DUACS DT2021 reprocessed altimetry improves sea level retrieval in the coastal band of the European Seas

More than 29 years of altimeter data have been recently reprocessed by the multi satellite Data Unification and Altimeter Combination System (DUACS) and made available under the name of DT2021 processing through the Copernicus Marine Service (CMEMS) and the Copernicus Climate Change (C3S) Service. New standards have been applied and various geophysical correction parameters were updated compared to the previous release in order to improve the product quality. This paper describes the assessment of this new release through the comparison of both all satellites and two satellites products with external in situ tide gauge measurements in the coastal areas of the European Seas. The aim is to quantify the improvements on the previous DT2018 processing version on the retrieval of sea level in the coastal zone. The results confirmed that the new DT2021 processing version better solves the signal in the coastal band. Moreover, the all satellites dataset provided more accurate sea level measurements when comparing with tide gauges respect to the climatic two satellites dataset due to the better performance of the former for the assessment of higher than climatic frequency signals. On the opposite, we found the two satellite dataset the most suitable product for the assessment of long term sea level time series in the coastal zone due to its larger stability to the detriment of the all satellites dataset.</p

Multi-platform experiments, numerical simulations and data science techniques for generation of new altimetric products: focus on mesoscale and sub- mesoscale variability (MANATEE – OSTST proposal)

Trabajo presentado en la Ocean Surface Topography Science Team Meeting (OSTST), celebrada online del 19 al 23 de octubre de 2020

Synergy Between Glider and Coastal Altimetry: Examples Along Envisat, Jason-1 and Jason-2 Tracks

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Improved description of the ocean mesoscale variability by combining four satellite altimeters

Data from four satellite altimeters are combined with the aim of improving the representation of the mesoscale variability in the Global Ocean. All missions [Jason-1, ERS-2/ENVISAT, Topex/Poseidon interleaved with Jason-1, and Geosat Follow-On] are cross-calibrated previously to produce weekly gridded maps. In areas of intense variability, the rms differences between a classical configuration of two altimeters and the scenario merging four missions can reach 10 cm and 400 cm2/s2 in SLA and EKE, respectively, which represents an important percentage of the signal variance. A comparison with surface drifters shows that the four altimeter scenario improves the recovery of mesoscale structures that were not properly sampled with Jason-1 + ERS-2/ENVISAT. Finally, the consistency between altimetric and tide gauge data is improved by about 25% when coastal sea level is estimated with 4 satellites compared to the results obtained with 2 altimeters. Copyright 2006 by the American Geophysical Union.Peer Reviewe

Synergy between Envisat satellite altimetry and glider data: case study in the Balearic Sea

Dynamics along the continental slopes are difficult to observe given the wide spectrum of temporal and spatial variability of physical processes which occur. Studying such complex dynamics requires the development of synergic approaches through the combined use of integrated observing systems. Thus, it is necessary to process and validate multi-sensor datasets especially dedicated to coastal ocean studies. This implies to fully understand their physical content in order to really assess their potential complementarities. In this context, we present the results of an intensive observational program conducted in the Balearic Sea combining coastal gliders and altimetry data from Envisat RA-2. The objectives of this experiment are 1) to investigate the limitations and potential improvements of different altimetric datasets in the coastal area, 2) to develop new methods to understand and increase the glider-altimetry synergy and 3) to test the feasibility of new technologies to study coastal dynamics. In the period comprised between July 2007 and June 2008, 4 glider missions have been performed in the Balearic Sea, collecting full CTD casts from the surface down to 180 m with a horizontal resolution of 500 m. The missions have been programmed to be simultaneous to the satellite passage along a selected Envisat tracks. Dynamic height and geostrophic velocity with a reference level of 180 m have been computed from the glider CTD data. New methodologies have been developed to compute consistent altimetric and glider velocities. On one hand, a novel technique has been applied to estimate absolute glider velocities by combining surface glider geostrophic velocities (ref. level 180 m) with integrated currents estimated by the glider (GPS locations every 6 hours), a complementary and relevant variable not fully exploited in other recent studies. On the other hand, the altimetric velocity computation has been improved, especially in coastal zone, by using high frequency along track sampling associated with new filtering and editing techniques. The approach used in this study proves to be very efficient for (1) homogenizing the physical contents of altimetry and glider data, (2) providing interesting insights of the present limitations and possible future improvements of coastal altimetry and (3) characterizing coastal dynamics in the Balearic Sea through a combined analysis of high resolution observing systems.Peer Reviewe

KaRIn Noise Reduction Using a Convolutional Neural Network for the SWOT Ocean Products

The SWOT (Surface Water Ocean Topography) mission will provide high-resolution and two-dimensional measurements of sea surface height (SSH). However, despite its unprecedented precision, SWOT’s Ka-band Radar Interferometer (KaRIn) still exhibits a substantial amount of random noise. In turn, the random noise limits the ability of SWOT to capture the smallest scales of the ocean’s topography and its derivatives. In that context, this paper explores the feasibility, strengths and limits of a noise-reduction algorithm based on a convolutional neural network. The model is based on a U-Net architecture and is trained and tested with simulated data from the North Atlantic. Our results are compared to classical smoothing methods: a median filter, a Lanczos kernel smoother and the SWOT de-noising algorithm developed by Gomez-Navarro et al. Our U-Net model yields better results for all the evaluation metrics: 2 mm root mean square error, sub-millimetric bias, variance reduction by factor of 44 (16 dB) and an accurate power spectral density down to 10–20 km wavelengths. We also tested various scenarios to infer the robustness and the stability of the U-Net. The U-Net always exhibits good performance and can be further improved with retraining if necessary. This robustness in simulation is very encouraging: our findings show that the U-Net architecture is likely one of the best candidates to reduce the noise of flight data from KaRIn

Assessing SARAL/AltiKa data in the coastal zone with HF radar observations. Marine Environmental Monitoring, Modelling And Prediction

Trabajo presentado en el International Liège Colloquium on Ocean Dynamics, celebrado en Liege, Bélgica, del 23 al 27 de mayo de 2015Peer Reviewe

META3.1exp: a new global mesoscale eddy trajectory atlas derived from altimetry

International audienceThis paper presents the new global Mesoscale Eddy Trajectory Atlases (META3.1exp DT all-satellites, https://doi.org/10.24400/527896/a01-2021.001, Pegliasco et al., 2021a; and META3.1exp DT two-satellites, https://doi.org/10.24400/527896/a01-2021.002, Pegliasco et al., 2021b), composed of eddy identifications and trajectories produced with altimetric maps. The detection method used is inherited from the py-eddy-tracker (PET) algorithm developed by Mason et al. (2014), and is optimized to efficiently manage large datasets, and thus long time series. These products are an improvement on the earlier META2.0 product, which was produced by SSALTO/DUACS and distributed by AVISO+ (https://aviso.altimetry.fr, last access: 8 March 2022) with support from CNES, in collaboration with Oregon State University and support from NASA, and based on the Chelton et al. (2011) code. META3.1exp provides supplementary eddy information, such as eddy shapes, eddy edges, maximum speed contours, and mean eddy speed profiles from the center to the periphery. The tracking algorithm is based on overlapping contours, includes virtual observations, and acts as a filter with respect to the shortest trajectories. The absolute dynamic topography (ADT) field is now used for eddy detection, instead of the previous sea level anomaly (SLA) maps, in order to better represent the dynamics in the more energetic oceanic regions and in the vicinity of coasts and islands. To evaluate the impact of the changes from META2.0 to META3.1exp, a comparison methodology has been applied. The similarity coefficient (SC) is based on the ratio of the eddy overlaps to their cumulative area, and allows for extensive comparison of the different datasets in terms of geographic distribution, statistics on the main physical characteristics, changes in the lifetimes of the trajectories, etc. After evaluating the impact of each change separately, we conclude that the major differences between META3.1exp and META2.0 are due to the change in the detection algorithm. META3.1exp contains smaller eddies and trajectories lasting at least 10 d; these were not available in the META2.0 product. Nevertheless, 55 % of the structures in META2.0 are similar to META3.1exp, thereby ensuring continuity between the two products and their physical characteristics. Geographically, the eddy distributions differ mainly in the strong current regions, where the mean dynamic topography (MDT) gradients are sharp. The additional information on the eddy contours allows for more accurate collocation of mesoscale structures with data from other sources, and so META3.1exp is recommended for multi-disciplinary application

Coastal and mesoscale dynamics characterization combining glider and altimetry: case study over the Western Mediterranean Sea

Comunicación presentada en la 38th COSPAR Scientific Assembly (Committee on Space Research), celebrada del 18 al 23 de julio de 2013 en Bremen (Alemania)Peer Reviewe