MOIST SOIL 'HOT SPOTS' MAY AFFECT RAINFALL
While the Earth is moistened by rainfall, scientists
believe that the water in soil can, in turn, influence
rainfall both regionally and globally. Forecasters, water
resource managers and farmers may benefit once this
connection is better understood.
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A NASA researcher led an effort that used a dozen computer
models to locate "hot spots" around the world where soil
moisture may strongly affect rainfall during northern
hemisphere summertime. The results appear in the August 20
issue of Science Magazine.
The "hot spots" appear in the central plains of North
America, the Sahel, equatorial Africa, and India. Less
intense hot spots show up in South America, central Asia and
China. These hot spots are, in a sense, analogous to ocean
areas where sea surface temperatures strongly affect climate
and weather, the most famous example being in the eastern
tropical Pacific, where El Ninos occur.
|Click on image to view animation.
"The study arguably provides the best estimate ever of the
areas where soil moisture changes can affect rainfall," said
Randal Koster, a researcher at NASA's Goddard Space Flight
Center in Greenbelt, Md. Koster led the international
computer modeling effort in collaboration with Paul Dirmeyer
and Zhichang Guo of the Center for Ocean Land Atmosphere
Studies, Calverton, Md.
In the Global Land-Atmosphere Coupling Experiment (GLACE),
Koster and colleagues duplicated the same experiment using 12
different computer models from around the world. With each
model researchers compared the rainfall behavior in two sets
of simulations: one in which the soil moisture differed
between the simulations, and one in which all simulations saw
the same soil moisture. Any increase in rainfall agreement in
the second set of simulations shows an impact of soil
moisture on the rainfall.
Although the model results differed, the simulations also
shared certain common features. By averaging together all the
findings, the researchers identified the common features, or
"hot spots" where soil moisture influences rainfall the most.
If soil moisture is assumed to affect rain locally, the hot
spots tell researchers who study land and atmosphere
interactions where to focus their measurements. NASA helps in
the design of satellites and instruments to measure soil
moisture. Currently, the Advanced Microwave Scanning
Radiometer for EOS on NASA's Aqua satellite measures the
moisture in surface soil down to a depth of a few
centimeters. In 2009, NASA plans to launch the Hydrosphere
State (HYDROS) mission that will provide the first global
view of the Earth's changing soil moisture down to 5
However, even if researchers could observe global soil
moisture levels at depths greater than a few centimeters, it
would be very hard to tell from the data alone how this
moisture contributes to precipitation. There are too many
factors involved. "Computer models are notorious for their
limitations. Still, given the overwhelming difficulty of
finding the hot spots through direct measurement, our study
provides the next best thing: a multi-model estimate of their
locations," Koster said.
In general, the hot spots have one thing in common: they
occur in transition zones between wet and dry regions. This
was expected. In wet climates, the Sun's energy and
cloudiness play a bigger role in determining evaporation
rates than soil moisture. In dry climates, the limited water
leads to limited evaporation rates, that are simply too small
to have a large impact on the atmosphere. The fact that
satellites cannot measure soil moisture through very dense
vegetation is therefore less of a problem. Dense vegetation
appears in wet regions, where the hot spots are typically not
Understanding soil moisture levels and their connection to
precipitation has important implications. It may improve
seasonal forecasting of rainfall vital to water managers, as
well as improve the accuracy of short-term weather forecasts.
Interest in the study is therefore high at national weather
centers, like the National Centers for Environmental
Prediction (NCEP), Camp Springs, Md.
"At NCEP, we are working on ways to specify soil moisture
accurately, in order to take advantage of the type of
connections examined in GLACE," said co-author Kenneth
Mitchell of NCEP. NCEP's soil moisture estimation project,
known as the Land Data Assimilation System, is run in
collaboration with scientists at NASA. Institutions from the
United States, Canada, the United Kingdom, Japan, and
Australia each funded use of their own model.
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