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August
27, 2007
LONG-TERM INCREASE IN RAINFALL
SEEN IN
TROPICS
NASA scientists have detected the first signs
that tropical
rainfall is on the rise with the longest and most complete data record
available.
Using
a 27-year-long global record of rainfall assembled by the international
scientific community from satellite and ground-based instruments, the
scientists found that the rainiest years in the tropics between 1979
and 2005
were mainly since 2001. The rainiest year was 2005, followed by 2004,
1998,
2003 and 2002, respectively.
"When we look at the whole planet over almost three decades, the total
amount of rain falling has changed very little. But in the tropics,
where
nearly two-thirds of all rain falls, there has been an increase of 5
percent," says lead author Guojun Gu, a research scientist at Goddard
Space Flight Center in Greenbelt, Md. The rainfall increase was
concentrated
over tropical oceans, with a slight decline over land.
Climate scientists predict that a warming trend in Earth's atmosphere
and
surface temperatures would produce an accelerated recycling of water
between
land, sea and air. Warmer temperatures increase the evaporation of
water from
the ocean and land and allow air to hold more moisture. Eventually,
clouds form
that produce rain and snow.
"A warming climate is the most plausible cause of this observed trend
in
tropical rainfall," says co-author Robert F. Adler, senior scientist at
Goddard's Laboratory for Atmospheres. Adler and Gu are now working on a
detailed study of the relationship between surface temperatures and
rainfall
patterns to further investigate the possible link. The study appears in
the
Aug. 1, 2007, issue of the American Meteorological Society's Journal of
Climate.
Obtaining a global view of our planet's rainfall patterns is a
challenging
work-in-progress. Only since the satellite era have regular estimates
of
rainfall over oceans been available to supplement the long-term but
land-limited record from rain gauges. Just recently have the many land-
and
space-based data been merged into a single global record endorsed by
the
international scientific community: the Global Precipitation
Climatology
Project, sponsored by the World Climate Research Program. Adler's
research
group at NASA produces the project’s monthly rainfall
updates, which are
available to scientists worldwide.
Using this global record, Gu, Adler and their colleagues identified a
small
upward trend in overall tropical rainfall since 1979, but their
confidence was
not high that this was an actual long-term trend rather than natural
year-to-year variability. So they took another look at the record and
removed
the effects of the two major natural phenomena that change rainfall:
the El
Niño–Southern Oscillation and large volcanic
eruptions.
El Niño is a cyclical warming of the ocean waters in the
central and eastern
tropical Pacific that generally occurs every three to seven years and
alters
weather patterns worldwide. Volcanoes that loft debris into the upper
troposphere and stratosphere create globe-circling bands of aerosol
particles
that slow the formation of precipitation by increasing the number of
small
cloud drops and temporarily shielding the planet from sunlight, which
lowers
surface temperatures and evaporation that fuels rainfall. Two such
eruptions –
El Chicon in Mexico
and
Mount Pinatubo in the Philippines
– occurred during the 27-year period.
The scientists found that during El Niño years, total
tropical rainfall did not
change significantly but more rain fell over oceans than usual. The two
major
volcanoes both reduced overall tropical rainfall by about 5 percent
during the
two years following each eruption. With these effects removed from the
rainfall
record, the long-term trend appears more clearly in both the rainfall
data over
land and over the ocean.
According to Adler, evidence for the rainfall trend is holding as more
data
come in. The latest numbers for 2006 show another record-high year for
tropical
rainfall, tying 2005 as the rainiest year during the period.
"The next step toward firmly establishing this initial indication of a
long-term tropical rainfall trend is to continue to lengthen and
improve our
data record," says Adler, who is project scientist of the Tropical
Rainfall Measuring Mission (TRMM), a joint mission between NASA and the
Japan
Aerospace Exploration Agency. The three primary instruments on TRMM are
currently providing the most detailed view of rainfall ever provided
from
space. Adler's group has been incorporating TRMM rainfall data since
1997 into
the global rainfall record.
NASA plans to extend TRMM's success of monitoring rainfall over the
tropics to
the entire globe with the Global Precipitation Measurement mission,
scheduled
for launch in 2013. This international project will provide
measurements of
both rain and snow around the world with instruments on a constellation
of
spacecraft flying in different orbits.
Writer: Stephen Cole, Goddard
Space Flight Center
##
Contact:
Lynn Chandler
NASA Goddard Space Flight Center
301-286-2806
Lynn.Chandler-1@nasa.gov
This
text is derived from:
http://www.nasa.gov/centers/goddard/news/topstory/2007/rainfall_increase.html
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