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Coastal Altimetry for the North-Eastern Atlantic Shelf
Luciana Fenoglio-Marc1, Salvatore Dinardo3, Christopher Buchhaupt2, Bernd Uebbing1, Remko Scharroo3, Jürgen Kusche1, Matthias Becker2, Jerome Benveniste4
1University Bonn, Germany; 2Technical University Darmstadt; 3EUMETSAT; 4ESA/ESRIN
Several studies have demonstrated the potential of conventional low resolution mode (LRM) altimetry in the coastal zone when waveforms are reprocessed and corrections optimized. SAR altimetry is expected to provide even better measurements of sea surface height, significant wave height and wind speed in the coastal zone than conventional altimetry Current projects like SCOOP (SAR Altimetry Coastal & Open Ocean Performance), SLCCI (Sea Level Climate Change Initiative) and the GOCE++Dycot (GOCE++ Dynamic Topography at the Coast and Tide Gauge Unification) are actually investigating the performance of coastal altimetry data produced by standard and improved processing schemes. Improved processing include consideration of sub-waveform CA re-trackers as well as various approaches in SAR altimetry, as Hamming Weighting Window on the burst data prior to the azimuth FFT, Zero-Padding Technique prior to the range FFT, doubling of the extension for the radar range swath, improved thermal noise estimated from beams in Stack data.
We investigate the North-Eastern Atlantic shelf from Lisbon to Bergen to verify the increase in coastal performances with SAR processing.
We consider SAR CryoSat-2 altimetry products from GPOD and SCOOP project, SAR altimetry Sentinel-3 PDGS products from ESA/EUM. We build pseudo-LRM (PLRM) from the SAR products mentioned above. As conventional altimetry data we use Envisat and Jason-2 data retracked by a sub-waveform retracker and the ESA Sea Level Climate Change Initiative products.
We inter-compare and compare the various products with in-situ sea level data from the German Federal Institute of Hydrology (BfG) and Système d'Observation du Niveau des Eaux Littorales (SONEL) databases to reach three specific goals. First we assess the improved accuracy of the SAR altimetry measurements by near-simultaneous comparison with the tide gauge stations Then we assess the long-term performance of SAR altimetry by comparison with tide gauge records corrected for the vertical motion, estimated by GPS. Finally we assess the contribution of SAR in deriving an improved coastal mean dynamic topography by point-wise comparison with mean sea level topography derived from in-situ observations.
11:15 - 11:35
Validation of CryoSat-2 and AltiKa Sea Level Anomaly in the Coastal Strip of the Gulf of Cadiz
Jesus Gomez-Enri1, Paolo Cipollini2, Stefano Vignudelli3, Josep Coca4
1University of Cadiz, Spain; 2National Oceanography Centre, UK; 3Consiglio Nazionale delle Ricerche, Italy; 4University of Cadiz, Spain
Four and half years of CryoSat-2 SIRAL data in SAR mode (at 18-Hz posting rate) and 2 years of AltiKa SARAL data (at 1 Hz) have been validated using two radar MIROS tide gauges in the Gulf of Cadiz: Huelva (HU), located close to the Tinto-Odiel river system, and Bonanza (BN), at the mouth of the main tributary in the study area, the Guadalquivir River. Tide gauge data were provided by Puertos del Estado (https://www.puertos.es/) at 5-minute interval. Data from CryoSat-2 SIRAL (processed according to baseline B) were provided by the European Space Agency (ESA) Grid Processing On-Demand (GPOD) SARvatore (SARversatile altimetric toolkit for ocean research & exploitation) service available at: https://gpod.eo.esa.int/. The data selection was made considering the track segments in a radius of 50 km from the tide gauges. AltiKa data, from two exact-repeat tracks in the vicinity of the tide gauge, were obtained from the Radar Altimeter Data System (RADS) available at: http://rads.tudelft.nl/rads/rads.shtml. Time series of Sea Level Anomalies (Cryo_SLA and AltK_SLA) were compared against in-situ SLA data: HU_SLA (Huelva) and BN_SLA (Bonanza). We estimated the root mean square error (rmse) between satellite and tide gauge data. Time series of Cryo_SLA were built selecting the locations at six different along-track distances to the coast: 1 - 3 - 5 -10 -15 - 20 km. Our results show that rmse increases getting closer to the coast at both stations. The rmse at Bonanza is twice than Huelva. The standard deviation observed in the 5-minutes time series at Huelva/Bonanza is 8/18 cm, respectively. This might indicate a larger signal due to the the Guadalquivir River where Bonanza tide gauge is located: part of the difference found between Cryo_SLA and BN_SLA might be attributed to this dynamic, which is concentrated in the estuary mouth and is not captured by the altimeter instrument as no valid Cryo_SLA data were found in that particular zone. The analysis of AltiKa data shows rmse smaller than 8 cm along the track segment close to Huelva station. The rmse is bigger than 18 cm in the track segment around Bonanza station, also suggesting the river output as source of the noise.
11:35 - 11:55
Corsica: A Multi-Mission Absolute Calibration Site
Pascal Bonnefond1, Olivier Laurain2, Pierre Exertier2, Thierry Guinle3, Pierre Féménias4
1Observatoire de Paris - SYRTE, France; 2Observatoire de la Côte d'Azur - Géoazur, France; 3CNES, France; 4ESA/ESRIN, Italy
In collaboration with the CNES and NASA oceanographic projects (T/P and Jason), the OCA developed a verification site in Corsica since 1996. CALibration/VALidation embraces a wide variety of activities, ranging from the interpretation of information from internal-calibration modes of the sensors to validation of the fully corrected estimates of Sea Surface Heights using in situ data. Now, Corsica is, like the Harvest platform (NASA side), an operating calibration site able to support a continuous monitoring with a high level of accuracy: a ’point calibration’ which yields instantaneous bias estimates with a 10-day repeatability of around 30 mm (standard deviation) and mean errors of 3-4 mm (standard error). For a 35-day repeatability (ERS, EnviSat, SARAL/AltiKa), due to a smaller time series, the standard error is about the double (~7 mm).
In-situ calibration of altimetric height (SSH for ocean surfaces) is usually done at the vertical of a dedicated CAL/VAL site, by direct comparison of the altimetric data with in-situ data. Adding the GPS-based sea level measurements to the “traditional” tide gauges ones, it offers the great opportunity to perform a cross control that is of importance to insure the required accuracy and stability. This configuration leads to handle the differences compare to the altimetric measurement system at the global scale: the Geographically Correlated Errors at regional (orbit, sea state bias, atmospheric corrections...) and local scales (geodetic systematic errors, land contamination for the instruments, e.g. the radiometer).
Our CAL/VAL activities are thus focused not only on the very important continuity between past, present and future missions but also on the reliability between offshore and coastal altimetric measurement. With the extension of the Corsica site (Capraia in 2004 and Ajaccio in 2005), we are now able to perform absolute altimeter calibration for ERS -2, EnviSat, HY-2A, CryoSat-2 and SARAL/AltiKa with the same standards and precision than for T/P and Jason missions. The Sentinel-3 mission is also naturally included in our CAL/VAL activities. This will permit to improve the essential link between all these long time series of sea level observation.
The presented results will include the full set of TOPEX/Poseidon, Jason-1 and Jason-2 GDR products. Updated values of the SSH bias for Jason-1 based on GDR-E will be also presented. However, this presentation will be focused on Jason-3 data and the comparison with Jason-2 over the Formation Flight Period. Preliminary results for Sentinel-3A will be also presented and the improvement thanks to SAR will be estimated.