New study: Freak summer weather and wild jet-stream patterns are on the rise because of global warming

Simulation of jet stream pattern July 22, 2018. (VentuSky.com)

by Jason Samenow, "The Capitol Weather Gang," The Washington Post, October 31, 2018

In many ways, the summer of 2018 marked a turning point, when the effects of climate change — perhaps previously on the periphery of public consciousness — suddenly took center stage. Record high temperatures spread all over the Northern Hemisphere. Wildfires raged out of control. And devastating floods were frequent.

Michael Mann, climate scientist at Pennsylvania State University, along with colleagues, has published a new study that connects these disruptive weather extremes with a fundamental change in how the jet stream is behaving during the summer. Linked to the warming climate, the study suggests this change in the atmosphere’s steering current is making these extremes occur more frequently, with greater intensity, and for longer periods of time.

The study projects this erratic jet-stream behavior will increase in the future, leading to more severe heat waves, droughts, fires and floods.

[Climate change supercharged a hot and dangerous summer]

The jet stream is changing not only because the planet is warming up but also because the Arctic is warming faster than the mid-latitudes, the study says. The jet stream is driven by temperature contrasts, and these contrasts are shrinking. The result is a slower jet stream with more wavy peaks and troughs that Mann and his study co-authors ascribe to a process known as “quasi-resonant amplification.”

The altered jet-stream behavior is important because when it takes deep excursions to the south in the summer, it sets up a collision between cool air from the north and the summer’s torrid heat, often spurring excessive rain. But when the jet stream retreats to the north, bulging heat domes form underneath it, leading to record heat and dry spells.

[‘Extreme’ jet stream pattern spurred a week of wild weather in U.S. this past summer]

If the excursions in the jet stream endure long enough, it can then set the stage for floods where the jet dips, and wildfires and drought where it ascends.

“What made these events [in the summer of 2018] so devastating was not just the extreme nature of the meteorological episodes but their persistence,” Mann said in a blog post discussing the implications of the new study.

The study, published Wednesday in Science Advances, finds that these quasi-resonant amplification events — in which the jet stream exhibits this extreme behavior during the summer — are predicted to increase by 50 percent this century if emissions of carbon dioxide and other greenhouse gases continue unchecked.

Whereas previous work conducted by Mann and others had identified a signal for an increase in these events, this study for the first time examined how they may change in the future using climate model simulations.

“Looking at a large number of different computer models, we found interesting differences,” said Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research and a co-author of the study, in a news release. “Distinct climate models provide quite diverging forecasts for future climate resonance events. However, on average they show a clear increase in such events.”

In an email, Mann said climate models aren’t fully capturing the phenomenon, and, for this reason, we should expect weather extremes “beyond what is typically projected” into the future.

Mann added the existing analyses that attempt to uncover the role of climate change in recent extreme events “are under-attributing the role that climate change is having … because they are not capturing the key mechanism responsible.”

Mann said in his blog commentary that he was particularly struck by the jet-stream behavior in the summer. “In summer 2018, I would argue, that signal was no longer subtle,” he said. “It played out in real time on our television screens and newspaper headlines in the form of an unprecedented hemisphere-wide pattern of extreme floods, droughts, heat waves and wildfires.”

Although model projections suggest these extreme jet-stream patterns will increase as the climate warms, the study concluded that their increase can be slowed if greenhouse gas emissions are reduced along with particulate pollution in developing countries. “[T]he future is still very much in our hands when it comes to dangerous and damaging summer weather extremes,” Mann said. “It’s simply a matter of our willpower to transition quickly from fossil fuels to renewable energy.”

Dr. Jennifer Francis, a climate researcher at Rutgers University who has published work exhibiting changing jet-stream behavior because of climate change, found the results of this new study compelling. “This work takes a big step toward understanding the spate of deadly extreme weather events during recent summers — heat waves, floods and droughts,” she said in an email.

https://www.washingtonpost.com/weather/2018/10/31/study-freak-summer-weather-wild-jet-stream-patterns-are-rise-due-global-warming/?noredirect=on&utm_term=.7afba03610ea

Lenders' Guide for Considering Climate Risk in Infrastructure Investments, January 2018

Acclimatise, Climate Finance Advisors (CFA), and Four Twenty Seven have released a new guidance document to increase the climate resilience of large infrastructure investments. The “Lenders’ Guide for Considering Climate Risk in Infrastructure Investments” clearly breaks down the ways in which physical climate risks might affect key financial aspects of prospective infrastructure investments. 

This guide provides a framework for questioning how revenues, costs, and assets can be linked to potential project vulnerability arising from climate hazards and draws attention to the potential opportunities emerging from resilience-oriented investments in infrastructure.

Ten sub-sectors, including airports, marine ports, gas and oil transport and storage, power transmission and distribution, wind-based power generation, data centers, telecommunications, commercial real estate, healthcare, and sports and entertainment, are analysed and illustrated with topical examples.

To learn more about this document, please visit our website and download the publication here.

Download the guide at this link:

http://www.acclimatise.uk.com/wp-content/uploads/2018/01/Lenders_Guide_for_Considering_Climate_Risk_in_Infrastructure_Investments.pdf

"Increased rainfall volume from future convective storms in the US" by Andreas Prein et al., Nature Climate Change, 7 (2017); doi: 10.1038/s41558-017-0007-7

Nature Climate Change, 7 (2017) 880–884; doi: 10.1038/s41558-017-0007-7

Increased rainfall volume from future convective storms in the US

• Andreas F. Prein, 
• Changhai Liu, 
• Kyoko Ikeda, 
• Stanley B. Trier, 
• Roy M. Rasmussen, 
• Greg J. Holland and 
• Martyn P. Clark
• 
  
• Abstract

Mesoscale convective system (MCS)-organized convective storms with a size of ~100 km have increased in frequency and intensity in the USA over the past 35 years1, causing fatalities and economic losses2. However, their poor representation in traditional climate models hampers the understanding of their change in the future3. Here, a North American-scale convection-permitting model which is able to realistically simulate MSCs4 is used to investigate their change by the end-of-century under RCP8.5 (ref. 5). A storm-tracking algorithm6 indicates that intense summertime MCS frequency will more than triple in North America. Furthermore, the combined effect of a 15–40% increase in maximum precipitation rates and a significant spreading of regions impacted by heavy precipitation results in up to 80% increases in the total MCS precipitation volume, focused in a 40-km radius around the storm center. These typically neglected increases substantially raise future flood risk. Current investments in long-lived infrastructures, such as flood protection and water management systems, need to take these changes into account to improve climate-adaptation practices.

https://www.nature.com/articles/s41558-017-0007-7

North American Storm Clusters Could Produce 80 Percent More Rain Say NCAR Scientists

by Floodlist News, November 28, 2017

Major clusters of summertime thunderstorms in North America will grow larger, more intense, and more frequent later this century in a changing climate, unleashing far more rain and posing a greater threat of flooding across wide areas, new research concludes.

The study, by scientists at the National Center for Atmospheric Research (NCAR), builds on previous work showing that storms are becoming more intense as the atmosphere is warming. In addition to higher rainfall rates, the new research finds that the volume of rainfall from damaging storms known as mesoscale convective systems (MCSs) will increase by as much as 80% across the continent by the end of this century, deluging entire metropolitan areas or sizable portions of states.

“The combination of more intense rainfall and the spreading of heavy rainfall over larger areas means that we will face a higher flood risk than previously predicted,” said NCAR scientist Andreas Prein, the study’s lead author. “If a whole catchment area gets hammered by high rain rates, that creates a much more serious situation than a thunderstorm dropping intense rain over parts of the catchment.”

“This implies that the flood guidelines which are used in planning and building infrastructure are probably too conservative,” he added.

The research team drew on extensive computer modeling that realistically simulates MCSs and thunderstorms across North America to examine what will happen if emissions of greenhouse gases continue unabated.

The study will be published November 20, 2017, in the journal Nature Climate Change. It was funded by the National Science Foundation, which is NCAR’s sponsor, and by the U.S. Army Corps of Engineers.

Hourly rain rate averages for the 40 most extreme summertime mesoscale convective systems (MCSs) in the current (left) and future climate of the mid-Atlantic region. New research shows that MSCs will generate substantially higher maximum rain rates over larger areas by the end of the century if society continues a “business as usual” approach of emitting greenhouse gases . Image: ©UCAR, Image by Andreas Prein, NCAR.

This satellite image loop shows an MCS developing over West Virginia on June 23, 2016. The resulting floods caused widespread flooding, killing more than 20 people. MCSs are responsible for much of the major flooding east of the Continental Divide during warm weather months. (Image by NOAA National Weather Service, Aviation Weather Center.)

A Warning Signal

Thunderstorms and other heavy rainfall events are estimated to cause more than $20 billion of economic losses annually in the United States, the study notes. Particularly damaging, and often deadly, are MSCs: clusters of thunderstorms that can extend for many dozens of miles and last for hours, producing flash floods, debris flows, landslides, high winds, and/or hail. The persistent storms over Houston in the wake of Hurricane Harvey were an example of an unusually powerful and long-lived MCS.

Storms have become more intense in recent decades, and a number of scientific studies have shown that this trend is likely to continue as temperatures continue to warm. The reason, in large part, is that the atmosphere can hold more water as it gets warmer, thereby generating heavier rain.

A study by Prein and co-authors last year used high-resolution computer simulations of current and future weather, finding that the number of summertime storms that produce extreme downpours could increase by five times across parts of the United States by the end of the century. In the new study, Prein and his co-authors focused on MCSs, which are responsible for much of the major summertime flooding east of the Continental Divide. They investigated not only how their rainfall intensity will change in future climates, but also how their size, movement, and rainfall volume may evolve.

Analyzing the same dataset of computer simulations and applying a special storm-tracking algorithm, they found that the number of severe MCSs in North America more than tripled by the end of the century. Moreover, maximum rainfall rates became 15-40% heavier, and intense rainfall reached farther from the storm’s center. As a result, severe MCSs increased throughout North America, particularly in the northeastern and mid-Atlantic states, as well as parts of Canada, where they are currently uncommon.

The research team also looked at the potential effect of particularly powerful MCSs on the densely populated Eastern Seaboard. They found, for example, that at the end of the century, intense MCSs over an area the size of New York City could drop 60% more rain than a severe present-day system. That amount is equivalent to adding six times the annual discharge of the Hudson River on top of a current extreme MCS in that area.

“This is a warning signal that says the floods of the future are likely to be much greater than what our current infrastructure is designed for,” Prein said. “If you have a slow-moving storm system that aligns over a densely populated area, the result can be devastating, as could be seen in the impact of Hurricane Harvey on Houston.”

Intensive Modeling

Advances in computer modeling and more powerful supercomputing facilities are enabling climate scientists to begin examining the potential influence of a changing climate on convective storms such as thunderstorms, building on previous studies that looked more generally at regional precipitation trends.

For the new study, Prein and his co-authors turned to a dataset created by running the NCAR-based Weather and Research Forecasting (WRF) model over North America at a resolution of 4 kilometers (about 2.5 miles). That is sufficiently fine-scale resolution to simulate MCSs. The intensive modeling, by NCAR scientists and study co-authors Roy Rasmussen, Changhai Liu, and Kyoko Ikeda, required a year to run on the Yellowstone system at the NCAR-Wyoming Supercomputing Center.

The team used an algorithm developed at NCAR to identify and track simulated MCSs. They compared simulations of the storms at the beginning of the century, from 2000 to 2013, with observations of actual MCSs during the same period, and showed that the modeled storms are statistically identical to real MCSs.

The scientists then used the dataset and algorithm to examine how MCSs may change by the end of the century in a climate that is approximately 5 degrees Celsius (9 degrees Fahrenheit) warmer than in the pre-industrial era — the temperature increase expected if greenhouse gas emissions continue unabated.

About the paper

Title: Increased rainfall volume from future convective storms in the US
Authors: Andreas F Prein, Changhai Liu, Kyoko Ikeda, Stanley B Trier, Roy M Rasmussen, Greg J Holland, Martyn P Clark

Source: University Corporation for Atmospheric Research

http://floodlist.com/america/usa/north-american-storm-clusters-could-produce-80-percent-more-rain-say-ncar-scientists

Catastrophic mesoscale convective system storms in the Sahel now three times more likely

Climate change brings more Sahel storms

Climate change is upsetting rainfall patterns and the frequency of flooding in West Africa as it makes the region's Sahel storms three times likelier.

by Tim Radford, Climate News Network, May 7, 2017

LONDON – Climate change has already made a difference to life in the West African Sahel, the arid belt of land stretching from the Atlantic to the Red Sea which separates the Sahara desert from the African savanna. It has made catastrophic storms three times more frequent.

And, according to a new study in the journal Nature, Sahel storms are among the most powerful on the planet. In 2009, one vast downpour deposited 263 mm of rain over Ouagadougou, the capital of Burkina Faso, claiming 8 lives, flooding half the city and forcing 150,000 people out of their homes.

Researchers believe the pattern of thunderstorms known as mesoscale convective systems will increase in frequency as global temperatures rise, as a consequence of increasing levels of carbon dioxide in the atmosphere, in turn driven by worldwide use of fossil fuels as sources of energy.

Mesoscale convective systems are big, bad, and very cold columns of thunderous cloud: up to 16 km high, covering an area of 25,000 square kilometres, and with temperatures at the highest altitude as low as minus 40 °C.

Between 1986 and 2005, Burkina Faso registered floods at a rate of little more than one a year. In the 11 years between 2006 and 2016, it was hit by 55 flood events.

Repeated warnings

Climate scientists have been warning for three decades that global warming will be accompanied by an increase in “extreme” events: in particular drought, flood, heat wave, and tropical cyclone.

Global warming has already been observed in the Sahel, and the consequences have not necessarily been bad: overall, precipitation has increased, and farmers have benefited, although in a dryland region south of the Sahara where people have endured a 2,000-year history of periodic drought, famine remains a constant hazard.

And now, so do massive downpours of rain: the Sahel storms. British and French scientists examined 35 years of satellite data and the rain gauges in the region to identify a rise in extreme daily rainfall totals. They found 85% of extreme rainfall cases coincided with satellite records of a passing mesoscale convection system.

They also examined the pattern of temperatures over the region and found that although the annual average temperatures have risen, the so-called “wet season” temperatures have remained steady. That is, locally warmer conditions alone have not brought more rainfall.

“Global warming is expected to produce more intense storms, but we were shocked to see the speed of changes taking place in this region of Africa”

Instead, they blame man-made global warming which has changed wind and rain conditions, and this will go on strengthening during this century, “suggesting the Sahel will experience particularly marked increases in extreme rain,” they conclude.

“Global warming is expected to produce more intense storms, but we were shocked to see the speed of changes taking place in this region of Africa,” said Christopher Taylor, a meteorologist at the UK’s Centre for Ecology and Hydrology, who led the study.

His co-author Douglas Parker, professor of meteorology at the University of Leeds in the UK, said: “African storms are highly organised meteorological engines, whose currents extract water from the air to produce torrential rain.

“We have seen these engines becoming more efficient over recent decades, with resulting increases in the frequency of hazardous events.” 

http://climatenewsnetwork.net/climate-change-brings-more-sahel-storms

Climate Code Red: Climate change pushing floods, cyclones to new extremes, with worse to come

by David Spratt, Climate Code Red, April 1, 2017

With Australia experiencing the aftermath of Cyclone Debbie and record-breaking rains and severe flooding in southeast Queensland and along the north coast of New South Wales, here’s a look at how global warming has, and will, push floods and cyclones to new extremes.

Flooding extremes

Warm air can be more humid than cold air, that is, it can hold more water vapour in absolute terms. And atmospheric water vapour content increases 7% for each 1-degree-Celsius increase in global average temperature, establishing the conditions for more intense rainfall events. 

Flash floods are likely to sweep across the Australian landscape with increasing intensity, particularly in urban or residential areas. Peak rainfall is predicted to soar with rising surface temperatures as Australia experiences ever greater extremes of heat.  

The frequency of major flood events (defined as events which caused extensive flooding within 50 kilometres of the coast, or inundation that extended 20 kilometres along the coast) along Australia's eastern seaboard has doubled in last 150 years, with climate change one of the possible factors, senior Bureau of Meteorology researchers say. 

Record-breaking heavy rainfall and a clear upward trend in downpours over the last 30 years fits in with global temperature rise caused by greenhouse gases. Statistical analysis of rainfall data from 1901 to 2010 around the globe, shows that from 1980 to 2010 there were 12% more of these intense events than would be expected in a climate without global warming. Wet regions generally saw a bigger increase in deluges and drier regions a smaller one. In southeast Asia, the observed increase in record-breaking rainfall events is as high as 56%.

Giant air streams pushing new extremes: The increase of devastating weather extremes in summer, including floods, is likely linked to human-made climate change, mounting evidence shows, with the recent discovery of giant airstreams circling the Earth, waving up and down between the Arctic and the tropics. These planetary waves transport heat and moisture. When these planetary waves stall, droughts or floods can occur. Warming caused by greenhouse-gases from fossil fuels creates favourable conditions for such events.

 “The unprecedented 2016 California drought, the 2011 U.S. heatwave and 2010 Pakistan flood, as well as the 2003 European hot spell, all belong to a most worrying series of extremes,” says Michael Mann, a lead author of the study. “The increased incidence of these events exceeds what we would expect from the direct effects of global warming alone, so there must be an additional climate change effect. In data from computer simulations as well as observations, we identify changes that favour unusually persistent, extreme meanders of the jet stream that support such extreme weather events. Human activity has been suspected of contributing to this pattern before, but now we uncover a clear fingerprint of human activity.”

Attribution studies show how the risk of a particular event may have changed due to the human influence on climate. Some attribution results surveyed by the World Meteorological Organisation include:

• The US National Oceanic and Atmospheric Administration determined that human-caused climate change increased chances of the fatal and record rains in Louisiana by at least 40% and could have nearly doubled the odds of such a storm.
• A scientific analysis of devastating 2014 floods in the United Kingdom, which cost an estimated $646 million in insurance losses, found that human-caused climate change has increased the chance of the extreme rain event by 43%.
• In May–June 2016, portions of northeast France received 6 full weeks of rain in 24 hours. A formal attribution study released June 9, 2016, found that such extreme rains are at least 40%—and as much as 90%—more likely in some areas of France.

Cyclone extremes

Cyclones, in part, draw their energy from the temperature of the ocean's surface waters, so a warming climate and ocean puts more energy into storms, including cyclones, loading them with more rainfall, and stronger winds pushing more of a storm surge.
The recent Climate Council brief notes, “Increasing temperature of the surface ocean affects the intensity of cyclones, both maximum wind speeds and in the intensity of rainfall that occurs in association with the cyclone.”  The force exerted on buildings and structures when cyclones make landfall increases disproportionately with wind speed.

The Council also notes that: “Tropical cyclones form most readily when there are very warm conditions at the ocean surface and when the vertical temperature gradient through the atmosphere is strong. As this vertical gradient weakens as the climate continues to warm, it is likely that fewer tropical cyclones will form.”

Whilst the best evidence scientists have suggests cyclones are unlikely to increase in number, a 2013 study challenges the status quo, suggesting they will occur more frequently, as well becoming more intense. 

In 2013, researchers reported that the stronger hurricanes in the North Atlantic, the South Pacific, and South Indian Oceans have become more intense.  The same year, the UN meteorological agency concluded that climate change is making super typhoons worse. 

In 2015, an international research team found that a warming planet is already stoking the intensity of tropical cyclones in the northwest Pacific, and their ferocity will continue to increase even with moderate climate change over this century.

More broadly, a 2010 study found that "future projections based on theory and high-resolution dynamical models consistently indicate that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11% by 2100...higher resolution modelling studies typically project substantial increases in the frequency of the most intense cyclones, and increases of the order of 20% in the precipitation rate within 100 km of the storm centre.

Recent records

With sustained wind speeds of more than 310 kilometres per hour, Typhoon Haiyan in the Philippines in November 2013 was the most powerful tropical cyclone to make landfall in recorded history. The previous record was held by Hurricane Camille, which in 1969 hit the state of Mississippi with wind speeds of just over 300 km/h. Data compiled from the US National Oceanic and Atmospheric Administration shows sea temperatures were about 0.5–1.0 degree Celsius above normal in the waters to the east of the Philippines as Haiyan began forming. The waters cooled in the storm's wake, an indication of how the storm sucked up energy.   

Hurricane Patricia which hit Mexico in October 2015 achieved a record peak intensity with maximum sustained winds 345 km/h, making it the most intense tropical cyclone on record in the Western Hemisphere, and the strongest globally in terms of 1-minute maximum sustained winds. Cyclone Winston in February 2016 was the strongest tropical cyclone to make landfall in Fiji and the South Pacific Basin in recorded history.

Attribution studies 

• Superstorm Sandy which hit the northeast coast of the USA with devastating effect in October 2012 was made worse by unusually warm waters which increased the hurricane’s intensity. As well, human-caused sea level rise added to the storm surge, and on the stretch of the Atlantic Coast that spans from Norfolk to Boston, sea levels have been rising four times faster than the global average. Researchers say that “It is possible that subways and tunnels may not have been flooded without the warming-induced increases in sea level and storm intensity and size.”  More broadly, the authors say that “‘snowmaggedon’ in February 2010, superstorm Sandy in October 2012 , supertyphoon Haiyan in November 2013, and the Boulder floods of September 2013 were all influenced by high sea-surface temperatures that had a discernible human component.
• The Climate Council reported that climate change exacerbated the damage caused by Cyclone Pam, which left a trail of destruction across Vanuatu in 2015.

Damage 

Reinsurance giant, MunichRe, says that "nowhere in the world are weather risks changing faster than in Eastern Asia," and concludes that "as a result of climate change... the intensity of typhoons will increase" in Eastern Asia.  On 11 November 2013, in the aftermath of super-typhoon Haiyan, MunichRe surveyed losses:

Eastern Asia has been hard hit by weather-related loss events in the past three decades. Their number has increased by more than a factor of four, causing overall losses from weather-related events of some US$ 700bn during this period. The insured losses of US$ 76bn amounted to only around 10% of overall losses, with 62% of these attributable to Japan. Floods caused 56% of the overall losses in Eastern Asia, but only 30% of insured losses. The number of floods has increased strongly and is expected to increase further in the coming decades. With insured losses of US$ 16bn, the 2011 Thailand floods caused the biggest-ever weather-related insured loss in the region. After floods, it is typhoons that cause the greatest weather-related losses. New analyses indicate a clear cycle of activity for typhoons, and increased typhoon activity is expected over the coming years..."

And in Australia, The Age reports that new modelling has shown that a cyclone the size of Debbie could have catastrophic consequences on the Gold Coast and as far as Brisbane, with winds of 260km/h, in areas where many homes and towers do not meet cyclonic safety standards. As climate change pushes cyclones further south, tens of billions of dollars worth of infrastructure is at risk. Actuaries, who predict and model scenarios for banks and insurers, have warned properties could become "uninsurable" as premiums rise up to 250% to meet this global warming challenge.

http://www.climatecodered.org/2017/04/climate-change-pushing-floods-cyclones.html

Precipitation extremes to worsen as the climate continues to warm

by Floodlist, March 8, 2017

A University of Connecticut climate scientist confirms that more intense and more frequent severe rainstorms will likely continue as temperatures rise due to global warming, despite some observations that seem to suggest otherwise.

In a research paper appearing this week in Nature Climate Change, UConn civil and environmental engineering professor Guiling Wang explains that data showing the intensity of severe rainstorms declining after temperatures reach a certain threshold are merely a reflection of climate variability. It is not proof that there is a fixed upper temperature limit for future increases in severe rains, after which they would begin to drop off.

“We hope this information puts things in better perspective and clarifies the confusion around this issue,” says Wang, who led an international team of climate experts in conducting the study. “We also hope this will lead to a more accurate way of analyzing and describing climate change.”

Climate scientists and policymakers closely monitor severe and prolonged rainstorms as they can have a devastating impact on local environments and economies. These damaging storms can cause catastrophic flooding, overwhelm sewage treatment plants, increase the risk of waterborne disease, and wipe out valuable crops.

Current climate models show most of the world will experience more intense and more frequent severe rainstorms for the remainder of the 21st century, due to hotter temperatures caused by global warming.

But whether this increase in extreme precipitation will continue beyond the end of the century, and how it will be sustained, is less clear.

Meteorological observations from weather stations around the globe show the intensity of severe rainstorms relative to temperature is like a curve — steadily going up as low to medium surface temperatures increase, peaking when temperatures hit a certain high point, then dropping off as temperatures continue rising.

Those observations raise the prospect that damaging rainstorms could eventually ease once surface temperatures reach a certain threshold.

However, Wang says the peaks seen in the observational data and climate models simply reflect the natural variability of the climate. As the Earth warms, her team found, the entire curve representing the relationship between extreme precipitation and rising temperatures is moving to the right. This is because the threshold temperature at which rain intensity peaks also goes up as temperature rises. Therefore, extreme rainfall will continue to increase, she says.

The relationship between precipitation and temperature is founded in science. Simply put, warmer air holds more moisture. Scientists can even tell you how much. A widely used theorem in climate science called the Clausius-Clapeyron equation dictates that for every degree the temperature goes up, there is an approximately 7 percent increase in the amount of moisture the atmosphere can hold. The intensity of extreme precipitation, which is proportional to atmospheric moisture, also increases at a scaling rate of approximately 7 percent, in the absence of moisture limitations.

The problem is that when scientists ran computer models predicting the likelihood of extreme precipitation in the future, and compared those results with both present day observations and the temperature scaling dictated by the so-called “C-C equation,” the numbers were off. In many cases, the increase in extreme precipitation relative to surface temperature over land was closer to 2 to 5 percent, rather than 7 percent. In their analysis, Wang’s team discovered that average local surface temperatures increase much faster than the threshold temperatures for extreme precipitation, and attributed the lower scaling rate to the fact that earlier studies compared extreme precipitation with average local temperatures rather than the temperature at the time the rainstorms occurred.

“There are a lot of studies where people are trying to determine why the scaling rate is lower than 7 percent,” says Wang. “Our study suggests that this is a wrong question to ask. If you want to relate rain intensity to temperature using the C-C relationship as a reference, you have to relate to the temperature at which the rain event occurs, not the mean temperature, which is the long term average.”

Kevin Trenberth, an expert on global warming and the lead author of several reports prepared by the Intergovernmental Panel on Climate Change, joined Wang in the current study. Trenberth is currently a Distinguished Senior Scientist in the Climate Analysis Section at the National Center for Atmospheric Research. He shared the 2007 Nobel Peace Prize with former Vice President Al Gore as a member of the IPCC. Trenberth explains the findings this way:

“In general, extreme precipitation increases with higher temperatures because the air can hold more moisture — although that depends on moisture availability. But beyond a certain point, it is the other way round: the temperature responds to the precipitation, or more strictly speaking, the conditions leading to the precipitation [such as extensive cloud cover or surface moisture]. The most obvious example of this is in a drought where there is no precipitation. Another example is in cloudy, stormy conditions, when it is wet and cool. By relating the changes in precipitation to the temperature where the relationship reverses – instead of the mean temperature as in previous studies — we can make sense of the differences and the changes. Moreover, it means there is no limit to the changes that can occur, as otherwise might be suspected if there were a fixed relationship.”

Source: University of Connecticut

Featured image: Storm system, US Pacific Coast from NOAA’s GOES-West satellite on Jan. 9, 2017. Credits: NASA/NOAA GOES Project

http://floodlist.com/climate/climate-study-intense-frequent-severe-rainstorms-likely-no-drop-off-expected

Eric Holthaus: America’s Latest 500-Year Rainstorm Is Underway Right Now in Louisiana

Observers are calling the record floods a “classic signal of climate change” — and high-resolution models predict another one to two feet of rain by Saturday evening.

Photo: Ines Hegedus-Garcia/Flickr.

by Eric Holthaus, Pacific Standard Magazine, August 12, 2016

By mid-morning on Friday, more than a foot of rain had fallen near Kentwood, Louisiana, in just a 12-hour stretch — a downpour with an estimated likelihood of just once every 500 years, and roughly three months’ worth of rainfall during a typical hurricane season. It’s the latest in a string of exceptionally rare rainstorms that are stretching the definition of “extreme” weather. It’s exactly the sort of rainstorm that’s occurring more frequently as the planet warms.

In response to the ongoing heavy rains, Louisiana Governor John Bel Edwards declared a statewide state of emergency on Friday, and local governments are distributing sandbags, conducting water rescues, and facilitating evacuations. The New Orleans Times-Picayune is maintaining a live blog of the latest developments. The Tickfaw River north of New Orleans soared 18 feet in about 12 hours to a new record crest on Friday morning, beating the water level of April 1983, and 5 feet higher than the high-water mark during Hurricane Isaac in 2012, the last hurricane to make landfall in Louisiana.

Meanwhile, a lot more rain is still on the way. High-resolution weather models predict an additional one or two feet of rain by Saturday evening, a total the local National Weather Service referred to as “scarily high.” The NWS has issued its highest alert for excessive rain and warned of “significant to catastrophic flash flooding.” A “flash flood emergency” is in effect for the hardest-hit regions, a warning reserved only for the direst and most life-threatening events.

An instant analysis from Climate Nexus refers to today’s Louisiana rainstorm as a “classic signal of climate change.” It’s right.

Obviously, this is no ordinary storm. Though the overall structure of this meteorological event does not meet the technical requirements for a tropical storm or hurricane (it’s attached to a stalled weather front, for example), the NWS is treating it roughly the same way, and the physics of the rain clouds themselves are similar. (Tropical rain clouds are generally more efficient at converting cloud moisture into raindrops.)

This storm’s tropical nature, in combination with record-warm water temperatures just offshore in the Gulf of Mexico, are creating a nearly perfect environment for extremely heavy rain and record flooding in one of the wettest places in the country. As the atmosphere warms thanks to greenhouse gas emissions, it can hold more water vapor — and this effect makes it exponentially more likely that extreme rainfall events will occur. The weather balloon released on Friday morning from the New Orleans office of the NWS measured near all-time record levels of atmospheric moisture, higher than some measurements taken during past hurricanes. The NWS meteorologist who reported this morning’s reading remarked simply, “obviously we are in record territory.”

An instant analysis from Climate Nexus refers to today’s Louisiana rainstorm as a “classic signal of climate change.” It’s right. The NWS maintains a statistical database used to calculate the “annual exceedance probability” of a given rainfall event — basically, the expected frequency this event would occur in any given year.

Today’s rainstorm in Louisiana is at least the eighth 500-year rainfall event across America in little more than a year, including similarly extreme downpours in Oklahoma last May, central Texas (twice: last May and last October), South Carolina last October, northern Louisiana this March, West Virginia in June, and Maryland last month.

And these were just the events that the agency decided to write a report on. One notable exception to this list is the Tax Day Flood in the Houston metropolitan area this April, at least the fourth major flood in that region in a span of a year. The local flood control district extrapolated the 23.5 inches of rain over 14.5 hours in Pattison, Texas, during the Tax Day Storm to be a one-in-10,000-year event.

Statistical calculations like these make a major assumption: That the climate of the past is the same as the climate of today. That’s no longer a very good assumption.

https://psmag.com/americas-latest-500-year-rainstorm-is-underway-right-now-in-louisiana-98acbdf435d0#.uum50kkps