Tag Archives: rain

To What Degree is Extreme Weather Linked to Climate Change?

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As flood waters continue to inundate Thailand and drought parches Texas, the Intergovernmental Panel on Climate Change and Goddard Institute for Space Studies Director James Hansen have both released new statements about the connection between extreme weather and climate change. Although linking extreme weather to climate change has generated controversy in the past, both of the new reports point plainly to a connection.The IPCC, an international organizational that represents the scientific consensus of hundreds of leading climatologists, put it this way in the executive summary of its new report.

It is very likely that there has been an overall decrease in the number of cold days and nights, and an overall increase in the number of warm days and nights, on the global scale, i.e., for most land areas with sufficient data. It is likely that these changes have also occurred at the continental scale in North America, Europe, and Australia.There have been statistically significant trends in the number of heavy precipitation events in some regions. It is likely that more of these regions have experienced increases than decreases, although there are strong regional and subregional variations in these trends.

There is medium confidence that some regions of the world have experienced more intense and longer droughts, in particular in southern Europe and West Africa, but in some regions droughts have become less frequent, less intense, or shorter, e.g., in central North America and northwestern Australia.There is evidence that some extremes have changed as a result of anthropogenic influences, including increases in atmospheric concentrations of greenhouse gases. It is likely that anthropogenic influences have led to warming of extreme daily minimum and maximum temperatures on the global scale. There is medium confidence that anthropogenic influences have contributed to intensification of extreme precipitation on the global scale.

There is limited to medium evidence available to assess climate-driven observed changes in the magnitude and frequency of floods at regional scales because the available instrumental records of floods at gauge stations are limited in space and time, and because of confounding effects of changes in land use and engineering. Furthermore, there is low agreement in this evidence, and thus overall low confidence at the global scale regarding even the sign of these changes.


Meanwhile, Hansen has released the draft of a new paper (pdf) that also tackles the topic of extreme weather and climate. He’s somewhat less equivocal in his summary of the state of the science:

The “climate dice” describing the chance of an unusually warm or cool season, relative to the climatology of 1951-1980, have progressively become more “loaded” during the past 30 years, coincident with increased global warming. The most dramatic and important change of the climate dice is the appearance of a new category of extreme climate outliers. These extremes were practically absent in the period of climatology, covering much less than 1% of Earth’s surface. Now summertime extremely hot outliers, more than three standard deviations (σ) warmer than climatology, typically cover about 10% of the land area. Thus there is no need to equivocate about the summer heat waves in Texas in 2011 and Moscow in 2010, which exceeded 3σ – it is nearly certain that they would not have occurred in the absence of global warming. If global warming is not slowed from its current pace, by mid-century 3σ events will be the new norm and 5σ events will be common.

Text by Adam Voiland. Lead image of flooding in Ayutthaya published originally by NASA’s Earth Observatory. Extreme weather curves published originally by the IPCC. Land trends over land published originally on James Hansen’s Columbia University website. 

A Closer Look at Dust

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                    A wide plume of dust blowing off the Saharan Desert toward the Canary Islands.  Credit: NASA Earth Observatory


Each summer, sandstorms lift millions of tons of dust from the Sahara, carrying plumes of it off the West Coast of Africa and over the Atlantic Ocean. Eric Wilcox, a researcher at NASA’s Goddard Space Flight Center, has been using data from NASA satellites to examine the impact such storms can have on rainfall patterns. Wilcox has a new paper about his findings in Geophysical Research Letters; Nature Geoscience also highlighted it in a recent issue. As a result, we sat down with Wilcox to discuss his new findings and some little-known details about dust.

WhatOnEarth: How does dust affect the atmosphere?

Wilcox: Well, we know that dust isn’t just a passive particle floating around. We know it can either absorb or scatter sunlight. Dust outbreaks surely cool the planet by reducing the amount of sunlight that reaches the surface. Likewise, we know dust can warm the atmosphere, although the magnitude is still uncertain.

WhatOnEarth: What causes a particle to absorb rather than scatter light?

Wilcox: It has to do with the color of the particle and, to some degree, the shape. We call this property the single scattering albedo, which is the probability that a photon of light will scatter versus getting absorbed. If the scatter is very high—say 0.99—then we’re confident that 99 out of 100 photons will scatter. If it’s lower, say 0.85, that means there’s a 15 percent chance that the proton will be absorbed.

WhatOnEarth: Where does Saharan dust fit in?

Wilcox: Some Saharan dust is quite bright and some is much darker. It depends on where the sand is coming from and what its mineralogy is like.

WhatOnEarth: Why does it matter if dust is absorbing light?

Wilcox: We’ve found that dust outbreaks, along with other factors, seem to be shifting tropical precipitation (which typically occurs in a narrow band where winds from the northern and southern hemispheres come together) northward by about four or five degrees, which is about 240 to 280 miles at the equator.

WhatOnEarth: Really? What does dust have to do with precipitation?

Wilcox: The main pathway for dust off the Sahara is usually well north of the band of tropical Atlantic rain storms. However, dust storms coincide  with a strong warming of the lower atmosphere, so the atmospheric circulation over the ocean responds to that warming by shifting wind and rainfall patterns northward during the summer. The rainfall responds to the passage of a dust storm even if the dust does not mix with the rain. 

WhatOnEarth: Will the upcoming Glory mission help you study this phenomenon? I know it has an instrument that will measure aerosols such as  dust? 

Wilcox: Definitely. Over bright reflective surfaces such as deserts — where it has been nearly impossible to distinguish aerosols from the surface — we’re at the point that any new information will be helpful. 

WhatOnEarth: What’s the significance of a northward migration of rainfall during dusty periods?

Wilcox: Certainly people local to the area have an interest in understanding how dust affects their rainfall patterns. The finding also lends support to an idea from one of my colleagues—Bill Lau—who studies the elevated heat pump.The idea is that aerosols from dust storms and air pollution actually affect monsoons. Space Flight Center.

Image Details: The lead image was acquired by the MODIS Land Rapid Response Team at NASA’s Goddard Space Flight Center in 2004.

— Adam Voiland, NASA’s Earth Science News Team

Fewer Southeastern Tornadoes Occur Following Dry Falls and Winters

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Rainfall irregularities as observed by NASA’s Tropical Rainfall
Measurement Mission  satellite. Credit: NASA

Perhaps Dorothy, from the famed film Wizard of Oz, should have hoped for a fall or wintertime drought. According to findings from a NASA-funded study published last June in Environmental Research Letters , dry fall and winter seasons in the southeastern United States mean it is less likely that Southern twisters will develop in springtime to sweep anyone off their feet.


Using rainfall data from NASA satellites, rain gauge information, and NOAA’s Storm Prediction Center tornado record dating back to 1952, University of Georgia meteorologists Marshall Shepherd and Tom Mote and Purdue University climatologist Dev Niyogi discovered a statistical tendency for drought-ravaged fall and winter seasons to pave the way for “below normal tornado days” in spring seasons that follow.

 

“This is conceptually similar to what Bill Gray’s been doing for more than 25 years when he predicts how active the hurricane season will be based on African rain,” said Shepherd, the study’s lead author, of the Colorado State University’s pioneer hurricane season forecaster.

They culled data from Northern Georgia and other parts of the southeast, but Shepherd and his colleagues believe their findings may have relevance for other regions. The new study also adds to the body of related work Shepherd and Niyogi are ushering, including their study earlier this year in the aftermath of Atlanta’s spring 2008 twister that linked urbanization and drought to tornado activity.

For Shepherd in particular, there’s no place like home when considering the geographical focus of much of his meteorological research. “Science is my proverbial yellow brick road,” explained Shepherd. “It’s taken me down some fascinating paths, especially in learning more in recent years about tornado phenomena in my own backyard.”

 

–Gretchen Cook Anderson, NASA’s Earth Science News Team

When It Rained,It Poured

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Satellite image
The image shows estimates of rainfall for the southeastern United States from
September 14–21. The estimates, acquired by multiple satellites, are calibrated
with rainfall measurements from NASA’s Tropical Rainfall Measuring Mission
(TRMM)satellite. The highest rainfall amounts—more than 300 millimeters
(11.8 inches)—appear in blue. The lightest amounts appear in pale green.
Credit: NASA/Jesse Allen

For the past four years, drought has parched the soil of north Georgia farms, nearly drained Atlanta’s 38,000-acre reservoir, and left area lawns brown. Then in just eight days in September 2009, the region’s weather took a turn that likely had residents asking if they were on a climate seesaw.

 

As millions of Georgians watched and as many fled for higher ground, meteorological forces coalesced to deliver heavy rains and flooding not seen in the southeastern U.S. in more than 100 years, according to the National Weather Service.

 

Millions of dollars of property were destroyed, and ten lives were lost. The flooding dominated local TV news and compelled the governor to declare a state of emergency across 17 counties.

 

Even before the rains ended, research meteorologist, Marshall Shepherd a professor of Atmospheric Sciences at the University of Georgia, and colleagues began piecing together rainfall and soil moisture data from NASA satellites, Doppler radar, weather reports and ground-level rain gauges to assemble a clearer picture of the climatological factors that fueled the flooding.

 

Though not yet peer reviewed, Shepherd’s initial findings suggest what may have prompted the downpours and floods. Moisture from the Gulf of Mexico was drawn into the southeast by a stalled low pressure system from the Mississippi Valley. The moist air and a series of meteorological disturbances merged with a key ingredient – a dramatic increase in urban land cover – to bring this historic weather event to the region for the second time in as many years.

 

“We had days and days of downpours and an extraordinary 24-hour rainfall event at the end of that period. With soil moisture already at a high, the rain could no longer infiltrate the soil and we reached a tipping point for flooding,” explained Shepherd, a native of metropolitan Atlanta who NASA has funded to investigate how urban land cover and pollutants affect rainfall and surface water changes. 

 

“The rain and the soil moisture content combined to overwhelm rivers and streams,” he said. “Add to that Atlanta’s impenetrable roads and sidewalks, which increase the volume of runoff, and you get the event of record we witnessed.”

 

According to Shepherd, the long drought in the southeast – which caused job losses in agriculture and lawncare and other water use hardships across many sectors of society — is now over.  

 

“In fact, October is normally the driest month in North Georgia,” Shepherd said. “But this year, sea surface temperature data from NASA and NOAA satellites tell us a moderate El Nino in the Pacific appears likely to lead to a cooler and wetter fall and winter in the Southeast.”

 

Curiously, drought may be the last of Atlanta residents’ weather worries in the next season or two to come.

 

–Gretchen Cook-Anderson, NASA’s Earth Science News Team