Associate Professor Xiaoli Deng

With dedicated coastal processing strategies and advanced Delay-Doppler technique, the quality of altimeter data from Low-Resolution Mode (LRM) and Synthetic Aperture Radar (SAR) mode altimeters in coastal areas have been greatly improved. In this study, we present a new 20-Hz along-track sea level anomaly (SLA) dataset of Jason-3 within 100 km to the global coastlines using the modified SCMR (Seamless Combination of Multiple Retrackers) processing strategy. The new reprocessed Jason-3 dataset, along with Sentinel-6 Michael Freilich (MF) SAR mode data, are evaluated and validated over global coastal oceans. The evaluation results show that the modified SCMR has significantly increases the data availability by 16%¿67% for Jason-3 when compared to the official SGDR MLE4 dataset, especially in the last 5 km to the coast. The resultant data availability retains >90% beyond 5 km to the coast and 80% within 5 km to the coast, which is slightly higher (2%¿10%) than that obtained by the Sentinel-6 MF. Most importantly, the modified SCMR mitigates the hump artifacts observed for the SLA spectrums of LRM altimeters, which makes the noise level of 20-Hz SLA estimates from Jason-3 (5.52 cm) comparable with that from Sentinel-6 MF (5.42 cm). This result demonstrates that the modified SCMR strategy would improve the LRM altimeters' capability of monitoring the mesoscale eddies. The evaluation results also show that the Sentinel-6 MF SAR mode data obtain higher data precision but lower data availability than reprocessed Jason-3 LRM data, especially in the 0¿5 km coastal strip and mid-to-high latitude (>40°N or < 40°S) regions. Although the quality of altimeter data in the 0¿5 km coastal strip has been significantly improved, the validation results against tide gauges demonstrate that the degraded performance still occurs when compared to the results beyond 5 km offshore. The significant discrepancy between tide gauge records and altimeter data is found in places such as sheltered bays and archipelagos where the land contamination is severe, and thus the development of dedicated coastal retrackers and corrections for SAR mode altimeters is still of great importance. Finally, the good consistency between reprocessed Jason-3 LRM and Sentinel-6 MF SAR mode altimeter datasets has been found by examining the inter-mission SLA biases (-0.12 ± 0.01 m).

To better understand the wave contribution to coastal sea level variation in the northern South China Sea (SCS), the total sea level time series, which is estimated as the sum of waves, tides, atmospheric surges and altimetric sea levels, over the period of 2002¿2022 are used in this study. It is found that tides and waves are two dominate contributors to coastal sea level changes at seasonal (45.7 % and 40.8 %) and interannual (58.6 % and 28.4 %) time scales. The sea level trend is dominated by the altimetric sea levels, but waves can also influence the trends in the Taiwan Strait and Beibu Gulf. The amount of sea level rise that doubles the frequency of the former 50-year return level is analyzed by using the Generalized Extreme Value Distribution (GEVD) model. The results show that the amount of sea level rise is closely related to the amplitude and variance of extreme sea levels. Because waves can significantly increase probability of occurrence of exceedingly large extreme sea levels, the amount of sea level rise (~1 m) that doubles the coastal flooding frequency is much greater than that (~10 cm) without the wave contribution. This result demonstrates the important role of waves in changing the coastal flooding frequency.

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the "Green" Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments' development and satellite missions' evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion.

Satellite altimetry has proven a valuable resource of information on river and lake levels where in situ data are sparse or non-existent. In this study several new methods for obtaining stable inland water levels from CryoSat-2 Synthetic Aperture Radar (SAR) altimetry are presented and evaluated. In addition, the possible benefits from combining physical and empirical retrackers are investigated.The retracking methods evaluated in this paper include the physical SAR Altimetry MOde Studies and Applications (SAMOSA3) model, a traditional subwaveform threshold retracker, the proposed Multiple Waveform Persistent Peak (MWaPP) retracker, and a method combining the physical and empirical retrackers. Using a physical SAR waveform retracker over inland water has not been attempted before but shows great promise in this study.The evaluation is performed for two medium-sized lakes (Lake Vänern in Sweden and Lake Okeechobee in Florida), and in the Amazon River in Brazil. Comparing with in situ data shows that using the SAMOSA3 retracker generally provides the lowest root-mean-squared-errors (RMSE), closely followed by the MWaPP retracker. For the empirical retrackers, the RMSE values obtained when comparing with in situ data in Lake Vänern and Lake Okeechobee are in the order of 2-5 cm for well-behaved waveforms. Combining the physical and empirical retrackers did not offer significantly improved mean track standard deviations or RMSEs. Based on these studies, it is suggested that future SAR derived water levels are obtained using the SAMOSA3 retracker whenever information about other physical properties apart from range is desired. Otherwise we suggest using the empirical MWaPP retracker described in this paper, which is both easy to implement, computationally efficient, and gives a height estimate for even the most contaminated waveforms.

Over inland waters the altimetry radar return is typically contaminated by both land and surrounding vegetation. Moreover, the radar returns corresponding to floodplain vegetation and exposed mud flats can have similar waveform shapes to those reflected from open water surfaces so that the derived water surface elevations are either inaccurate or difficult to extract. This letter presents a robust and automated method based on image analysis of satellite imagery for the selection of altimetry waveforms over inundated zones. The altimetry footprint is assessed as being inundated if the radiometric response of an image kernel within the footprint conforms to known remotely sensed responses for water. The method was applied in the lower middle Fly floodplain of the Fly River in the Western Province of Papua New Guinea (PNG). Altimetry waveforms were assessed for inundation extent and vegetation cover, with those that met threshold levels being flagged for further retracking and water surface elevation monitoring. The results show that the method accurately identified> 90% of inundated sites along altimeter ground tracks and correctly selected waveforms reflected from water surfaces.

A new approach for improving the accuracy of altimetry-derived sea level anomalies (SLAs) near the coast is presented. Estimation of SLAs is optimized using optimal waveform retracking through a fuzzy multiple retracking system and the most appropriate detiding method. With the retracking system, fuzzy-retracked SLAs become available within 5 km of the coast; meanwhile it becomes more important to use pointwise tide modelling rather than state-of-the-art global tidal models, as the latter leave residual ocean tide signals in retracked SLAs. These improvements are demonstrated for Jason-2 waveforms in the area of the Great Barrier Reef, Australia. Comparing the retrieved SLAs with in situ tide gauge data from Townsville and Bundaberg stations showed that the SLAs from this study generally outperform those from conventional methods, demonstrating that adequate waveform retracking and detiding are equally important in bringing altimetry SLAs closer to the coast. © 2014 Taylor & Francis.

It is broadly acknowledged that the precision of satellite-altimeter-measured instantaneous sea surface heights (SSH) is lower in coastal regions than in open oceans, due partly to contamination of the radar return from the coastal sea-surface state and from land topography. This study investigates the behavior of ERS-2 and POSEIDON altimeter waveform data in coastal regions and estimates a boundary around Australia's coasts in which the altimeter range may be poorly estimated by on-satellite tracking software. Over one million 20 Hz ERS-2 (March to April 1999) and POSEIDON (January 1998 to January 1999) radar altimeter waveform data were used over an area extending 350 km offshore Australia. The DS759.2 (5' resolution) ocean depth model and the GSHHS (0.2 km resolution) shoreline model were used together to define the coastal regions. Using the 50% threshold retracking points as the estimates of expected tracking gate, we determined that the sea surface height is contaminated out to maximum distance of between about 8 km and 22 km from the Australian shoreline for ERS-2, depending partly on coastal topography. Using the standard deviation of the mean waveforms as an indication of the general variability of the altimeter returns in the Australian coastal region shows obvious coastal contamination out to about 4 km for both altimeters, and less obvious contamination out to about 8 km for POSEIDON and 10 km for ERS-2. Therefore, ERS-2 and POSEIDON satellite altimeter data should be treated with some caution for distances less than about 22 km from the Australian coast and probably ignored altogether for distances less than 4 km.