The Mean Dynamic Topography (MDT) is a key reference surface for altimetry. It is needed for the calculation of the ocean absolute dynamic topography, and under the geostrophic approximation, the estimation of surface currents. Those are required for a wide range of applications as the management of fishery resources, the monitoring of potential pollution, maritime security… Also, the MDT is the missing component for the optimal assimilation of altimeter data into operational ocean system as those run under the Copernicus Marine Environment Monitoring Services (CMEMS).

CNES-CLS Mean Dynamic Topography solutions are calculated by merging information from altimeter data, GRACE and GOCE gravity data and oceanographic in-situ measurements from drifting buoy velocities and hydrological profiles. The objective of this communication is to present the newly updated CNES-CLS18 MDT. The main novelties compared to the previous CNES-CLS13 solution is the use of updated input datasets: the GOCO05S geoid model (instead of DIR4) is used together with all drifting buoy velocities (SVP-type and Argo floats) and hydrological profiles (CORA database) available from 1993 to 2017 (instead of 1993-2012). The new solution also benefits from improved data processing (in particular, a new Ekman model is used to extract the geostrophic component from the buoy velocities) and methodology (in particular, the correlation scales used for the multivariate mapping have been revised).

An evaluation of this new solution compared to the previous version and to other existing MDT is done through comparison to independent in-situ data. Further validation by “super-users” such as monitoring and forecasting centres have also been performed. Compared to the CNES-CLS13 solution, the new CNES-CLS18 MDT shows improved performance everywhere and more significantly in coastal areas and in strong western boundary currents.

The DUACS multimission system curretly merges data from 5 satellites (Jason-3, Sentinel-3A, SARAL-DP/Altika, and Cryosat-2, OSTM/Jason-2) in Near Real Time to deliver high quality altimetry Sea Level products for oceanographic applications. This constellation is foreseen to be completed with Sentinel-3B in the interleaved orbit in early 2019. This high number of altimeter missions contributes to the robustness of the system and the product quality, but also opens new perspectives in terms of resolution. Using the global Synthetic Aperture Radar mode (SARM) coverage of Sentinel-3A and B, allow us to fully exploit the fine-scale content of the altimetric missions.
In parallel, altimeter standards and processing are involving in order to reduce as much as possible the altimeter errors, in particular at small mesoscales. Previous studies underlined the significant contribution of the MSS in the altimeter errors at wavelengths shorter than ~100km, avoiding us to take full benefit of the noise-reducing SARM technology. The up-to-date MSS errors indeed represent nearly 30% of the SLA variance along uncharted track as the Sentinel-3A and Sentinel-3B interleaved. In that context, nearly past 2 years of Sentinel-3A measurement were used to build a new local MSS (also known as mean profile) along the new ground track, leading to a significant reduction of the errors for critical wavelengths ranging from 30 to 100 km. The use of a precise mean profile thus contributes to the full exploitation of the measurement at small spatial scales. This contribution became all the more important when considering measurement processed with new algorithms, as the new stacking method LR-RMC that allow to reduce the red noise signal up-to-now observed on the SAR measurements.

The impact of forthcoming wide-swath altimetry missions on the ocean analysis and forecasting system was investigated by means of OSSEs (observing system simulation experiments). These experiments were performed with a regional data assimilation system,implemented in the Iberian–Biscay–Ireland (IBI) region, at 1/12° resolution using simulated observations derived from a fully eddy-resolving free simulation at 1/36° resolution over the same region. The objective of the experiments was to assess the ability of different satellite constellations to constrain the ocean analyses and forecasts, considering both along-track altimeters and future wide-swath missions; consequently, the capability of the data assimilation techniques used in the Mercator Ocean operational system to effectively combine the different kinds of measurements was also investigated. These assessments were carried out as part of a European Space Agency (ESA) study on the potential role of wide-swath altimetry in future versions of the European Union Copernicus programme. The impact of future wide-swath altimetry data is evident for investigating the reliability of sea level values in OSSEs. The most significant results were obtained when looking at the sensitivity of the system to wide-swath instrumental error: considering a constellation of three nadir and two “accurate” (small instrumental error) wide-swath altimeters, the error in ocean analysis was reduced by up to 50 % compared to conventional altimeters. Investigating the impact of the repetitivity of the future measurements, the results showed that two wide-swath missions had a major impact on sea-level forecasting, increasing the accuracy over the entire time window of the 5-day forecasts – compared with a single wide-swath instrument. A spectral analysis underlined that
the contributions of wide-swath altimetry data observed in ocean analyses and forecast statistics were mainly due to the more accurate resolution, compared with along-track data, of ocean variability at spatial scales smaller than 100 km. Considering the ocean currents, the results confirmed that the information provided by wide-swath measurements at the surface is propagated down the water column and has a considerable impact (30 %) on ocean currents (up to a depth of 300 m), compared with the present constellation of altimeters. The ocean analysis and forecasting systems used here are those currently used by the Copernicus Marine Environment and Monitoring Service (CMEMS) to provide operational services and ocean reanalysis. The results obtained in the OSSEs considering along-track altimeters were consistent with those derived from real data (observing system experiments, OSEs). OSSEs can also be used to assess the potential of new observing systems, and in this study the results showed that future constellations of altimeters will have a major impact on constraining the CMEMS ocean analysis and forecasting systems and their applications.

Thanks to its current accuracy and maturity, altimetry is considered as a fully operational observing system dedicated to scientific and operational applications. In order to access the targeted ocean signal, altimeter measurements are corrected for several geophysical parameters among which the ocean tide correction is one of the most critical. Global ocean and loading tide models GOT and FES are operationally used in present altimeter GDRs to remove the barotropic tide component, but the internal tides signature at the surface are not yet corrected globally.

Internal tides can have a signature of several cm at the surface with wavelength about 50-250 km for the first mode and even smaller scales for higher order modes. The perspective of SWOT mission and high-resolution ocean measurements make the correction of these small scale signals a challenge, as we need to be able to separate all tidal variability from other oceanic signals.

A detailed comparison and validation of several internal tide models developed by different scientific teams (Egbert and Erofeeva 2015, Ray and Zaron 2016, Zaron 2016, Zhao et al. 2012, Dushaw 2015, Ubelmann-pers comm, Ngodock et al. 2016) has been performed recently; the analysis is focused on the coherent internal tide signal for the main tidal components M2, K1, O1 and S2 frequencies.
The validation process is mostly based on the use of a multi-missions altimetry database including Jason-2, AltiKa and Cryosat-2 data, and on a combined statistical and spectral analysis approaches. Some in situ data have been also used in some regions.
The results show a significant altimeter variance reduction when using such a new internal tide correction on all ocean regions where internal tides are generating/propagating.
We will present the last results of this inter-comparison analysis and some insight on the estimation of the non-stationary part of internal tides in different regions studied.

Passive microwave (PMW) observations of sea surface temperature (SST) benefit global ocean and SST analyses because of their near-all-weather capability. Present PMW SST observations have real aperture-limited spatial resolution in excess of 50 km, limiting the spatial fidelity with which SST features reflecting ocean dynamics can be captured. This contrasts with the target resolution of global analyses of 5 to 10 km. Moreover, continuity for PMW SST observations is not presently assured. The Copernicus Imaging Microwave Radiometer (CIMR) is a mission concept under consideration as a high-priority candidate mission for expansion of the Copernicus space programme (the Sentinels). The concept includes deployment of a ~7 m mesh antenna capable of real aperture resolution <15 km at 6.9 GHz (required for cool-water SST estimation). CIMR is motivated by the EU Arctic policy, whereby monitoring of the physical properties of the changing high-latitude environment is essential for sustainability, economic development and environmental protection. The CIMR mission will deliver both high-latitude and global data with near global coverage every day. CIMR SSTs will therefore improve the observability of ocean dynamics reflected in SST compared to an infra-red-only SST dataset, particularly in areas of persistent cloud: the Arctic, southern ocean, north Pacific and Atlantic oceans, marine stratiform regions and the inter-tropical convergence zone including the tropical warm pool. CIMR will have low radiometric uncertainty and both forward and nadir scans will be available, meaning that CIMR SSTs will have simultaneously higher spatial resolution and lower noise uncertainty than present PMW SSTs. The presentation will give results of a comparative study of SST uncertainty and feature resolution with and without the availability of CIMR in the future SST-observing satellite constellation, based on a detailed simulation of CIMR plus infra-red observations and the processing of global SST analysis at 0.05 deg. Improvements in SST uncertainty and the fidelity of ocean-dynamic SST signatures will be quantified.

The S1 Level-2 WV OCN product contains three geophysical components: the radial surface velocity (RVL), the ocean surface wind field (OWI) and the ocean swell wave spectra (OSW). The RVL radial velocity is estimated from the difference between measured Doppler centroid and predicted geometric Doppler shift from pointing and attitude information. Unfortunately, the accuracy of attitude information onboard Sentinel1 proved to be not sufficient for the expected performance of RVL product and a new vicarious calibration methodology based on a combination of raw Gyro and Star trackers information together with Doppler measurements is discussed. Resulting calibrated RVL product is then interpreted in terms of surface current thanks to a new estimates of the sea state Doppler based on a cross spectral parameter. The main difference between the former ASAR WM Level 2 product and S1 OCN product is higher range bandwidth (74.5 MHz versus 16 MHz) and larger imagettes (20x20km2) versus 10x5km2) allowing improved detection of very short waves and very long swell systems. As part of the Level 2 processing, full resolution cartesian cross spectra (CCS) are generated by inter-looking the SLC image providing complex spectra with range spectral components down to around 5 meters. This CCS is shown to provide a useful proxy for Sea state contribution to the Doppler centroid anomaly (DCA). Finally the resulting surface current is compared to drifter data at global scale from wave mode data and at regional scale over large western boundary currents such as the Agulhas current from IW mode data.