cyclone yasi

Force of nature

1 August 2011

Between 80 and 100 tropical cyclones develop each year, but it is the few intense systems that strike land that make the news and cause extensive damage such as Cyclone Yasi which struck Queensland Australia in February 2011. Julian Heming, Met Office Tropical Prediction Scientist, examines the nature and prediction of tropical cyclones.

Tropical cyclones are driven by heat and moisture, which is why they develop over the vast tropical oceans. At certain times of year, conditions are right in the atmosphere and oceans to cause them to develop. With stronger winds, some become hurricanes (a name used in the Atlantic and east Pacific) or typhoons (a name used in the west Pacific). In other regions they are known as cyclones.

In the northern hemisphere, storms usually develop between June and November and in the southern hemisphere between November and April. The number and intensity of tropical cyclones can vary greatly from year to year due to cyclical phenomena such as El Niño and La Niña. For example, there were 28 tropical storms in the Atlantic in 2005, but just nine in the following season.

Forecasting centres such as the Met Office attempt to predict the track, or path, of tropical cyclones by using numerical weather prediction models. Short period forecasts have improved vastly over the years with five-day forecasts now better than three-day forecasts were 20 years ago. In most cases, computer models give several days warning that a storm will develop. For example, the Met Office model predicted the formation of Cyclone Yasi3,500 km away from its eventual landfall location over Queensland. The prediction of landfall six days later was accurate to within 50 km.

Beyond a week in advance, the Met Office uses ensemble techniques to predict the likely track of tropical cyclones. This involves running 24 separate forecasts from slightly different starting conditions. The spread of the forecast tracks enables a probabilistic forecast of the track to be made. These forecasts run out to 15 days ahead.

Responsibility for issuing forecasts of tropical cyclones to the public and emergency services rests with a number of designated forecasting agencies based in the tropics such as the US National Hurricane Center. However, Met Office forecasts are shared with these agencies contributing to warnings for tropical cyclones worldwide. Met Office forecasts also inform the Foreign and Commonwealth Office and aid agencies of the likelihood of tropical cyclones in their areas of interest.

On a seasonal timescale (up to six months ahead) it is impossible to predict the formation and track of individual tropical cyclones. However, it is possible to predict whether tropical cyclone activity across a whole season and whole region is expected to be above, near or below normal. Since 2007, we have issued forecasts of Atlantic tropical cyclone activity using our GloSea model and the model of the European Centre for Medium Range Weather Forecasts (ECMWF). These forecasts are useful to the reinsurance market which needs to assess the risk of financial losses from hurricane strikes months in advance.

The Met Office uses its climate model to predict trends in tropical cyclone activity on long timescales. In 2010, Met Office scientist Doug Smith and his team won the Lloyd's Science of Risk Research Prize for groundbreaking work on the prediction of Atlantic hurricane frequency on decadal timescales. Research using the Met Office climate model contributes to the ongoing international effort to predict the impact of climate change on tropical cyclone frequency and intensity.

A team at the Met Office strives to predict tropical cyclones across the timescales. This work involves evaluating and improving numerical model predictions, providing products and services to other meteorological agencies, industry and the public. It also forms part of the global research effort to improve understanding and prediction of the force of nature which is the tropical cyclone.

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