http://www.climatecentral.org/news/biggest-rain-and-snowstorms-likely-to-get-bigger-study-finds-15832

Heavy, Dam-Busting Rainstorms To Increase, Study Finds

• Published: April 5th, 2013 , Last Updated: April 5th, 2013

By Andrew Freedman

Boosted by the added moisture from warming air and ocean temperatures, the heaviest precipitation events — those that can cause dams to fail, rivers to spill over their banks, and cities to flood — are likely to become significantly heavier by the end of this century, according to a new study. The study, published in the journal Geophysical Research Letters, warns engineers and planners that are designing long-lasting, critical infrastructure that planning for only the current types and severity of extreme events is likely to underestimate the actual risk. 

This is the first study to focus on changes in the heaviest possible rain or snowstorms, and it builds upon the findings of previous research that found that extreme precipitation events are already becoming more common in some parts of the world, and that global warming has increased the odds of particular flooding events, such as record flooding that struck parts of England and Wales in 2000. 

The study examined changes in what is known as the "potential maximum precipitation," which is the maximum amount of precipitation that is "potentially possible" in a particular area given ideal conditions.

That calculation is not something that will be familiar to viewers of the daily TV weather report, but it helps engineers guide infrastructure planning for dams and other water management structures, many of which are constructed to last 50 to 100 years. Many of the dams and levees in use today along the Mississippi River were designed in the early 1900s, for example.

"We looked very specifically at the biggest storms," said Kenneth Kunkel,  a senior research professor at North Carolina State University's Cooperative Institute for Climate and Satellites and lead author of the study. "What we're saying is the big event on the extreme tail of the distribution . . . that event is likely to be bigger by the end of this century; quite a bit bigger."

The paper looked at three factors that can affect the potential maximum precipitation: the amount of moisture, or water vapor, in the atmosphere, the vertical motion of air, and horizontal winds. The researchers used climate models to simulate how increasing amounts of greenhouse gases in the atmosphere, such as carbon dioxide, would alter those factors. Kunkel said the researchers were particularly interested in seeing whether the other factors would offset the influence of the already observable upward trend in water vapor.

The study found just the opposite — the only factor that will change significantly in a warming world is the maximum moisture in the air, and it won't be counterbalanced by changes in the other variables.

Using climate models that incorporated two different scenarios of future greenhouse gas emissions, the researchers found that the largest precipitation events could be as much as 20 to 30 percent larger across the lower 48 states by the 2071-2100 period. The biggest increases are slated for the West, the study found, although the maximum precipitation values are generally lower in the West than they are in the more humid eastern half of the country.

Heavy precipitation events are already becoming more frequent and/or severe in parts of the U.S. This chart shows extremes in 1-day precipitation in the Northeast during the Fall season.
Click image to enlarge. Credit: NOAA/NCDC.

The study also noted that the plumes of rich, tropical moisture known as "atmospheric rivers," which have been responsible for many of the historic floods along the West Coast, may grow more intense — making them more analogous to atmospheric rapids.

Dam failures can have catastrophic consequences. In 1889, a dam failure in Pennsylvania killed more than 2,200 people in Johnstown, and more recently, dam and levee failures were responsible for much of the deadly flooding in New Orleans when Hurricane Katrina made landfall in 2005.

Other studies have shown that there already is about 4 percent more water vapor in the air over the planet's oceans compared to the preindustrial era, which translates into extra moisture for storms of all sizes to wring out as rain or snow. Water vapor is forecast to continue to increase as temperatures rise throughout the rest of the 21st century, since warmer air holds more moisture, and warmer ocean temperatures allow for more evaporation to occur. Published research also shows that global warming may yield wetter and more powerful hurricanes in the North Atlantic Basin, although there may be fewer storms per hurricane season.

How much temperatures and water vapor will go up depends in part on the amount and pace of any greenhouse gas emissions reductions.

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