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For more information contact:

Gretchen Cook-Anderson
Headquarters, Washington
August 19, 2004
(Phone: 202/358-0836)

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

Viewable Images

Caption for Items 1 and 2: Relationships Between Soil Moisture and Rainfall

Item 1: Soil moisture plays an important role in summer rainfall, particularly in transition zones between very wet and very dry climates. The warm, moist air rises until it encounters colder air high above the Earth's surface, leading to afternoon rainshowers. The water remains in the ground through the cool night, and the cycle repeats the next day.

Item 2: Dry soil has the opposite effect on rainfall. As the temperature rises during the day, the air near the Earth's surface heats up and rises, but does not contain enough moisture to form rainclouds. As each day passes more moisture is removed from the ground, enhancing the effect. Credit: Earth Observatory, animations courtesy Susan Byrne, NASA GSFC

Caption for Item 3: U.S. Soil Moisture (July 1 to 15, 2002)

This animation shows changes in soil moisture from July 1 through 15, 2002, derived from a land surface computer model. Soil moisture is important for knowing how much water is contained in the soil, which is vital for crop production, flood prediction, and evapotranspiration estimates. The blue colored areas represent wetter soils. Credit: Images by Robert Simmon, NASA Earth Observatory, NASA/GSFC and Univ. of Maryland-Baltimore County.

Caption for Item 4: "Hot Spots" Where Soil Moisture Changes Can Affect Rainfall

The red areas are "Hot Spots" where soil moisture changes can affect rainfall, according to the GLACE multi-model study. The bars in the insets show the individual results for each of the 12 climate models, called Atmospheric General Circulation Models (AGCMs), averaged over the indicated regions. According to the insets, the models clearly do not show perfect agreement in the "strength" of the hot spots. Still, many independent models place the hot spots in the same place. The results pertain to Northern Hemisphere summer months, June, July and August. Red areas show the highest connection between soil moisture and rainfall. The units for the insets are the same as those for the color bar. Credit: Koster et. al, 2004, Copyright Science

Caption for Item 5: Animation of Global Soil Moisture

This animation shows global soil moisture from December 1999 through June 2000 from the NASA Seasonal-to-Interannual Prediction Project (NSIPP) global climate model. The model was driven by observations of sea surface temperatures, which were used to estimate global soil moisture. The brown areas depict very dry soil, while the greenest areas indicate wet, saturated soil. Credit: NASA/GSFC Scientific Visualization Studio


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August 19, 2004 - (date of web publication)


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.

image from animation of wet soil  

still from animation of dry soil

Items 1 and 2
Click on images to view animations

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.


still from animation of changes in soil moisture in early July 2002

Item 3

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.


This image shows a plot of the "hot spots" where soil moisture changes can affect rainfall.

Item 4

Click on image to enlarge.
Click here for higher resolution image.

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.



still from animation of global soil moisture

Item 5


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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