August 19, 2004

Gretchen Cook-Anderson 
Headquarters, Washington
(Phone: 202/358-0836)

Krishna Ramanujan
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 607/273-2561)

RELEASE: 04-272

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. 

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.

"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 centimeters. 

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 found.

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. 

For more information and images on the Internet, visit: 

http://www.gsfc.nasa.gov/topstory/2004/0819soilrain.html 

	
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