Abstract
Abstract. 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.
Highlights
Satellite altimetry measurements, based on nadir radar altimeters, have made a fundamental contribution to the un-Published by Copernicus Publications on behalf of the European Geosciences Union.A
The contribution of wide-swath altimetry data to ocean analyses and forecasts was assessed in the IBI region over a 1year period (2009) by means of observing system simulation experiments (OSSEs)
Five different experiments were designed simulating different constellations of satellites, composed of nadir and wide-swath altimeters, and the results were compared with the NR
Summary
Satellite altimetry measurements, based on nadir radar altimeters, have made a fundamental contribution to the un-. The calibration of OSSEs with respect to OSEs is important for obtaining robust results from OSSEs (Halliwell et al, 2014, 2017; Kourafalou et al, 2016) In this sense, the choice of the NR, AR, the data assimilation scheme (DAS) and the errors to be considered for the synthetic observations have to be carefully analysed to avoid unrealistic forecast and analysis errors in the OSSEs. In this study we investigated the potential impact of future constellations of satellite altimeters, based on nadir and wide-swath missions, using a regional ocean analysis and forecasting system implemented in the Iberian–Biscay–Ireland (IBI) region at a spatial resolution of 1/12◦.
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