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Session 2D - Application and validation over Ocean
4:10pm - 5:10pm
Session Chair: Walter H F Smith
Chair: G. Costa
4:10pm - 4:30pm
The Great Value of Cryosat-2 SAR-in for Coastal Sea Level Monitoring
Ole Baltazar Andersen1, Martina Idzanovic2, Vegard Ophaug2, Adil Abulaitijiang1
1DTU space, Denmark; 2NMBU, Norway
Cryosat-2 operates in SAR-in for selected coastlines in the world. We perform an evaluation and cross-comparison with conventional altimetry and coastal tide gauges to demonstrate the great stop forward in accurate coastal sea level mapping that Cryosat-2 SAR-in data can provide.
The investigation of SAR-in data in Greenland adds an entire new dimension to coastal altimetry. An amazing result of the investigation is the ability of Cryosat-2 to detect and recover sea level even though the coast (sealevel) is up to 15 km away from the nadir location of the satellite.
This ability of capture and use returns from outside the main (-3Db) loop in theory enables Cryosat-2 SAR-in to map sea level height of fjords more frequently than the 369 days repeat.
4:30pm - 4:50pm
Evaluating the Performance of Sentinel-3 SRAL SAR Altimetry in the Coastal and Open Ocean, and Developing Improved Retrieval Methods – The ESA SCOOP Project
David Cotton1, Thomas Moreau2, Eduard Makhoul Varona3, Paolo Cipollini4, Mathilde Cancet5, Francisco Martin6, Luciana Fenoglio-Marc7, Marc Naeije8, M Joana Fernandes9, Marco Restano10, Américo Ambròsio11, Jérôme Benveniste12
1Satellite Oceanographic Consultants, United Kingdom; 2Collecte Localisation Satellites, France; 3isardSAT, UK; 4National Oceanography Centre, UK; 5Noveltis, France; 6Starlab, UK; 7University of Bonn, Germany; 8Delft University of Technology, The Netherlands; 9University of Porto, Portugal; 10SERCO / ESRIN, Italy; 11DEIMOS / ESRIN, Italy; 12ESA-ESRIN, Italy
The ESA Sentinel-3 satellite, launched in February 2016 as a part of the Copernicus programme, is the second satellite to operate a SAR mode altimeter. The Sentinel 3 Synthetic Aperture Radar Altimeter (SRAL) is based on the heritage from Cryosat-2, but this time complemented by a Microwave Radiometer (MWR) to provide a wet troposphere correction, and operating at Ku and C-Bands to provide an accurate along-track ionospheric correction.
SCOOP (SAR Altimetry Coastal & Open Ocean Performance) is a project funded under the ESA SEOM (Scientific Exploitation of Operational Missions) Programme Element, started in September 2015, to characterise the expected performance of Sentinel-3 SRAL SAR mode altimeter products, in the coastal zone and open-ocean, and then to develop and evaluate enhancements to the baseline processing scheme in terms of improvements to ocean measurements. There is also a work package to develop and evaluate an improved Wet Troposphere correction for Sentinel-3, based on the measurements from the on-board MWR, further enhanced mostly in the coastal and polar regions using third party data, and provide recommendations for use.
At the end of the project recommendations for further developments and implementations will be provided through a scientific roadmap.
In this presentation we provide an overview of the SCOOP project, highlighting the key deliverables and discussing the potential impact of the results in terms of the application of delay-Doppler (SAR) altimeter measurements over the open-ocean and coastal zone. We also present the initial results from the project, including:
Key findings from a review of the current “state-of-the-art” for SAR altimetry,
Specification of the initial “reference” delay-Doppler and echo modelling /retracking processing schemes,
Evaluation of an initial Test Data Set in the Open Ocean and Coastal Zone, processed from Cryosat FBR data, using a processing scheme designed to be equivalent to the Sentinel-3 baseline processor
Overview of modifications planned to the reference delay-Doppler and echo modelling/ re-tracking processing schemes.
This work builds on findings from the Cryosat Plus for Oceans (CP4O) study, in which new processing schemes for Cryosat SAR mode data were developed and evaluated with a view to supporting a range of open ocean and coastal zone applications , and continues to be highly relevant to further exploitation of Cryosat data in theseapplications.
The SCOOP test data sets and relevant documentation are available to external researchers on application to the project team.
4:50pm - 5:10pm
Using Cryosat to Improve the Observation of Global Oceanic Internal Tides
University of Washington, United States of America
Our recent work demonstrates the usefulness of Cryosat altimeter data in observing global oceanic internal tides (Zhao 2016 JGR). It has long been known that TOPEX/Poseidon (T/P) can detect internal tides via their centimeter-scale sea surface height (SSH) displacements, which correspond to tens of meters of subsurface displacements. T/P altimetric observations, however, are limited to 254 discrete tracks, leaving most of the ocean unsampled. Even worse, the cross-track spacing of T/P (200~300 km) is wider than the wavelength of M2 internal tides (~150 km). For comparison, Cryosat has a much higher spatial resolution with 10688 tracks around the globe; this orbit configuration determines that Cryosat is superior to T/P for monitoring global internal tides.
Traditional harmonic analysis is incapable of extracting internal tides from Cryosat altimeter data. The Cryosat data series at any given point is short, due to its 369-day-long repeat period. We take up this challenge by developing a plane wave fit method (Zhao 2016 JGR). Using our new technique, we extract internal tides by fitting plane waves in large horizontal windows covering multiple tracks, instead of at individual points. We demonstrate that M2 internal tides can be extracted using 4 years of Cryosat data from 2011 to 2014 (CryoSat4yr). Likewise, M2 internal tides are extracted using 60 satellite years of SSH measurements made by exact-repeat missions (T/P--Jason-1/2, ERS-1/2--Envisat, and GFO) from 1992 to 2012 (MultiSat20yr). CryoSat4yr and MultiSat20yr are in good agreement in the central North Pacific, although they are from satellite data of different sampling patterns (1998 versus 10688 tracks) and different observational periods (60 versus 4 years). Further comparisons reveal that the internal tide field is subject to significant seasonal and interannual variability.
Internal tides have drawn great research interest in recent years, because they play an important role in a variety of ocean processes. The satellite altimeter’s near global coverage offers a great improvement over moored and shipboard measurements. Previous studies mainly observe global internal tides using exact-repeat satellite altimeter missions. The unique Cryosat dataset, combined with the newly developed analysis technique, has a potential to better observe internal tides on a global scale.