Coastal sea level anomalies and associated trends from Jason satellite altimetry over 2002–2018

Climate-related sea level changes in the world coastal zones result from the superposition of the global mean rise due to ocean warming and land ice melt, regional changes caused by non-uniform ocean thermal expansion and salinity changes, and by the solid Earth response to current water mass redistribution and associated gravity change, plus small-scale coastal processes (e.g., shelf currents, wind & waves changes, fresh water input from rivers, etc.). So far, satellite altimetry has provided global gridded sea level time series up to 10–15 km to the coast only, preventing estimation of sea level changes very close to the coast. Here we present a 16-year-long (June 2002 to May 2018), high-resolution (20-Hz), along-track sea level dataset at monthly interval, together with associated sea level trends, at 429 coastal sites in six regions (Northeast Atlantic, Mediterranean Sea, Western Africa, North Indian Ocean, Southeast Asia and Australia). This new coastal sea level product is based on complete reprocessing of raw radar altimetry waveforms from the Jason-1, Jason-2 and Jason-3 missions

Sea level along the world’s coastlines can be measured by a network of virtual altimetry stations

For nearly 30 years, space-based radar altimetry has been routinely measuring changes in sea level at global and regional scales. But this technique designed for the open ocean does not provide reliable sea level data within 20 km to the coast, mostly due to land contamination within the radar echo in the vicinity of the coast. This problem can now be overcome through dedicated reprocessing, allowing the retrieval of valid sea level data in the 0-20 km band from the coast, and then the access to novel information on sea level change in the world coastal zones. Here we present sea level anomalies and associated coastal sea level trends at 756 altimetry-based virtual coastal stations located along the coasts of North and South America, Northeast Atlantic, Mediterranean Sea, Africa, North Indian Ocean, Asia and Australia. This new dataset, derived from the reprocessing of high-resolution (300 m) along-track altimetry data from the Jason-1, 2 and 3 missions from January 2002 to December 2019, allows the analysis of the decadal evolution of coastal sea level and fills the coastal gap where sparse sea level information is currently available

Retrieval of Metop-A/IASI N2O Profiles and Validation with NDACC FTIR Data

International audienceThis paper reports atmospheric profiles of N2O retrieved from Metop/IASI with the Software for the Retrieval of IASI Data (SOFRID) for the 2008–2018 period and their validation with FTIR data from 12 stations of the Network for the Detection of Atmospheric Composition Changes (NDACC). SOFRID retrievals performed in the 2160–2218 cm−1 spectral window provide 3 independent pieces of information about the vertical profile of N2O. The FTIR versus SOFRID comparisons display a better agreement in the mid-troposphere (MT, 700–350 hPa) than in the lower (LT, Surface–700 hPa) and upper (UT, 350–110 hPa) troposphere with correlation coefficients (R) in the 0.49–0.83 range and comparable variabilities (3–5 ppbv). The agreement for oceanic and coastal stations (R > 0.77) is better than for continental ones (R < 0.72). The SOFRID MT N2O mixing ratios are significantly biased high (up to 16.8 ppbv) relative to FTIR at continental stations while the biases remain below 4.2 ppbv and mostly unsignificant when oceanic data are considered. The average MT decadal trends derived from SOFRID at the 8 NDACC stations with continuous observations during the 2008–2018 period (1.05 ± 0.1 ppbv·yr−1) is in good agreement with the corresponding FTIR trends (1.08 ± 0.1 ppbv·yr−1) and the NOAA-ESRL trends from surface in-situ measurements (0.95 ± 0.02 ppbv·yr−1). In the Northern Hemisphere where they are clearly detected, the N2O MT seasonal variations from SOFRID and FTIR are phased (summer minima) and have similar amplitudes. SOFRID also detects the UT summer maxima indicating independent MT and UT information. The global MT N2O oceanic distributions from SOFRID display low geographical variability and are mainly characterized by enhanced tropical mixing ratios relative to mid and high latitudes

CTOH studies for extending the range of altimetry applications over the ocean and continental surfaces

The Center for Topography of the Oceans and Hydrosphere (CTOH) is a French Observation Service created in 1989 and dedicated to satellite altimetry studies. It focuses on the development and promotion of new processing approaches of the altimetric data for emerging research domains (coastal ocean, oceanic sub-mesoscale phenomenons, continental surface water, sea ice, polar caps). It works in close relationship with space agencies (mainly CNES and ESA) at different levels for satellite altimetry missions: preparation of new missions, definition of the user’s needs, CAL/VAL studies, signal analysis & data reprocessing, development of thematic products, teaching and outreach. In terms of data distribution, the CTOH maintains a global GDR data base for almost all altimetry missions since Topex/Poseidon. All the products are made homogeneous (addition of the most recent parameters / corrections for the old missions) and provided in netcdf format. A new version of ERS-2 data, reprocessed by the CTOH for hydrological applications is also available (it includes ICE-1 and ICE-2 retrackers). Both 1Hz and 10/20/40Hz data are available globally. For some products, the CTOH database contains also L1 products (waveforms). In addition, the CTOH develops new altimetry products: • Fine-resolution ocean products: position of the main Southern Ocean polar fronts and global climatology of near-inertial current characteristics. • Coastal products : X-TRACK along-track SLA time series and tidal harmonic constants, reprocessed with a software designed for coastal altimetry, but also the recent high-resolution (20-Hz) X-TRACK/ALES sea level product, now reaching a distance of 3-4 km from the coast on average (see Leger et al., OSTST 2022). These products are distributed by AVISO+ and by ESA, respectively. • Continental hydrology products : including Topex/Poseidon reprocessed by the CASH project and water level maps developed for 5 rivers: the Amazon, Orenoque, Gange-Bramhapoutre, Congo and Mekong.It also contributes to the development of sea-ice products distributed by AVISO+ (Arctic Sea Ice Thickness and Snow depth monthly maps over sea ice) and to the “Hydroweb” data base for monitoring river and lake levels

Coastal sea level rise at Senetosa (Corsica) during the Jason altimetry missions

International audienceAbstract. In the context of the ESA Climate Change Initiative project, we are engaged in a regional reprocessing of high-resolution (20 Hz) altimetry data of the classical missions in a number of the world's coastal zones. It is done using the ALES (Adaptive Leading Edge Subwaveform) retracker combined with the X-TRACK system dedicated to improve geophysical corrections at the coast. Using the Jason-1 and Jason-2 satellite data, high-resolution, along-track sea level time series have been generated, and coastal sea level trends have been computed over a 14-year time span (from July 2002 to June 2016). In this paper, we focus on a particular coastal site where the Jason track crosses land, Senetosa, located south of Corsica in the Mediterranean Sea, for two reasons: (1) the rate of sea level rise estimated in this project increases significantly in the last 4–5 km to the coast compared to what is observed further offshore, and (2) Senetosa is the calibration site for the TOPEX/Poseidon and Jason altimetry missions, which are equipped for that purpose with in situ instrumentation, in particular tide gauges and a Global Navigation Satellite System (GNSS) antenna. A careful examination of all the potential errors that could explain the increased rate of sea level rise close to the coast (e.g., spurious trends in the geophysical corrections, imperfect inter-mission bias estimate, decrease of valid data close to the coast and errors in waveform retracking) has been carried out, but none of these effects appear able to explain the trend increase. We further explored the possibility that it results from real physical processes. Change in wave conditions was investigated, but wave setup was excluded as a potential contributor because the magnitude was too low and too localized in the immediate vicinity of the shoreline. A preliminary model-based investigation about the contribution of coastal currents indicates that it could be a plausible explanation of the observed change in sea level trend close to the coast

AlTiS Software for generating Time-Series of Water Levels from Radar Altimetry Data

Satellite radar altimetry can be used to determine water level height of continental water bodies, if used with appropriate processing that depends on the size and geometrical configuration of the targets. AlTiS (Altimetry Time Series) is software designed to visualize and process radar altimetry data with the goal of generating time series data of radar altimetry data over water bodies of different sizes such as river, lakes and wetlands. Even if its major goal is the creation of time-series of water levels derived from the altimetry range, it can also be used to generate time series of any other altimetry parameters (e.g., corrections applied to the range, backscattering coefficients, or brightness temperatures).Through a Graphical User Interface (GUI), without any skills in data processing, the user can handle altimetry data in order to: • Display several parameters of altimetry data like surface height, altimetric range, atmospheric corrections (ionosphere and wet and dry troposphere path delay corrections) and also to display some characteristic parameters of the waveform like the backscatter coefficient, and peakiness. • Graphically select altimetric measurements to remove outliers and easily done owing to Landsat background image. • Generate water height time series from the valid altimetry data previously selected • Export the time series into various files format as CSV and HydroWebAlTiS accepts CTOH altimetry products (Level 2 GDR supplied by the CTOH). CTOH GDR data have been specifically conditioned to optimize the data size by making a geographical selection and includes the right altimetry parameters for hydrological studies.AlTiS can process several altimetric data products from followed missions : Jason-1/2/3, ERS-2, ENVISAT, SARAL, Sentinel-3A/B, and soon, Sentinel-6/Jason-CS and the nadir altimeter onboard SWOT. They are supplied for free through a web request form on the CTOH website (http://ctoh.legos.obs-mip.fr/applications/land_surfaces/altimetric_data/altis).AlTiS is mainly employed for hydrological applications and can be used for training courses on radar altimetry at bachelor or master levels. It is also a very convenient tool to analyse the radar altimetry data contained in the GDR over any type of land surfaces.AlTiS is a software developed by CTOH as part of its activities as a National Observation Service. AlTiS is a free software and it is released as an open source under the CeCill License. Altis is working under python3 environment and tested for GNU/Linux, Windows 10/11.AlTiS is available on GitLab : https://gitlab.com/ctoh/alti

The X-TRACK/ALES multi-mission processing system: New advances in altimetry towards the coast

In the context of the ESA Climate Change Initiative project, a new coastal sea level altimetry product has been developed in order to support advances in coastal sea level variability studies. Measurements from Jason-1,2&3 missions have been retracked with the Adaptive Leading Edge Subwaveform (ALES) Retracker and then ingested in the X-TRACK software with the best possible set of altimetry corrections. These two coastal altimetry processing approaches, previously successfully validated and applied to coastal sea level research, are combined here for the first time in order to derive a 16-year-long (June 2002 to May 2018), high-resolution (20-Hz), along-track sea level dataset in six regions: Northeast Atlantic, Mediterranean Sea, West Africa, North Indian Ocean, Southeast Asia and Australia. The study demonstrates that this new coastal sea level product called X-TRACK/ALES is able to extend the spatial coverage of sea level altimetry data ∼3.5 km in the land direction, when compared to the X-TRACK 1-Hz dataset. We also observe a large improvement in coastal sea level data availability from Jason-1 to Jason-3, with data at 3.6 km, 1.9 km and 0.9 km to the coast on average, for Jason-1, Jason-2 and Jason-3, respectively. When combining measurements from Jason-1 to Jason-3, we reach a distance of 1.2–4 km to the coast. When compared to tide gauge data, the accuracy of the new altimetry near-shore sea level estimations also improves. In terms of correlations with a large set of independent tide gauge observations selected in the six regions, we obtain an average value of 0.77. We also show that it is now possible to derive from the X-TRACK/ALES product an estimation of the ocean current variability up to 5 km to the coast. This new altimetry dataset, freely available, will provide a valuable contribution of altimetry in coastal marine research community