Back to basics for climate models


Basic physics and statistic tools could offer a simpler and more meaningful way to model key elements of the Earth's climate, according to researchers at the University of Leeds and Brown University.

The research, published in Physical Review Letters, shows that a technique called direct statistical simulation accurately models fluid jets, fast-moving flows that form naturally in oceans and in the atmosphere. The weather of Britain and northern Europe, for example, is heavily dominated by one of these fluid jets commonly known as “The Jet Stream”.

Currently, scientists use a complex method, known as direct numerical simulation, to model the Earth’s climate.

“This involves taking a weather model and running it through long periods of time, having to take into account every individual weather event, for example the rainfall, temperature and wind speed at any one point. As these models often span thousands, if not millions of years, they have to be run on some of the world’s most powerful computers,” explained Professor Steve Tobias from the School of Mathematics at the University of Leeds, one of the authors of the study.

This new statistical physics approach could dramatically reduce the time and brute-force computing that the current simulation techniques require. It focuses on the fundamental forces that drive climate rather than on understanding each and every change in the air and water that make up fluid jets.

Brad Marston, professor of physics at Brown University, USA, and co-author of the paper, said: “It’s a bit like the approach physicists use to describe the behaviour of gases. If you wanted to describe the air in a room, one way to do it would be to run a giant supercomputer simulation of all the positions of all of the molecules bouncing off of each other. But another way would be to develop statistical mechanics and find that the gas actually obeys simple laws you can write down on a piece of paper: PV=nRT, the gas equation. That’s a much more useful description, and that’s the approach we’re trying to take with the climate.”

The study simulated the jets that form as a fluid moves on a hypothetical spinning sphere to investigate whether direct statistical simulation provided a useful way of describing their formation and characteristics. The researchers modelled the fluid using both the traditional numerical technique and their statistical technique, and then compared the output of the two models, finding that both models generally arrived at similar values for the number of jets that would form and the strength of the airflow.

However there were limits to what the statistical model could do. The study found that as the pace of adding and removing energy to the fluid system increased, the statistical model started to break down. Professors Tobias and Marston are currently working on improvements of their technique to make it a suitable replacement for larger, more complex models.

“At this early stage, the simulations have only been applied to hypothetical atmospheres with one or two layers - the Earth’s atmosphere is far more complex. The approach could also be fruitful for describing the `climate’ of stars and exoplanets. For these, we are never going to get an accurate weather forecast – but we may be able to say something about the statistics,” added Professor Tobias.

Image credit: NASA / DVIDS

For more information

The paper Direct statistical simulation of out-of-equilibrium jets was published online by Physical Review Letters on 5th March 2013.

Professor Steve Tobias is available for interview.

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