Waves near the coast

Improving assessment of wave height near the coast

 

Researchers at PML have created an innovative methodology to compare the accuracy of different satellite radar technologies in producing wave height data in coastal areas.

Accurate wave height data is particularly important around coastal areas, where activities such as ship routing, offshore engineering and predicting the transport and dispersal of floating objects are all affected. Traditionally, these data were obtained in-situ by moored buoys, but these provide limited spatial coverage.

Altimetry involves emitting radio pulses from satellite-borne radar altimeters towards Earth’s surface and using the shape and magnitude of the reflected pulses to obtain data including wave height, with better coverage than from the buoys alone.

Using a new methodology that combined satellite and in-situ observations with innovative use of modelling, PML researchers were able to assess conventional and new processing modes of the altimetry signals.

One issue with the conventional processing method is that it is known to have less accurate results in coastal areas due to reflections from the land contaminating the signal. The new mode was expected to avoid this issue by having a smaller ground footprint, which allows the altimetry to focus on more specific areas close to land, without the land interfering with the signal.

To test these expected improvements, altimeter observations from the European Space Agency satellite Sentinel-3A were compared against in-situ observations from a network of 17 buoys around the coast of Southwest England.

These buoys provide a continuous time series of hourly values for wave height, period and direction. They are also distributed along the coastline and so represent a broad range of different coastal conditions.

The key advance of this investigation was in using a model that could judge which buoys would be best to compare with each satellite track. The buoys closest to the track would not always be most representative, so the model could couple the satellite data with the most appropriate in-situ buoy data for that area.

By combining these three elements – satellite, in-situ and modelling – the researchers demonstrated that the new fine resolution mode outperformed the conventional one in the critical region within 15km of the coast.

Dr Francesco Nencioli noted: "This comparison methodology could be used in other future studies to assess observations over different combinations of coastal regions and altimeters.”

He added: "Our study confirms that the new altimetry processing really does show improved recovery of wave height information in the critical coastal zone where the impacts are most keenly felt through coastal inundations and damage to harbour defences."

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