French scientists are using images from the European Space Agency‘s (ESA) Envisat satellite to learn lessons about the origin and movement of waves which reached heights of up to 36 feet before devastating France’s Reunion Island in the Indian Ocean May 12.

The ESA reported that the waves that slammed across the southern port of Saint Pierre to leave two fishermen missing, ruin several piers and flood numerous homes and businesses originated south of Cape Town, South Africa, and travelled northeast for nearly 2,500 miles over a period of three days before reaching landfall at Reunion Island.

Bertrand Chapron of the French Research Institute for Exploitation of the Sea and Fabrice Collard of Boost Technologies located and tracked the swells using standard processed 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 achieve only a few feet, Collard explained.

"Because 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 Reunion Island had a 19-second range and initially originated from very intense storm winds May 8.

Approaching the coastline, the wave system slows down and individual waves increase to reach at least two times the mean average of their initial wave height easily; thus a 16- foot significant wave height system can hit the coast at a height of 33 feet.

Chapron and Collard intend to make data for global swells available this year as a demonstration for weather centers to complement the accuracy of their sea forecast models.

Envisat’s advanced SAR can acquire 6.2- by 3.1-mile small images, or "imagettes," of the sea surface every 62 miles along the satellite orbit. Depictions of individual wave heights are then mathematically broken down to analyze wave energy and direction, called ocean-wave spectra, which ESA will make available to scientists and weather centers.

— J.J. McCoy