Heavy weather: Subtle forces may be shaping the patterns of our local climate

www.hero.ac.uk: Higher Education & Research Opportunities in the United Kingdom, 26 March 2004

NEXT TIME THE WEATHER FORECAST is wrong, don't rush to blame the meteorologists.  They may not have been able to factor in your local inertia-gravity waves.

Researchers at Oxford University have discovered that small-scale fluctuations, which are widespread in the atmosphere, may have a greater impact on weather systems than previously thought. The results could have important implications for accurate weather forecasting in the future.

The fluctuations are known as inertia-gravity waves because they are sustained by a combination of inertial and gravitational forces. They are prominent in the bottom 15 km of the atmosphere, and can often be detected from the surface of the Earth as 'stripy' features in clouds. Their horizontal wavelengths can be as short as 5 km - too small to be picked up by current weather prediction models, which divide the surface of the Earth into grid-boxes measuring around 50 km by 50 km.

Meteorologists have until now always had to assume that inertia-gravity waves do not significantly interact with weather systems, such as warm and cold fronts, but this assumption had never been rigorously tested.

Following laboratory experiments, which seemed to challenge the standard models, Dr Paul Williams and Professor Peter Read at the Clarendon Laboratory, Oxford University, developed a computer model of a simple fluid system resembling the atmosphere. Inertia-gravity waves, which can be highly irregular, chaotic and transient were represented as random noise in the model. The researchers found that the system could behave differently when the inertia-gravity wave representation was activated.

Notably, the fluid could undergo spontaneous transitions to quite different states, with a dramatic shift in the patterns of low and high pressure. Translating these results to the real atmosphere suggests that inertia-gravity waves could be a cause of significant errors in current weather forecasts. The surprise implication of the research is that adding random noise to the forecast might actually help improve things.

"It seems that we have observed a phenomenon which might have the potential to affect the accuracy of weather forecasts," said Dr Williams. "More research is needed to find out exactly how bad the forecast error might be, but the preliminary results are very exciting. It sounds bizarre to suggest that adding random noise to a forecast might help to improve it, but science is always full of surprises."

The team's findings are published in the journal Nonlinear Processes in Geophysics.