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February 11, 2004
NASA Predicts More Tropical Rain in a
Warmer World
As the tropical oceans continue to heat up,
following a 20-year trend, warm rains in the
tropics are likely to become more frequent,
according to NASA scientists.
In a study by William Lau and Huey-Tzu Jenny
Wu, of NASA’s Goddard Space Flight Center,
Greenbelt, Md., the authors offer early proof of
a long-held theory that patterns of evaporation
and precipitation, known as the water cycle, may
accelerate in some areas due to warming
temperatures. The research appears in the current
issue of Geophysical Research Letters.
The study cites observations from NASA’s
Tropical Rainfall Measuring Mission (TRMM)
satellite showing the rate that warm rain
depletes clouds of water is substantially higher
than computer models predicted. This research may
help increase the accuracy of models that
forecast rainfall and climate. The rate that
water mass in a cloud rains out is known as the
precipitation efficiency. According to the study,
when it comes to light warm rains, as sea surface
temperatures increase, the precipitation
efficiency substantially increases as well.
“We believe there is a scenario where in
a warmer climate there will be more warm rain.
And more warm rain will be associated with a more
vigorous water cycle and extreme weather
patterns,” Lau said.
The process that creates warm rain begins when
water droplets condense around airborne particles
and clouds are created. The droplets collide,
combine and grow to form raindrops. The raindrops
grow large and heavy enough to fall out as warm
rain. The study claims, for each degree rise in
sea surface temperature, the rate a cloud loses
its water to moderate-to-light warm rainfall over
the tropical oceans increases by eight to 10
percent.
Cold rains are generally associated with heavy
downpour. They are generated when strong updrafts
carry bigger drops higher up into the atmosphere,
where they freeze and grow. These drops are very
large by the time they fall. Once updrafts take
these large drops high enough, and freezing takes
place, the process of rainfall is more dependent
on the velocity of the updraft and less on sea
surface temperatures. Since the process of
condensation releases heat, warm rains heat the
lower atmosphere. More warm rains are likely to
make the air lighter and rise faster, creating
updrafts producing more cold rain.
The study found warm rains account for
approximately 31 percent of the total global rain
amount and 72 percent of the total rain area over
tropical oceans, implying warm rains play a
crucial role in the overall water cycle. Light
warm rains appear to occur much more frequently,
and cover more area, than cold rains, even though
they drop less water per shower. The total
precipitation from all types of warm rains
accounts for a substantial portion of the total
rainfall.
In a warmer climate, it is possible there will
be more warm rain and fewer clouds. If the amount
of water entering into clouds stays constant and
rainfall efficiency increases, then there will be
less water in the clouds and more warm rains.
More study is needed to better understand the
relationship between increased warm-rain
precipitation efficiency and a rise in sea
surface temperatures, and to determine how cold
rain might be affected by an increase in warm
rain and a decrease in cloud water amounts.
NASA’s Earth Science Enterprise is
dedicated to understanding the Earth as an
integrated system and applying Earth System
Science to improve prediction of climate, weather
and natural hazards using the unique vantage
point of space.
For more information and images related to the
study on the Internet, visit:
http://www.gsfc.nasa.gov/topstory/2003/
1224rainfall.html
For information about NASA on the Internet,
visit:
http://www.nasa.gov
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Contacts:
Elvia H. Thompson
Headquarters, Washington
(Phone: 202/358-1696)
Krishna Ramanujan
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 607/273-2561)
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Distribution of Warm Clouds and Cold
Clouds in El Niño Year, 1998
These two images show typical average
distributions of warm clouds (top image, mostly
in green) and cold clouds (bottom image, mostly
in orange) for January 1 to 3, 1998. 1998 was an
El Niño year with warmer than average
tropical sea surface temperatures. This El
Niño year offers an example of warm and
cold cloud coverage when tropical oceans warm.
The top image shows a much greater distribution
of warm clouds globally, as compared with cold
clouds in the bottom image. The units are in
millimeters of liquid water in clouds. Warm
clouds tend to be thinner and lower and
precipitate a light warm rain. Cold clouds are
thicker and higher in the atmosphere, and produce
larger raindrops. Analyses of images like these
with respect to sea surface temperature provide
useful information regarding the redistribution
of warm clouds vs. cold clouds as a result of
global climate change.
These images were derived using data from the
TRMM satellite. Credit: Lau and Wu, NASA GSFC
High-Resolution
Image
Clouds from Space
This image over Southern Brazil, taken from the
space shuttle by an astronaut in February 1984,
shows a mixture of cold and warm clouds. Warm
clouds, consisting of individual cumuli clouds
are found near the bottom of the image. The
image does not show whether they produced rain.
The tops of these warm clouds are generally
found below 3.1 miles (5 kilometers).
Cold clouds, with large anvils rising typically
above 6.2 miles (10 km) dominate the top and
left portions of the image. The feathery edge of
the anvil indicates a cold rain process, called
glaciation. These clouds are capable of producing
very heavy rain and thunderstorms.
Near the center, about a third and half way from
the bottom are cumulus congestus clouds, which
can produce rain, and can rise to just above the
freezing level near 3.1 miles (5 km), and are
still considered warm clouds.
Notice the way warm clouds are also found
underneath cold clouds, as indicated near the
center of the picture. Credit: NASA-JSC
High-Resolution
Image
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