Supplementary material to “Challenges in Coastal Satellite Radar Altimetry”


Published 30 June 2009


Jérôme Benveniste, European Space Agency Center for Earth Observation (ESRIN), Frascati, Italy

Stefano Vignudelli, Consiglio Nazionale delle Ricerche, Pisa, Italy

Citation:

Benveniste, J., and S. Vignudelli (2009), Challenges in Coastal Satellite Radar Altimetry, Eos Trans. AGU, 90(26), 225. [Full Article (pdf)]

Recommendations from the workshop sub‐topics (sessions at the workshop):


1 – USER REQUIREMENTS

The community recommends that the user list be broadened to include users who may not be aware of altimetry as a potential solution to their daily tasks in coastal oceanography, as well as present altimetry users.

The community recommends that outreach and community building presentations be made at major conferences such as AGU, EGU, etc., along with regional venues of opportunity, to inform and get feedback from a wide range of potential coastal altimeter data users.

The community recommends that at the 3rd Coastal Altimetry Workshop (Frascati, Italy, 16‐18 Sept 2009), selected representative users with coastal applications be invited to the ‘showcase’ part of the event.

More specifically on product requirements (see also session 5):

2 – RETRACKING

The community agrees that the retracking of coastal altimetry data requires higher resolution coastlines and land DEMs than available currently from global models, and therefore recommends that better global coastlines and DEMs should be provided. Alternatively, local hi‐resolution DEMs should be used where available. Coastal retrackers should be applied over an area with sufficient overlap with open‐ocean retrackers, not just within few km from coast. This will allow users to assess their performance, analyze the transition between deep and coastal ocean regions, allow a careful evaluation of biases and thus ‘inspire confidence’. Coastal retrackers should be intercompared and compared against data on test sites. These include sites with the availability of accurate DEMs, along with fields of SSH and SSH gradients derived from glider and/or HF radar data.

3a – CORRECTIONS: WET TROPOSPHERIC

Requirements on the wet tropo path delay are identical as for the open ocean or even more stringent, because of shorter‐scale atmospheric phenomena in coastal regions, producing changes in path delay equivalent to several cm over 30' / 50 km. Three main methods are being developed: (I) dynamic extrapolation methods, using high‐resolution atmospheric models; (II) GNSS measurements of ZTD (Zenith Total Delay) (and met correction to ZWD — Zenith Wet Delay) and (III) land decontamination method. Continue focused efforts on this correction, which is the main source of error but shows very encouraging developments. Validation of the new techniques is a priority. Future missions should include radiometers with higher spatial resolution (standard frequencies with larger antennas or higher frequencies around 183 GHz). They should also consider scanning radiometers for swath altimeters and/or radiometers with nadir + far side focal points.

3b – CORRECTIONS: IONOSPHERIC

While the Total Electron Content (TEC) is not affected by land/ocean transitions, the dual‐frequency ionospheric correction is affected by the coast: the C‐band (or S‐band) footprint of the altimeter “sees” the coasts earlier than the Ku‐Band footprint. The DORIS Beacon network coverage appears not to be dense enough to include all coastal areas; moreover research on this system has stalled on the last 10 years. And GPS‐derived GIM models have been shown to represent the TEC more accurately, but they will have problems during periods of high solar activity in the normal 11‐year solar cycles. GIM maps should be used over DORIS (especially during high solar activity). Space agencies should promote further work on this important contribution.

3c – CORRECTIONS: TIDES

Recent progress has been made with global and coastal modelling (GOT 4.7, EOT08a), including developments in advanced assimilation techniques. There are issues to do with high‐resolution needs (short tidal wavelengths) and major omission errors (a limited number of constituents) for coastal tides, where errors are ~10‐20 cm. The community recommends that better fields of global coastal bathymetry be made available. These should have horizontal resolutions of at least 1 km, and preferably 200 m or 0.1 nautical miles, from the 200 m isobath to the coast. Higher resolution will still be needed in particular areas where nested tide and surge models are required e.g. large estuaries.

3d – CORRECTIONS: HF/IB

The first recommendation is the same as in 3c — we need better global coastal bathymetry with higher horizontal resolution (at least 1 km and preferably 200 m or 0.1 nautical mile resolution, from the 200 m isobath to the coast). Higher resolution will still be needed in particular areas where nested tide and surge models are required e.g. large estuaries. We also recommend that a compilation of outputs from local models is made available, as also suggested by GOOS.

4 – WAVES AND SEA STATE BIAS

Need for more modelling:

5 – DATA PRODUCTS, QUALITY AND DISSEMINATION

6 – SYNERGY WITH OTHER DATA AND MODELS

Users have started already to adopt altimetry (even if not specifically processed for coastal regions) in their coastal applications, and in many cases are eager to get data of higher quality (see also section 1). Altimeter + Tide Gauges can provide continuous alongtrack SSH to the coast. Alongtrack SSH and crosstrack geostrophic velocity may resolve positions of alongshore fronts and jets with offshore scales of 10+ km. We need further investigation of this and research on methods for computing gridded SSH and velocity fields from the alongtrack and ancillary data. Possible improvements are to add scatterometer wind Ekman components to produce total surface velocities, combine with MCC and Coastal Radar Surface Velocities, assimilate into coastal models. These combinations are needed to resolve features with shorter time scales: filtered tide gauge and model time scales are ~ 2+ days, while altimeter alone time scales are 20+ days. A community‐wide effort should be made to integrate forthcoming coastal altimetry products into observing systems and other applications, quantifying the improvement resulting from the addition of the coastal altimetry data.

7 – FORTHCOMING TECHNOLOGIES

A number of technological developments in current and forthcoming altimetry missions can contribute to the improvement of coastal altimetry. These include:

We need better (possibly more quantitative) definitions of end‐user requirements to drive future mission and instrument designs. There are different requirements for different applications, e.g. NRT near‐shore significant wave height is only conceivable with constellations of nadir‐viewing altimeters. Future missions should have better on‐board DEMs.

8 – INTERNATIONAL COOPERATION

Space Agencies must to ensure that instrument information is available to all Coastal Zone data processors. For instance see AVISO documentation and RA-2 Product Control Service.

Space Agencies should disseminate waveform products in NRT (ftp).

We need to make provisions for merging all the data from future missions CryoSat, Altika, etc. into a single archive.

We need to deliver findings to other groups and conferences, like GODAE follow‐ons, OceanObs'09, EGU, AGU, COSPAR (see also section1). We especially need two‐way interactions (workshops, short courses, etc.) with those working in coastal fisheries, search and rescue, navigation, hazardous spills, harmful algal blooms, etc., to educate altimeter experts on the needs of the users and to educate the users on the capabilities of altimetry (and other remotely sensed fields).

We need sustained capacity building effort in countries like India, China, Africa, South America, etc. (ALTICORE‐India and ALTICORE‐Africa are good examples).

The whole Group is encouraged to make further recommendations for Coastal Zone Oceanography on:

Paolo Cipollini (National Oceanography Centre, Laboratory for Satellite Oceanography, Southampton, UK) and Ted Strub (Cooperative Institute for Oceanographic Satellite Studies, Oregon State University, Corvallis) contributed to the writing of this report.