A little-understood weather phenomenon called “atmospheric rivers” — water carried across the Atlantic by strong winds as long streams of vapour as well as clouds — is key to understanding the severe flooding affecting Britain in the past two weeks, according to a leading US weather expert.
Modelling based on the work of Professor Marty Ralph at the Scripps Institution of Oceanography in San Diego shows that a third such weather event is set to hit Britain at the end of this week, although the rainfall is not likely to be quite as intense as last weekend’s deluge.
The UK Environment Agency has warned of further flooding risks across England and Wales with heavy rain falling on saturated ground, adding to the volumes of water in swollen rivers in the wake of two storms — Dennis and Ciara — that hit the British Isles on successive weekends.
Prof Ralph has led research into atmospheric rivers that flow in from the Pacific Ocean across western North America and is now looking to raise awareness of their importance over the Atlantic and how climate change increases the threat they pose.
“Projections from climate models show ARs becoming a bit larger and some of them being more intense, partly because a warmer atmosphere can hold more water in these storms,” he said. “It is quite plausible that we will be seeing somewhat larger and longer lived ARs and those are the ones that create the big problems for infrastructure.”
The significance of this weather phenomenon — and the potential to improve flood forecasting by understanding it better — has only recently recognised. Atmospheric rivers were first defined by the American Meteorological Society in 2017.
Since then Prof Ralph, working with other US weather experts, has developed a hurricane-style warning system published last year to characterise the strength and impact of atmospheric rivers.
The scale ranges from category one or weak, deemed as beneficial rainfalls lasting 24 hours, to category five or exceptional, where the rainfall lasting over 100 hours is deemed hazardous. One such level five event in California lasted five days over the 1997 new year, causing $1bn in damage.
Looking at his computer model of Storm Dennis that hit the UK last weekend, after presenting his findings to the American Association for the Advancement of Science meeting in Seattle, Prof Ralph told the Financial Times: “That is a five.”
A typical atmospheric river is 300km to 500km wide and thousands of kilometres long, flowing in the atmosphere far below the jet stream, and it may persist for several days.
Large ones carry prodigious amounts of water vapour, which condenses and causes heavy rain when landmasses push the air flow upwards. The amount of water moving across Britain last weekend was at least 50 times the flow of the Mississippi River, Prof Ralph estimated.
He said the role played by atmospheric rivers from the North Atlantic in the water cycle of north-west Europe had received relatively little attention even from professional meteorologists. Next month he will visit the European Centre for Medium-Range Weather Forecasts in Reading to discuss how global meteorology could benefit from taking more account of atmospheric rivers.
Atmospheric rivers are closely related to features present in familiar meteorological charts such as depressions, isobars and weather fronts, but Prof Ralph says taking explicit account of them can improving forecasting.
They always involve strong winds but often form well away from the low-pressure centres that people associate with bad weather. In the case of Storm Dennis the atmospheric river was flowing across Britain more than 1,500km south-east of the intense depression near Iceland which was in conventional terms at the heart of the storm.
Research into the phenomenon will get a boost this month with the US Air Force and US National Oceanic and Atmospheric Administration flying their fleet of “hurricane hunter” aircraft, which are normally used to fly into tropical cyclones, into atmospheric rivers over the Pacific.
“With the ‘AR Recon’ missions we are working to improve forecasts of where atmospheric rivers will make landfall and better understand how much precipitation they may bring, which has impacts on water supply, flood mitigation and more,” said Prof Ralph.
