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The impact of soil moisture on precipitation has historically been hard to quantify. Some limitations in determining this impact include a lack of observational data and the fact that the strength of this impact can vary between separate computational estimates. A numerical experiment was conducted among twelve climate groups to estimate areas where soil moisture changes affect rainfall during Northern Hemisphere summer. Improved seasonal rainfall forecasts is a potential benefit of this study.

The GLACE (Global Land-Atmosphere Coupling Experiment) experiment focuses on the ability of land surface conditions to affect rainfall and the atmosphere. For GLACE, NASA researchers joined numerous international groups in using their weather and climate models to determine the strength of the land-atmosphere coupling. An additional benefit is that the determination of coupling strengths between the models would aid in the interpretation of the many land-atmosphere interaction studies now appearing in the literature.

This figure shows the land-atmosphere coupling strength for the summer, averaged across the 12 models participating in GLACE. Areas shaded in orange into red are regions where summer seasonal rainfall is significantly dependent on soil moisture conditions. These areas, or "hot spots", are in the central Great Plains of North America, India, equatorial Africa and the Sahel region of Africa. Other regions where soil moisture has a weaker effect on rainfall include China, in South America, and in central Asia. For these regions, dry soil moisture conditions tend to result in less local precipitation, while wet soil moisture conditions usually mean greater rainfall, in the summer. Outside of these "hot spots", the soil moisture conditions tend not to be a strong enough predictor of summer precipitation.

The insets to the figure show the individual areally-averaged coupling strength for the 12 models over the outlined "hot spot" regions.

More information can be found in the Science paper "Regions of strong coupling between soil moisture and precipitation" by Koster et al. published in the 20 August 2004 issue.

The following two links also contain more information:

Earth Observatory: "Hot Spots"

Goddard News: "Hot Spots"

(Submitted by David Mocko/SAIC, General Sciences Operation)