Friday 01 Dec 2017From Determinism to Probability: Mitigating chaos in weather and climate prediction with the ensemble forecast system.

Tim Palmer - Oxford

Newman Red 12:30-13:30

Humankind has attempted to forecast weather at least since the dawn of civilisation. However, attempts to forecast weather from scientific principles date from the early 20th Century and only came of age with the dawn of the electronic computer after the Second World War. Since then, the development of faster computers, more accurate numerical algorithms and space-based observations of weather has led to a steady improvement in weather forecast skill. However, there is an Achilles Heel which undermines such scientific and technical developments: chaos - the notion that the unobservable flap of a butterfly’s wings can change the course of weather. However, a closer look at chaotic systems reveals that predictability is itself a state dependent quantity. Hence a way to mitigate the effects of chaos is to develop ensemble-based forecast systems that can predict temporal and spatial variations in predictability. Ensemble forecasting, based on multiple integrations of a stochastic weather or climate model, has changed the character of weather and climate prediction over the last 25 years, from one of determinism to one of probability. The development of such ensemble forecast systems, and how they have begun to affect the way weather and climate forecasts are used in practice, will be reviewed.

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