Conference Agenda

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Session Overview
S6: Applications II - Sea Level and Extreme Events
Thursday, 23/Feb/2017:
11:30 - 12:50

Session Chair: P David Cotton
Session Chair: Kaoru Ichikawa
Session Chair: Luciana Fenoglio-Marc
Session Chair: Clara Lázaro

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11:30 - 11:50

Validating Altimeter Estimates of Sea level Along the Southern Coast of Australia

Madeleine Louise Cahill1, Benoit Legresy1, Hugo Bastos de Oliveira1,2

1CSIRO, Australia; 2University of Tasmania, Australia

The southern coast of Australia along the Great Australian Bight is an east-west shelf extending for thousands of kilometers. The shelf is a known waveguide for large amplitude Coastally Trapped Waves (CTWs) that often contribute to creating extreme sea levels in the more populated gulf regions. At the eastern end of the shelf, the Bonney Coast, is Australia’s only deep-reaching coastal upwelling system, drawing water from over 300m and extending along 800km of coastline. Upwelling occurs 2-3 times during the summer at the Bonney Coast, principally due to upwelling favourable winds but events are enhanced by the passage of correctly phased CTWs.

In previous work we found that estimates of sea level along the Bonney Coast, using AVISO’s PEACHI product, were highly self-consistent in identifying upwelling events. The work has been extended now to compare altimetric sea level with data from two (National Tidal Centre) tide gauges along the southern coast, each of which is in the path of both an ascending and descending AltiKa pass. The propagating nature of the CTW signal provides the opportunity to incorporate heights from all altimeter tracks crossing the shelf – greatly increasing the number of altimeter/tide gauge comparisons that can be made.

11:50 - 12:10

UK Sea Level Space Watch – Monitoring Regional Sea Level Variability around the UK from Satellite Altimetry

David Cotton1, Ellis Ash1, Paolo Cipollini2, Francisco Mir Calafat2

1Satellite Oceanographic Consultants Ltd, United Kingdom; 2National Oceanography Centre, United Kingdom

UK Sea Level SpaceWatch is a service designed to support the UK agencies responsible for the management and planning of national flood defences and for the preservation of coastal habitats threatened by sea level change.

Using data from satellite altimeters together with tide gauge data, Sea Level SpaceWatch provides, through an easy to use web-interface, the latest figures on observed sea level around the UK, supported by careful analyses of these data in terms of long-term trends, regional variability and confidence intervals showing the lower and upper limit for the current mean sea levels. The service complements and supplements the sea level change scenario information available from UK Climate Projections, offering planners the opportunity to verify the regional variability of sea level around the UK at multiple time scales and observe the presence of any significant inter-annual changes.

In the past it has been difficult to retrieve useful data from satellite altimeters close to the coast, because of land contamination of the return waveform. To address this problem, NOC has developed the “ALES” altimeter re-tracker for coastal regions. With this re-tracker, altimeter data from Jason-1, Jason-2, Envisat and Altika has been reprocessed to generate a 14 year times series of sea level data for the UK coastal zone (2002-2015).

This multi-year sea level data set has been validated against tide gauge data and then further analysis carried out to provide a characterisation of regional variability in sea level, in terms of the annual cycle, and inter-annual variability.

Key findings are:

- There is preliminary evidence of a geographical structure in the long-term trend, larger on the South and East than in the North-West.

- There was clear consistency between the annual cycle parameters (amplitude and phase) from tide gauge and altimeter data, with some localised differences. The annual cycle peaks between early October in the south-east and early November in the west coast and has an amplitude ranging from 5 to 9 cm.

- There is good agreement between the de-trended de-seasoned sea level from altimetry and from the tide gauges.

- There is significant spatial coherence in sea level on inter-annual timescales, with the leading EOF capturing over 50% of the variability.

The Development of Sea Level Space Watch has been funded by the UK Space Agency under the Space for Smarter Government Programme.

12:10 - 12:30

The Estimation of Sea Level Rise Impact on Coastal Zones of the Eastern Adriatic Sea

Marijan Grgić1, Robert Steven Nerem2, Tomislav Bašić1

1University of Zagreb, Faculty of Geodesy, Croatia; 2University of Colorado Boulder, Colorado Center for Astrodynamics Research, USA

The mean rate of global sea level rise is estimated to be ~3.1 mm yr−1. Those rates are routinely calculated from tide gauge measurements and satellite altimetry. However, regional rates that are driven by local geophysical processes can vary significantly. Thus, the impact of sea level change in the areas that adjoin the oceans should be evaluated through the local perspective. That includes modeling the sea level change with regards to the local vertical land motion and coastal relief combined with specific local sea level change rates. This study aims to model the future sea level change in relation to the coastal specifics of the eastern Adriatic Sea, which mostly covers Croatian coastline. The area of interest covers the northernmost part of the Mediterranean Sea, which contains over 1300 islands and is one of the most indented sea coasts in the world. The study encompasses sea level modeling from tide gauge measurements and satellite altimetry along with its comparison to the Shuttle Radar Terrain Model (SRTM) of the proposed area combined with vertical land movement trends. The satellite altimeter data processed include measurements captured by the Topex/Poseidon, Envisat, Jason-1, Jason-2, (Jason-3), Cryosat-2, and Saral altimetry missions. These data were combined with monthly solutions from the tide gauge measurements. The vulnerability of the coastal areas was evaluated through the analysis of the coastal relief, land cover in the coastal areas and terrain slopes. Finally, an insight into the expected absolute and relative sea level change and its impact throughout the 21st century is provided for the coastal areas of the eastern Adriatic area. The study provides numerical and graphical analyses along with the advantages and disadvantages of the employed modeling method.

12:30 - 12:50

Coastal SAR and PLRM Altimetry in the German Bight and West Baltic Sea with Sentinel-3A and CryoSat-2

Salvatore Dinardo1, Christopher Buchhaupt2, Luciana Fenoglio-Marc2,3, Remko Scharroo6, Joana Fernandes4, Matthias Becker2, Jérôme Benveniste5

1HeSpace, Eumetsat, Darmstadt, Germany; 2TU Darmstadt, Institute of Geodesy, Physical and Satellite Geodesy, Darmstadt, Germany; 3University of Bonn, Institute of Geodesy, Bonn, Germany; 4University of Porto, Faculty of Science, Porto, Portugal; 5ESA-ESRIN, Frascati, Italy; 6EUMETSAT, Darmstadt, Germany

The scope of this study is a regional analysis and inter-comparison between CryoSat-2 and Sentinel-3 SAR altimeter data against in-situ data and regional model results at distances to coast smaller than 10 km. The in-situ data are from a network of tide gauges and GNSS stations. The validated geophysical altimeter parameters are the sea surface height above the ellipsoid (SSH), the significant sea wave height (SWH) and wind speed (U10).

We have carried out, from the CryoSat-2 FBR (L1a) product, a Delay-Doppler processing and waveform retracking tailored specifically for coastal zone by applying Hamming Window and Zero-Padding, using an extended vertical swath window in order to minimize tracker errors and a dedicated SAMOSA-based coastal retracker, named SAMOSA+. SAMOSA+ accepts the mean square slope as a free parameter and the epoch’s first guess fitting value is decided according to the peak in correlation between 20 consecutive waveforms, in order to reduce land off-ranging effect.

Exactly the same processing baseline has been considered to process Sentinel-3A L0 data using the most accurate possible orbits (POE) and platform files (PCP) available.

Since the highest remaining uncertainties in the altimeter parameters derived in coastal shallow waters arise from residual errors in the applied corrections, we use a regional ocean tide and high resolution geoid and mean sea surface models (TPXO8 for tides, EIGEN-6C4 for the geoid and DTU13 for the mean sea surface). We also apply a regional improved wet tropospheric correction computed from the GNSS-derived Path Delay Plus (GPD+) algorithm at the University of Porto.

In parallel with SAR measurements, in order to quantify the improvement with respect to pulse-limited altimetry, we build PLRM (pseudo-LRM) data from CryoSat-2 FBR and Sentinel-3 L1a and retrack them with a numerical convolutional Brown-based retracker. PLRM is used as a proxy for real pulse-limited products (LRM), since there is no direct comparison of SAR and LRM possible otherwise. Both the CryoSat-2 and the Sentinel-3A SAR L2 ocean data are generated and extracted from the ESA-ESRIN GPOD service, whereas the CryoSat-2 and Sentinel-3A PLRM data are built and retracked by the Technical University of Darmstadt (TUDa). The regions of interest are the German Bight and West Baltic Sea, the latter being a very challenging area due to its complex coastal morphology and high tide dynamics. The epoch of interest is the complete mission duration for Cryosat-2 and and as much data as we can gather before the Workshop for Sentinel-3A.

The analysis will be based both on geometric parameters, such as the distance-to-coast parameter and the sea floor bathymetry and radar waveform parameters (misfit and entropy).

The final objective is to verify the ability of SAR Altimetry to measure accurately in the coastal zone the sea level annual cycle and the sea level trend. By the time of workshop, we will use 6 years of data for CryoSat-2 and the longest available dataset for Sentinel-3A to attempt the first assessment of whether Sentinel-3A and CryoSat-2 measure the same sea level annual cycle in open ocean and coastal zone.

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Conference: Coastal Altimetry 2017
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