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Atmospheric Rivers Grow, Causing Worse Floods Ahead

By Paul Brown, Climate News Network

LONDON – Heavy and prolonged rainfall will cause both more frequent and more severe flooding across the United Kingdom and the rest of north-west Europe as the atmosphere continues to warm, say British and American scientists.

A study in IOP Publishing’s Environmental Research Letters of what are known as atmospheric rivers pins the blame for the increasing flood risk firmly on man-made climate change and says the same problem will afflict other parts of the planet.

Don’t go out without your umbrella. More downpours are on the way due to larger atmospheric rivers from a warming climate.
Credit: Roger Price via Wikimedia Commons

Researchers at the University of Reading near London, and the U.S. University of Iowa, describe how atmospheric rivers carry vast amounts of water vapor around the Earth, delivering heavy and prolonged rainfall, particularly to mountainous areas. They were responsible for the protracted winter and summer floods in the UK in 2012, which caused an estimated $1.6 billion in damage.

In a warming world the atmosphere can carry more water and the research showed that the rivers, typically running a kilometer (.62 miles) above the earth, 300 kilometers (186 miles) wide and thousands of kilometers long, would become larger and capable of delivering even bigger quantities of prolonged rainfall.

Longer danger period

An example of their potential danger is the atmospheric river that caused the severe flooding on November 19, 2009 over north-west Britain. As it approached the coast it was transporting a moisture volume 4,500 times the average gauged flow of the river Thames through London.

In California, where atmospheric rivers (ARs) have already been assessed, the climate models predict that the number of years with these features will increase. To discover what could happen in Europe the models were tested against the known flooding events between 1980 and 2005, and the researchers found that they could accurately simulate what actually happened.

This gave them confidence to test what would happen in the future. All the models showed that with more greenhouse gases emitted by humans there would be a doubling of the number of atmospheric rivers later this century compared with the 1980 to 2005 period. Most of these events occur in the winter, but in a warmer world the danger period is extended.

Effects will be widespread

Because of the way the warmer atmosphere is able to carry more water and deliver much higher rainfall totals, the potential for far worse floods from each of these rainfall events is much increased.

Because of the way the warmer atmosphere is able to carry more water and deliver much higher rainfall totals, the potential for far worse floods from each of these rainfall events is much increased.
Credit: flickr/U.S. Geological Survey

The head of the research, Dr. David Lavers, from the department of meteorology at the University of Reading, said: “ARs could become stronger in terms of their moisture transport. In a warming world, atmospheric water vapor content is expected to rise due to an increase in saturation water vapor pressure with air temperature. This is likely to result in increased water vapor transport.”

“The link between ARs and flooding is already well established, so an increase in AR frequency is likely to lead to an increased number of heavy winter rainfall events and floods. More intense ARs are likely to lead to higher rainfall totals, and thus larger flood events.”

The paper points out that while the scientists were specifically looking at the atmospheric rivers that caused heavy rainfall in Europe; these storms affect many temperate regions of the planet. As the atmosphere warms, it is likely that they will increase the risk of flooding elsewhere.

Paul Brown is a joint editor at Climate News Network. Climate News Network is a news service led by four veteran British environmental reporters and broadcasters. It delivers news and commentary about climate change for free to media outlets worldwide.


By Eric Peterson (Front Royal, VA 22630)
on July 27th, 2013

“Atmospheric river” is another word for the jet stream flowing from a warm moist area of the ocean.  It is a completely ordinary phenomenon, not some new manifestation of global warming.  One might argue that the warm moist areas at the sources of the jet will get warmer on average, with global warming.  But the moisture transport and in particular the rainfall at the other end is determined by dynamics, not moisture and warmth.  For example if there is no lift the “atmospheric river” will pass overhead with no or little rainfall.

There is very little understanding of how dynamics will change, if at all, with global warming.  One piece of consensus science up to the late 2000’s was that the jet stream would be stronger and therefore stay further north and therefore create fewer “atmospheric rivers”.  Now work by Francis and others challenges that consensus but it is still speculative and not at all settled.

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By M J Murcott
on July 28th, 2013

Why do rivers flood and burst their banks and what can we do to reduce the risks of river flooding. -

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By Dave (Basking Ridge, NJ 07920)
on July 29th, 2013

I had not heard of atmospheric rivers (ARs) before. Thousands of miles long, massive amounts of water transport, relatively low altitude – it all sounds very interesting. Clearly they are not part of the jet stream because at the very least the typical altitude and speed are all wrong. Anyway I searched and found some references which I found to be an interesting introduction – perhaps others will too:

1. The full pdf for a 2011 paper by the same lead author of the article referenced here – and on the same topic. It gives a bit more of the technical background on ARs in this British context than is available from the abstract of the current article.

“Winter floods in Britain are connected to atmospheric rivers”

2. NOAA has some web pages:

3. A slide presentation by a professor of climate science at Scripps Inst of Oceanography (UCSD) with some interesting imagery:

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By Robert Marston
on July 29th, 2013

@ Eric P

What you’ve missed is that the research found that global warming increases to the hydrological cycle (rate of evaporation) increased the size and frequency of atmospheric rivers to produce more powerful rainfall events. You should also know that the same mechanism—increased evaporation—also results in more numerous and severe droughts.

Water, in a warmer world, goes up into the air faster and when it comes down, it comes down harder.

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By Eric Peterson (Front Royal, VA 22630)
on July 30th, 2013

Dave, this is from wikipedia: “Pineapple Express is a non-technical term for a meteorological phenomenon characterized by a strong and persistent flow of atmospheric moisture and associated with heavy precipitation from the waters adjacent to the Hawaiian Islands and extending to any location along the Pacific coast of North America. A Pineapple Express is an example of an atmospheric river, which is a more general term for such narrow corridors of enhanced water vapor transport at mid-latitudes around the world.”

Then they say “A Pineapple Express is driven by a strong, southern branch of the Polar jetstream and is marked by the presence of a surface frontal boundary which is typically either slow or stationary, with waves of low pressure traveling along its axis. Each of these low pressure systems brings enhanced rainfall”

There are two ingredients: a jet stream from an area from a high moisture source and some dynamics like the short waves rippling along the jet.  While the “atmospheric river” is not a jet, an atmospheric river requires a jet.

Robert Marston, increased average size is possible driven the increase in temperature from added CO2.  But like temperatures, evaporation varies widely with weather so there may be no increase in evaporation at the source of “atmospheric rivers” nor an increase in precipitation at the end.  Why?  Because those are primarily driven by atmospheric dynamics, not heat and moisture.

The dynamics, in a nutshell, are that the jet stream has sections of higher and lower speeds.  The higher speed areas (divergence) draw warm air up from the surface.  That forms a kink or short wave that moves along the jet which then allows more convergence and divergence to drop the rain on the target area.  The one factor related to global warming is some extra rising warm air (on average) ahead of the short wave.  But that is only one factor of several.  The most important factor is the jet itself.

As I state above, consensus science is that there will be less dips and meandering in the polar jets,, therefore fewer atmospheric rivers.

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By Dave (Basking Ridge, NJ 07920)
on July 30th, 2013

I also came across this page of animations of the past few days of satellite microwave data. It provides mapped concentrations of precipitable water over time. Although wind speeds measurements are not included, the accepted inference appears to be that the animations identify AR locations. The page includes regional options.

Eric: Yes I have read that Wikipedia entry too in relation to the so-called pineapple express AR. The point is that ARs are not the jet stream as you first stated – or perhaps mistated. It is also however not clear to me that ARs would require a jet in order to form. In that respect it would be nice to be able to overlay the coincident track of major jets on the AR satellite imagery to see whether or not or where there are clear interactions.

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By Eric Peterson (Front Royal, VA 22630)
on July 31st, 2013

Dave, I misstated, an AR is not a jet.  But an AR does require a jet stream which coincides with the surface front or is parallel to it, otherwise the good moisture (well above the surface) will not move north or northeast.

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By Joe Witte (Arlington, VA 22209)
on August 1st, 2013

Check out video of NOAA’s AR work using NASA’s Global Hawk

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