Analysis of Altimetry Range and Correction in a Flat Coastal Environment at Aix Island Sea-Level Observatory, France
1LEGOS, France; 2LIENSs, France; 3SYRTE, France; 4GEOAZUR, France
Satellite altimetry in coastal areas is a challenging task due to both altimeter and radiometer contamination by land. However, the ability to use satellite altimetry data as close as possible to the coast would be invaluable and would benefit numerous applications such as local sea-level monitoring or hydrodynamic modeling and coastal oceanography.
Aix island is located between the two flat elongated islands of Ré and Oléron, which define a >10km wide inlet, sufficiently large for most altimetry mission to provide some measurements.
The Ile d’Aix sea-level observatory provides in-situ data, such as tide gauge and GPS, which can be used to validate final altimetric heights and improve specific corrections. In this work, we will investigate the quality of the SSB corrections provided by the altimeter by comparison with significant wave height provided by a local hydrodynamic model and an offshore wave gauge. Due to the site configuration and the sheltering of Ré and Oléron islands, methods based on the extrapolation of corrections from the less-contaminated offshore zones is not adequate.
We also look at the improvement of the tropospheric delay correction when using local tropospheric delay modeled from the GPS stations of Ile d’AIX (ILDX) and La Rochelle (LROC), with respect to using the tropospheric delay estimated from the land-contaminated radiometer.
Enhance Coastal Tide Modeling Using Cryosat-2: A Feasibility Study
During the last years significant improvements have been achieved in coastal altimetry. Advances in observational techniques, data processing and corrections brought to a higher accuracy in sea level estimations, reaching remarkable results within few kilometers from the shore. In particular, geophysical corrections have a significant impact on coastal products and therefore their constant improvements are crucial. Tide correction is the principal contributor in sea level determination and its solutions have dramatically improved through the last years. However, discrepancies of around 1 meter are still found among different tide models at short distances from the coast. With the unprecedented design of CryoSat’s radar altimeter, high-performance observations over littoral areas are reached, showing higher resolution and Signal to Noise Ratio up to 2 km from the coast. Indeed, these promising features appear as a good opportunity for improving coastal tide representation. The aim of this work is to investigate on the possibility to exploit CryoSat-2 data within a tide model, focusing on the enhancement over coastal areas. This study is based on the Empirical Ocean Tide (EOT11a) model released by DGFI-TUM in 2012, which was developed with the spherical harmonic method. The difficulty in implementing this technique with CryoSat stays in the fact that short-repeat orbits are required in order to compute the aliasing periods of single constituents. In the case of CryoSat the same principle cannot be applied because of its long repeat cycle. However, alternative approaches such as a gridding strategies and the comparison of local sampling with major tide oscillations will be presented. The preliminary experiment will be carried out in the open ocean and the results will be compared with external models and values from short-repeat missions.
Evaluation of the Dry and Wet Tropospheric Corrections for CryoSat-2 and Sentinel-3 Over Inland Waters
1Universidade do Porto, Faculdade de Ciências, Porto, Portugal; 2Centro Interdisciplinar de Investigação Marinha e Ambiental (CIIMAR/CIMAR), Universidade do Porto, Porto, Portugal
Errors of up to few centimetres are presently associated with the dry and wet tropospheric corrections (DTC and WTC, respectively) given in Level 2 (L2) altimeter products over inland water regions, mainly because these corrections are usually referred to other than the appropriate height reference. These errors have been analysed in the scope of the Sentinel-3 Hydrologic Altimetry PrototypE (SHAPE) project, aiming at developing improved corrections for CryoSat-2 (CS-2) and Sentinel-3 (S3) L2 products. For this evaluation, regions of interest (ROI) covering rivers and lakes where CS-2 is operating in SAR and SAR-In modes, e.g. Amazon and Danube rivers and Titicaca and Baikal lakes have been selected.
For CS-2, the tropospheric corrections provided by Level 1B products have been compared against corrections computed from the ECMWF operational model at: i) the level of ECMWF model orography; ii) the level of the ACE2 digital elevation model; iii) the mean lake level, derived from Envisat data, or river profile derived in the scope of this project by AlongTrack (ATK) from Jason-2 data. Whenever GNSS data are available in the vicinity of the ROI, a GNSS-derived WTC is also generated and used for comparison. For S3, model-derived WTC are also compared to Envisat derived WTC in the central parts of the lakes, where radiometer data are valid.
The obtained results show that the model-derived corrections present in CS-2 products are referred to the model orography, which can depart from the mean river profile or mean lake level by hundreds of metres. ACE2 DEM is a better reference when compared to ECMWF orography, but height errors of the same magnitude are still found for some of the analysed regions e.g. Amazon and Lake Titicaca. Systematic errors up to 3-5 cm in the DTC are generally found for most of the selected regions, which can induce significant errors e.g. in the determination of mean river profiles or lake level time series. The effect of ECMWF surface pressure uncertainty propagation into DTC error has been found to be negligible from the analysis of surface pressure observations. Height-dependent errors of smaller magnitude, up to a few centimetres, have been found for the WTC for the selected ROI.
Results from this evaluation show that in general the magnitude of DTC and WTC errors is around 1 centimetre, when the corrections are referred to the proper reference: mean river profile, derived using the height of the closest point in the profile, or mean lake level determined from satellite altimetry. The evaluation of model-derived WTC errors using GNSS and MWR derived path delays is in progress and shall be extended to regions of higher WTC variability.
This analysis is currently being extended to S3 data samples accessed under the scope of Sentinel-3 Validation Team project ID 13769, titled “Validation Of Coastal ALtimetry from Sentinel-3 (VOCALS3)” and the first results will also be presented.
Recent Improvement in MSS and Gravity field in Coastal and Arctic Regions
DTU Space, Denmark
The latest release of the global high resolution mean sea surface and free air gravity from DTU Space are increasing dependent of the Cryosat-2 LRM, SAR and SAR-In data with its more than 6 years in orbit.
Recently the SARAL/AltiKa was shifted from its nominal 35 days repeat track into a geodetic orbit. With its smaller footprint compared to conventional altimeters. As this satellite continue to collect 1-2 years of geodetic data it will further improve coastal mplete 1-2
Here we perform sevel local coastal studies of the influence and importance of Cryosat-2 and the first SARAL/AltiKa for coastal and Arctic region in preparation for the release of the global high resolution DTU16/DTU17 MSS and Free Air Anomaly maps.
Tidal Downscaling in a 3D (Structured) Circulation Model: A New Approach Based on Tailored 2D (Unstructured) Simulations
1CNES/LEGOS, France; 2CNRS/LEGOS, France; 3CNRS/LA, France; 4Celad/LEGOS, France
Modeling the 3D ocean circulation in coastal areas requires an accurate representation of the tidal dynamics. This is particularly true in the Bay of Biscay, where tides are highly energetic over the shelf, with tidal ranges reaching 6m at the coast. Although tidal dynamics are dominated by semi-diurnal constituents, nonlinear interactions occurring between these constituents and the topography result in the generation of overtides such as M4. Downscaling the tidal dynamics in a coastal model from a larger scale solution raises several methodological issues, especially in terms of currents. In particular, the choice of the large scale solution is crucial; the impact of likely inconsistencies in bathymetry and grid resolutions between the large scale model and the coastal one should be
In this study, we propose a new approach to tidal downscaling for coastal modelling by using two numerical models, T-UGO and SYMPHONIE, on the same rectangular mesh and bathymetry at the nodes. The unstructured grid model T-UGO 2D spectral model is adapted to perform these simulations, and provide tidal boundary conditions to the 3D circulation model SYMPHONIE. The latter is set-up on a variable mesh grid that allows us to represent the different physical processes, including tides, that influence the dynamics of the bay, from the deep plain scale to the estuarine scale. The horizontal grid resolution varies between approximately 3km at the oceanic open boundary and less than 300m in the Gironde estuary and the Pertuis Charentais. Three types of tidal boundary conditions are tested for the SYMPHONIE model: the FES2012 atlas, T-UGO 2D spectral simulations, and SYMPHONIE 2D clamped simulations. Complex errors, taking into account both the amplitude and the phase of the M2 tidal constituent, are reduced by more than 75% with a regional forcing (SYMPHONIE or T-UGO 2D), compared to a global forcing (FES2012).