Predicting ocean waves

NIWA scientist Richard Gorman predicts wind-forced wave conditions in the ocean using computer modelling. This is important because big waves are dangerous for ships, and they can cause beach erosion.

Ocean waves

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Ocean waves rolling onto the beach at Piha. When talking about waves, most people think of ocean waves.

Rights: Aleksei Potov, image licensed through 123rf.com

Computer modelling predicts wave conditions

Richard uses mathematics and computer modelling to predict wave conditions. He uses the weather forecast that is done by other researchers at NIWA as data in his modelling. His model covers the whole world – he works on large-scale predictions regarding waves that are created on the other side of the Pacific and travel right across the ocean to reach New Zealand and vice versa.

The area studied is divided into cells of 120 km2, and the model predicts average wave conditions in each cell. Richard also uses a smaller grid of 12 km2 to do smaller-scale predictions that look at what is happening closer to New Zealand. At an even smaller scale, Richard models waves in estuaries.

The prediction of wave conditions forms one part of a series of forecasts on weather, waves, tides, storm surge, river flow and other weather-related events. Organisations such as port companies, oil exploration companies and regional councils subscribe to these forecasts.

Energy transfer: the basic idea behind the models

The computer model is a mathematical description of how energy is transferred from wind to waves, how the energy travels across the ocean and how it dissipates. The energy originally comes from the wind blowing over the water, which makes ripples that grow into swells.

There is potential energy contained in waves as they rise and fall, and there is kinetic energy contained in the moving water. The model contains a mathematical representation of the distribution of energy in the waves and at what rate the wind puts energy into the waves.

Predicting wave conditions

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Numerical modelling is used by NIWA to predict wave conditions around New Zealand. The chart shows predicted wave heights (colours) and direction (arrows) for 13 April 2007, when heavy seas hit the Southland coast.

Rights: NIWA

Predicting wave conditions

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Numerical modelling is used by NIWA to predict wave conditions around New Zealand. The chart shows predicted wave heights (colours) and direction (arrows) for 13 April 2007, when heavy seas hit the Southland coast.

Rights: NIWA

The model also tracks the travel of the waves – how wave energy moves across the ocean. Another part of the model looks at how the energy is dissipated. When waves break, they appear to lose energy. In fact, there is transformation of energy where the energy lost changes into turbulent motion of the water or surf.

Richard’s modelling of waves also takes into account the interaction of waves with the seabed through refraction, bed friction and shoaling. Reflection of waves is not a major process unless there is a cliff or steep beach at the coast, and it does not make much difference at the larger scales for which the model is used.

Confirming predictions about waves

Richard confirms his computer-modelled wave predictions by using data from buoys set up in various locations. Buoys give information on wave height, wave period and wave direction. He also looks at data from radar and satellites. If there are anomalies, he will check his computer modelling and change it to align with the physical data and go through the modelling cycle again.

I think that is one of the big benefits of working in a cross-disciplinary way – bringing people who are used to one particular way of thinking or one set of theories into a new area and seeing if those ideas are relevant.

Richard’s approach to doing science

During his career, Richard has crossed over from particle physics research into wave prediction research. This has been important for his approach to doing science, which is “looking at a particular problem, seeing what other people have done before and [making] connections with something else that I have seen done before … A lot of the time, my style of approach is the cross-fertilisation of ideas, which come through different streams of research.”

Richard does not work in isolation. He is part of an informal international community of scientists who work on different aspects of the complex model that predicts weather and waves. Collaboration enables Richard to modify and improve the modelling process. He says, “I can’t do it all. In this area, there is a lot of data flowing around.”

Richard is also collaborating with colleagues, such as Dr Rob Bell, in Wellington to develop a new model for wave forecasting. This model works on a more compact scale to predict effects of smaller events (such as a cyclone) on wave conditions.