMSE) below 30 mm/km. For smaller, mountain rivers (San, Dunajec) the preliminary errors were slightly larger (RMSE ~100 mm/km). We also compared our accuracies with those of the slopes based on DEM models, lidar data, ICESat-2 altimetry, and SWORD database. In general, the multi-mission approach revealed the highest accuracy. The research is supported by the National Science Centre, Poland, through the project no. 2020/38/E/ST10/00295.

How to cite: Schwatke, C., Halicki, M., and Scherer, D.: High-resolution water surface slope of Polish rivers from two decades of multi-mission satellite altimetry measurements, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11956, https://doi.org/10.5194/egusphere-egu23-11956, 2023.