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August 19, 2004
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 Niños 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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Contacts:
Gretchen Cook-Anderson
Headquarters, Washington
Phone: 202/358-0836
Krishna Ramanujan
Goddard Space Flight Center, Greenbelt, Md.
Phone: 607/273-2561
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Relationships Between Soil Moisture and Rainfall
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.
Relationships Between Soil Moisture and Rainfall
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
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.
“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
High-Resolution
Image
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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