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Supplementary material to “Coastal Altimetry Progresses Toward Applications”
2 February 2010
Stefano Vignudelli, Consiglio Nazionale delle Ricerche, Pisa, Italy
Jérôme Benveniste, European Space Agency Centre for Earth Observation (ESRIN), Frascati, Italy
Citation:
Vignudelli, S., and J. Benveniste (2010), Coastal altimetry progresses toward applications, Eos Trans. AGU, 91(5), 45. [Full Article (pdf)]
A full report containing findings and recommendations from the workshop topics (sessions at the workshop) can be found at www.coastalt.eu. Here we report the main outcomes:
1 – Coastal waveform and retracking
Status
Contributions focused on contamination of Envisat waveforms, validation of Jason-1 retracked data with tide gauges and novel algorithms to retrack waveforms.
Open Issues
- What factors does contamination near the coast depend upon? We need a uniform metric to quantify distance from coast. We have to consider that different sensors may have different waveforms.
- What happens when the waveform is very far from the model? We need more work on the operational processing of waveforms in the coastal zone.
- Which are the real limits for retrackers? No limits, but noisier waveforms might introduce additional random errors in the estimated parameters.
- How much is the improvement of the retracked outputs compared to original (SGDR) data? We have to define how to measure quality.
- What about validation of retracking techniques? We need to consider the context (e.g., point-wise vs footprint averages, colocation with the nearest point, etc.) and take care of differences in the measuring systems.
- Is intercalibration of retrackers still an issue? YES as we need to make sure that we set the right parameters.
Recommendations
There is no magic bullet, but a consistent methodology should be applied to more cases of oceanographic interest, and findings reported at the next workshop.
2 – Corrections
Status
Probably not an issue:
- Dry tropospheric correction
- Worry about "elevation contamination" in pressure fields
- Ionospheric correction
- Dual-frequency will be affected very near to coast
- GPS-based 3-hourly TEC fields with high-enough resolution experience no coastal contamination
- Pre-GPS: climatological TEC fields
- Solid earth tide
- Generally, no coastal issues
- Ocean loading tide
- Relatively small, with small coastal gradients
More problematic:
- Wet tropospheric correction
- Radiometer footprint size very large (up to 40 km)
- Land contamination correction models have been developed / are being developed
- Alternative approaches (i.e. GPS-based) also very promising
- Inverse barometer correction
- Now modelled dynamically, correcting high-frequency component
- Higher-resolution models are under development
- Mean sea surface
- State-of-the-art models (based on altimetry) have very high resolution (1'x1')
- Much less "trackiness" than previous models
- Produce better mean ocean dynamics when compared to geoid
- Ocean tide correction
- Global models generally do a well-enough job for the constituents that they already contain, but lack the more "esoteric" tides that are prominent in coastal areas
- Local models still have benefits in terms of resolution and inclusion of locally important tidal constituents
Open Issues
- Corrections: multiple choices for the user - single or blended product?
- Tidal corrections: more work on global tidal models, blending in local tide models and constituents like M3.
- Ionospheric correction: the GPS method works fairly well. Need for systematic procedure.
- Wet tropospheric correction: Possible further validation of the various methods with ground-based water vapour radiometers and GPS?
- Local corrections: make "library" of local models so many people can use them
Recommendations
The community asked for quality flags and clear documentation on the characteristics and limitations of the individual corrections. Studies of the variability of all individual geophysical corrections in the coastal zone are recommended.
3 – Sea level
Status
There is interest in 'bringing altimetry to the coastline' to better understand the error budget in global sea level rise, when altimeters are tied to coastal tide gauges for calibration.
Open Issues
There is still no clear explanation why coastal and island tide gauge measurements made over the past century show sea level rising at ~1.7 mm/yr, while satellite altimeter measurements made over the past 17 years give a rate of ~3.0 mm/yr, roughly 50% higher. Recent studies comparing coastal and open ocean altimeter measurements (e.g. Holgate and Woodworth 2004, Prandi, et al., 2009) show the rate in the coastal zone, defined as extending 200 km from land, is nearly twice as fast as in the open ocean in the interval 1993–2002. However, when the past 17 years are considered there is no significant difference between the coastal and open ocean rates.
Because of land-water interference problems caused by the relatively large sizes of the radiometer and radar footprints, the comparison with tide gauges is never exactly coincident. The radar problem may become more complicated by on-going efforts to use special waveform re-tracking algorithms near the coast that might introduce subtle biases not present in open ocean tracking algorithms.
Holgate S.J. and Woodworth P.L. (2004), Geophys. Res. Lett., 31, L07305, doi:10.1029/2004GL019626.
Prandi, P., A. Cazenave, and M. Becker (2009), Geophys. Res. Lett., 36, L05602, doi:10.1029/2008GL036564.
Recommendations
The community recommends keeping "sea level" as a major topic in future coastal altimetry workshops. The effects of waveform re-tracking, tidal modelling, and wet troposphere correction on altimeter-derived estimates of sea level need to be better assessed. There is a clear consensus that long, consistent and homogeneous multi-mission time series of altimeter observations are also required in the coastal zone.
4 – Waves
Status
With reference to the along track profiles at 10/20 Hz rate, these data might contain physical information (at least till 5 km from the coast) below 7 km.
Open Issues
- Are the raw along-track high resolution data too noisy to retrieve good wave information? YES, we need some spatial filtering.
- Can altimetry contribute to wave models through improved coastal high resolution wave data for validation and assimilation? We need Operational Sensor Data Record (OSDR) products at higher resolution than 7 km but lower than 350 m.
- How close to the coast can altimetry provide good wave measurements? Wave retrieval requires improvement of the retracking techniques.
- How can accurate wave models contribute to the empirical SSB modelling? We can get from accurate models better description of the wave age.
Recommendations
The main recommendation from the workshop is to reinforce the link between those estimating waves from altimetry and the wave modeling community.
5 – New products
Status
The PISTACH and COASTALT products are still very new and assessment by users is still far from complete.
Issues
- Short time span is general limiting factor for full exploitation
- Lack of documentation for the scientific community is a major concern
- Validation is a prerequisite for coastal applications
- Categories of users with different knowledge levels (e.g., light or expert) and different objectives (e.g. NRT or off-line)
Recommendations
The community recommends the continuation of projects to produce these coastal altimeter products automatically and systematically. There is a general consensus that products must be simple to use for inexpert users. Production of a comprehensive User Manual should be an immediate priority.
6 – Future of coastal altimetry
Status
Next generation altimeters, particularly well suited to Coastal Altimetry, include AltiKA (ka-band, high resolution altimeter) and SAR/DelayDoppler altimeters.
Wide-Swath altimeter concepts currently envisaged suffer from the lack of accurate estimation of SWH, required for coastal applications.
Recommendations
Some features related to the next generation of altimeters and in particular their effect on the accuracy of the measurements need to be investigated as early as possible, when first space-based data become available.
7 and 8 – Applications using standard and coastal-processed products
Status
The coastal oceanography community makes use of existing satellite altimetry, even if not specifically processed properly for coastal regions.
Open Issues
- Knowledge of the altimeter error budgets in coastal regions remains an important topic of research and discussion.
- For applications requiring absolute dynamic topography enhanced resolution of the geoid and Mean Dynamic Topography is needed for specific regions of interest.
Recommendations
Given the shorter spatial (across-shelf) and temporal scales in coastal regions, along-track altimeter data are most useful in combination with ocean circulation models and/or other types of data.
We recommend intensive evaluation, analysis, calibration/validation, etc. for different types of coastal configurations.
Similarly, we need to entrain more research scientists in analysis and data assimilation of coastal altimetry.
9 – Capacity building
Status
Topic was added because of its strategic importance for the developing countries. New initiatives just started in Thailand, India and South Africa.
Open Issues
- Providing access to data in these countries is not the final solution, but there is a need to identify case-studies of coastal applications.
- The expertise of the users is a non-negligible (limiting) factor in the correct processing and exploitation of altimeter data in the coastal zone.
Recommendations
A priority in capacity building concerns training in how to access and deal with data before exploiting scientifically correct information.
Acknowledgments: This report was collated by Jérôme Benveniste, Paolo Cipollini, Laury Miller and Stefano Vignudelli, with contributions by, Lifeng Bao, Florence Birol, Wolfgang Bosch, Phil Callahan, Luciana Fenoglio-Marc, David Griffin, Kaoru Ichikawa, Sylvie Labroue, Gilles Larnicol, Constantin Mavrocordatos, Franck Mercier, Remko Scharroo, Ted Strub, John Wilkin, and by the rest of the community present at the Frascati workshop.