'Rivers' in air could boost flooding

Cockermouth in 2009 The flooding in Cockermouth in 2009 was caused by one such atmospheric river

Winter floods could intensify in Britain, according to new research into powerful weather systems called "atmospheric rivers".

Only identified about 20 years ago, atmospheric rivers are intense bands of moisture that flow through the air.

Known to be responsible for heavy rainfall, they have been blamed for severe flooding in California and the UK.

The new study suggests that warmer conditions could create more rivers - and make them more severe.

The paper is published by the Institute of Physics in Environmental Research Letters.

Atmospheric rivers are up to 300km wide and can stretch in length for over 1,000-2,000km. They flow invisibly between 1-2.5km above the surface of the ocean.

One atmospheric river is believed to have been behind the violent flooding that hit Cockermouth in Cumbria on 19 November 2009.

The flooding claimed the life of a policeman, PC Bill Barker, who died after a bridge collapsed.

The researchers, led by Dr David Lavers of the University of Iowa, have estimated the staggering volume of moisture carried by this particular atmospheric river.

They calculate that at its peak it was transporting almost 300,000 tonnes of moisture every second.

By comparison, the River Thames carries about 65 tonnes of water through London over the same period.

Remain on course

If the rivers make landfall and encounter a steep rise in terrain, the air is forced upwards where it cools and releases the moisture in the form of rain.

On top of that, if the river remains on the same course for 24 hours - as it did over Cumbria in 2009 - it will deliver a continuous flow of heavy rain over the same area.

The most closely-studied atmospheric river, which flows towards the California coast, has been dubbed the "Pineapple Express" because it usually originates from the region of Hawaii.

Infographic

It has been linked to a number of extremely damaging storms along the US West Coast.

Over the last 30 years, there has been an average of 9-11 of the strongest atmospheric river events hitting Britain every year.

In this latest study, the researchers examined five different modelling scenarios to simulate possible conditions this century and found that a warming climate - which allows the atmosphere to hold more moisture - made the rivers more likely.

Dr Lavers said: "All five models suggest that there could be a doubling of atmospheric river events in the period 2074-99 and most of those could be expected to make landfall in the UK.

"One of the big things is that these are the most relevant feature of winter flooding in Britain and the work is certainly suggesting an increase in strength and frequency."

Computer modelling

Among the uncertainties about the research are the reliability of the models used to generate the future scenarios and possible shifts in the patterns of the winds - a change of course away from the UK would reduce the risk.

It was research in the 1970s that first identified "conveyor belts" of moisture travelling through the atmosphere, with later studies in the early 1990s detecting much narrower bands of intense vapour that became known as atmospheric rivers.

Dr Richard Allan of Reading University, also an author of the paper, said: "What this shows is that the dominating factor is the increase in water vapour which means that if you've got more moisture - and the winds don't change -then you've got a much bigger potential for flooding.

"These are really massive flows of invisible water which can feed clouds and cause rainfall if forced up over mountains."

The researchers say the study could help guide forecasters trying to give warning of future flood risks.

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