189 research outputs found

    JASON-1 CALVAL experiences in Cape of Begur and Ibiza island

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    The direct and indirect calibration experiences made at the Cape of Begur area in 1999, 2000 and 2002, for Topex/Poseidon and at the Ibiza island in 2003 have contributed to the international campaigns made at Harvest (USA), Corsica (France) and Bass (Australia). The main objective of IBIZA 2003 campaign has been the determination of the instantaneous sea surface/marine geoid gradient along Jason-1 tracks using a GPS catamaran and a network of GPS located in Portinatx and Ibiza and San Antonio harbours. The marine geoid will be used to relate the tide gauge coastal data with the altimeter data. We present the first results obtained with static and kinematic analysis of the data using different softwares.Peer ReviewedPostprint (published version

    Improved satellite altimeter mapped sea level anomalies in the Mediterranean Sea: A comparison with tide gauges

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    The new gridded Mediterranean sea level anomaly product recently released by AVISO (DT14) is evaluated and compared with the earlier version (DT10) at which it is aimed to substitute. Differences between the two products are found along coastal regions, where the new version captures more variability (up to 10% more) and trends locally differ by up to 1 mm/yr for the altimetric period. Coastal tide gauge observations have therefore been used as the basis for quantifying changes in DT14. Correlation and variance reduction in available monthly tide gauge time series are improved in more than 80% of the selected sites by up to 0.2 and 5 cm2, respectively. This resulted in an overall higher skill to recover coastal low frequency (with periods larger than a few months) sea level signals. Results for higher/lower order percentiles were also explored and showed different performances depending on the site, although with a slight overall improvement. A comparison with tide gauges on a daily basis using wavelet analysis reveals that altimetry gridded products are not capable of recovering higher frequency (a few days) coastal sea level signals despite some advances have been achieved thanks to the daily temporal sampling of DT14M. Marcos acknowledges a “Ramon y Cajal” contract funded by the Spanish Ministry of Economy. Funding from the MyOcean-Follow-On project (PRE-OPERATIONAL MARINE CONTINUITY IN TRANSITION TOWARDS COPERNICUS- H2020-SPACE/0003) is acknowledgedPeer Reviewe

    Validation of CryoSat-2 SIRAL sea level data in the eastern continental shelf of the Gulf of Cadiz (Spain)

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    We present the validation of sea level measurements taken over the eastern shelf of the Gulf of Cadiz (southwestern Iberian Peninsula) by the European Space Agency CryoSat-2 satellite’s SIRAL altimeter in SAR mode. Time series of sea level anomaly (SLA) at 20-Hz posting rate (corresponding to approximately 350 m along the satellite track) are compared against sea level measurements from two tide gauge stations located along the Spanish coast of the study area. Due to the long repeat cycle of CryoSat-2 (369 days), data selection is performed by including all the tracks falling in a radius of 50 km from the location of the tide gauge stations. Then, time series of SLA are formed during the observational time period (August 2010–December 2014) by taking the altimeter measurements located at growing distances from the coast (1, 3, 5, 10, 15 and 20 km) as well as around four isobaths (10, 25, 50 and 75 m). The validation of AltiKa’s SARAL altimeter 20-Hz data, using its two 35-day-repeat ground tracks available in the area is also presented. Overall, CryoSat-2 20-Hz SLAs selected in the coastal strip from 3 to 20 km agree well with data from the western station (Huelva) with rmse ranging from 11.4 to 6.4 cm. The analysis at the eastern station (Bonanza) yields rmse ranging from 16.8 to 14.6 cm. The comparisons made with CryoSat-2 20-Hz SLAs are in agreement with the rmse estimated using AltiKa 20-Hz SLAs at distances to the coast higher than 5 km, and much better in the 1–5 coastal band. The differences in quantity of SAR mode data observed at the two stations and in their agreement with the tide gauges can be explained by (1) the more complex coastal morphology around Bonanza station with the altimetry tracks almost parallel to the coast; (2) the noisier hydrodynamics around Bonanza station due to the presence of the estuary mouth of the Guadalquivir River; and (3) residual (uncorrected) tidal signals in the Bonanza gauge record. We conclude that the quality of SAR altimetry is comparable or even slightly better than conventional altimetry, thus qualifying SAR data for exploitation in oceanographic studies and sea level monitoring. An example is the analysis made of the surface signal due to a heavy Guadalquivir River discharge event

    The status of measurement of the Mediterranean mean dynamic topography by geodetic techniques

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    We review the measurement of the mean dynamic topography (MDT) of the Mediterranean using ellipsoidal heights of sea level at discrete tide gauge locations, and across the entire basin using satellite altimetry, subtracting estimates of the geoid obtained from recent models. This ‘geodetic approach’ to the determination of the MDT can be compared to the independent ‘ocean approach’ that involves the use of in situ oceanographic measurements and ocean modelling. We demonstrate that with modern geoid and ocean models there is an encouraging level of consistency between the two sets of MDTs. In addition, we show how important geodetic MDT information can be in judging between existing global ocean circulation models, and in providing insight for the development of new ones. The review makes clear the major limitations in Mediterranean data sets that prevent a more complete validation, including the need for improved geoid models of high spatial resolution and accuracy. Suggestions are made on how a greater amount of reliable geo-located tide gauge information can be obtained in the future

    Comparison of sea-ice freeboard distributions from aircraft data and cryosat-2

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    The only remote sensing technique capable of obtain- ing sea-ice thickness on basin-scale are satellite altime- ter missions, such as the 2010 launched CryoSat-2. It is equipped with a Ku-Band radar altimeter, which mea- sures the height of the ice surface above the sea level. This method requires highly accurate range measure- ments. During the CryoSat Validation Experiment (Cry- oVEx) 2011 in the Lincoln Sea, Cryosat-2 underpasses were accomplished with two aircraft, which carried an airborne laser-scanner, a radar altimeter and an electro- magnetic induction device for direct sea-ice thickness re- trieval. Both aircraft flew in close formation at the same time of a CryoSat-2 overpass. This is a study about the comparison of the sea-ice freeboard and thickness dis- tribution of airborne validation and CryoSat-2 measure- ments within the multi-year sea-ice region of the Lincoln Sea in spring, with respect to the penetration of the Ku- Band signal into the snow

    Comparison of the X-TRACK altimetry estimated currents with moored ADCP and HF radar observations on the West Florida Shelf

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    The performance of coastal altimetry over a wide continental shelf is assessed using multiple-year ocean current observations by moored Acoustic Doppler Current Profilers (ADCP) and high frequency (HF) radar on the West Florida Shelf. Across track, surface geostrophic velocity anomalies, derived from the XTRACK along-track sea level anomalies are compared with the near surface current vector components from moored ADCP observations at mid shelf. The altimeter derived velocity anomalies are also directly compared with the HF radar surface current vector radial components that are aligned perpendicular to the satellite track. Preliminary results indicate the potential usefulness of the along-track altimetry data in contributing to descriptions of the surface circulation on the West Florida Shelf and the challenges of such applications. On subtidal time scales, the root mean square difference (rmsd) between the estimated and the observed near surface velocity component anomalies is 8 11 cms-1, which is about the same magnitude as the standard deviations of the velocity components themselves. Adding a wind-driven Ekman velocity component generally helps to reduce the rmsd values
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