ENVISAT tracks waves hit Reunion Island

ENVISAT tracks waves hit Reunion Island

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France, 16 May 2007: The origin and movement of waves reaching up to 11 metres that devastated France’s Reunion Island in the Indian Ocean on evening of May 12, have been detected with ESA’s Envisat satellite.

The waves that thrashed the southern port of Saint Pierre, causing several piers to collapse and flooding several homes and businesses, originated at south of Cape Town, South Africa, and travelled northeast for nearly 4000 km over a period of three days before slamming into Reunion Island.

Dr Bertrand Chapron of IFREMER, the French Research Institute for Exploitation of the Sea, and Dr Fabrice Collard of France’s BOOST Technologies located and tracked the swells using Synthetic Aperture Radar (SAR).

“Swells are still surprise factors, which can unfortunately be deadly,” Chapron said. “The SAR Wave Mode product allows us to locate and systematically track swells globally. In the near future we anticipate using SAR wave data to predict their arrival time and intensity.”

“Although waves were forecast to hit Reunion Island, their intensity was predicted to be only a couple of metres”, Collard explained.”Strong swells are preceded by calm water, it is impossible to detect their arrival from shore,” Collard said. “SAR is the swell instrument and can typically observe swell periods in the range of 12 to 25 seconds.”

A larger wave period correlates to a more extreme wind event. The one that hit Saint Pierre, Reunion Island, had a 19-second range and initially originated from very intense storm winds on 8 May.

Envisat is equipped with an advanced version of the SAR instrument, Advanced Synthetic Aperture Radar (ASAR), flown on the ERS-1 and ERS-2 missions. Its wave mode acquires 10 by 5 km small images, or ‘imagettes’, of the sea surface every 100 km along the satellite orbit. These small ‘imagettes’, which depict the individual wave heights, are then mathematically transformed into averaged-out breakdowns of wave energy and direction, called ocean-wave spectra, which is made available to scientists and weather centres.