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GCOS Ocean Surface ECV Definition: Sea State - A description of the properties of sea surface waves at a given time and place. This might be given in terms of the wave spectrum, or more simply in terms of the significant wave height and some measure of the wave period. (from the AMS Glossary of Meteorology) Introduction: Observations of sea state are particularly relevant to coastal and offshore impacts on human activities, but also affect climatically important air-sea exchanges and can also provide complementary information of relevance to monitoring changes in the marine environment, e.g., in winds, storms, air-sea fluxes and extreme events. Although sea state (variables relating to the height, direction, wavelength and time period of waves) has been observed from satellites, there is at present no coordinated and sustained global observing effort for sea state. Present best estimates of sea state are computed from model reanalysis and analysis systems.
Issues relative to sea state observations and analysis include:
To address the issues raised above, it is proposed that the JCOMM Expert Team on Waves and Surges implement wave measurement systems as part of the Surface Reference Mooring Network. Satellite Observations: Sea state and wind speed govern air-sea fluxes of momentum, heat, water vapour and gas transfer. The state of the sea and surface pressure are two features of the weather that are important to commercial use of the sea (e.g. ship routing, warnings of hazards to shipping, marine construction, off-shore drilling installations and fisheries). Information on surge height at the coast is key to the protection of life and property in coastal habitats. These data are also important for climate purposes as they are needed for the correct representation of turbulent air-sea fluxes. Wave height is influenced by wind speed and direction, the wind ‘fetch’ and its rate of change. In the nowcasting context, ocean wave models are driven by NWP predictions of surface wind. However, errors in waves generated at large distances can accumulate. Improvements in forecasts, especially of long wavelength swell, can be achieved by assimilating observations from different sources. These are currently available from isolated buoys, satellite altimeter and scatterometer data. In the absence of direct observations, initial wave state is deduced from the wind history. This is currently available over the sea from isolated buoys and from low-Earth-orbiting satellite scatterometer and microwave instruments. For global NWP, ships and buoys provide observations of acceptable frequency that are acceptable to marginal accuracy, but coverage is marginal or absent over large areas of the ocean. Altimeters on polar satellites provide information on significant wave height with global coverage and good accuracy, but horizontal/temporal coverage is marginal. Information on the 2D wave spectrum is provided by SAR instruments with good accuracy, but marginal horizontal/temporal resolution. SAR instruments can accurately measure changes in ocean waves and winds, including wavelength and the direction of wave fronts, regardless of cloud, fog or darkness. The AMI SAR on ERS-2 has been operating in both wave and image mode, and the ASAR on Envisat continues to provide the ERS wave mode products, but with improved quality. PALSAR on JAXA’s ALOS mission provides data on sea surface wind and wave spectrum required for oil spill analysis and for studies of coastal topography-air-sea interaction. The ScanSAR wave data supplied by RADARSAT will continue to be provided by RADARSAT-2. Europe’s Sentinel-1 mission will also ensure future provision of SAR data supply.Information from radar altimeters, such as that on the Jason-1 mission, is limited to data on significant wave height.The GCOS IP recognises that altimetry and SAR measurements useful for sea state measures (wave height, direction, wavelength and time period) have been continuously available since 1991 and will be maintained in the future, but no consolidated data product has ever been produced. GCOS proposes that new altimeter (wide-swath) and SAR technologies are needed to advance retrieval of near-shore sea state parameters. CEOS agencies propose to cooperate with the user community to support efforts aimed at building on the decade-long satellite sea state records and making a comprehensive use of future altimeter- and SAR-bearing missions. (Satellite Missions) (Source: CEOS EO Handbook - Earth Observations Plans by Measurement) Additional Information:
References:
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