“Monster” waves in the open ocean were often dismissed as fishermen’s yarns until a 25-m tall wave hit a North Sea oil platform 12 years ago. Now, physicists in Spain and Germany have developed software that can track the progress of these deadly waves, and might serve as an early warning system for sailors or other seafarers.
Big waves are known to form, for example, when smaller waves move into a strongly opposing current near shore. However, scientists have been unable to explain why monster waves, which can tower over 30 m higher than surrounding waves, sometimes appear in the open ocean.
Although their origin cannot be predicted, Jose Nieto Borge from the University of Alcala in Madrid, Spain, together with a team led by Wolfgang Rosenthal at the GKSS research centre in Germany, has designed software that can predict a monster wave’s likely course. Their software interprets radar images taken at time increments to monitor the evolution of monster waves in time and space, and so might supplant existing techniques using buoys, which can only monitor wave height at fixed points in the ocean.
Complex signals
Radar signals from the ocean’s surface do not give a one-to-one map of wave height, but depend on several factors including sea surface roughness and wave tilt. To remove these unwanted contributions, Nieto Borge and colleagues’ software uses a computer “3D Fourier transform” to convert the raw, time-evolving radar data into a spectrum of frequency and wavenumber.
According to the theory of fluid dynamics, there should be a dependency between frequency and wavenumber for waves, known as the dispersion relation. By checking the validity of this relation for each component, an algorithm extracts only the contributions from real waves. In the final step, an inverse 3D Fourier transform converts the spectrum back into a time-evolving map, which is calibrated by comparing the energy of the waves with the background noise.
The software is currently being commercialized by OceanWaveS, a spin-off company of GKSS. Ina Tränkmann, project manager at OceanWaveS, told physicsworld.com that it should be ready for sale next year. But the software may not limited to tracking monster waves — Nieto Borge is now looking at how it could be used to predict the trajectory of oil spills.
