News•September 12, 2012

Dry-Soil Phenomenon Triggers More Storms, Not Less

Common sense says that you should get more rainstorms in areas with moist soils, since that’s where there’s the most water evaporating from the ground. But common sense isn’t always the best guide to scientific truth, and a paper released Wednesday in Nature has cast doubt on this bit of conventional wisdom.

In fact, say a team of European scientists, parched soil is more likely than damp soil to trigger some kinds of rainstorms — and the fact that climate models assume the opposite may lead them to make unrealistic projections.

Credit: Ashleyv/flickr

“In terms of future climate, it could mean that you simulate longer droughts than would happen in reality,” said co-author Phil Harris, of the U.K.’s National Environmental Research Council Centre for Ecology and Hydrology in an interview.

The first hints of this counter-intuitive effect came in computer models that focused on just this single aspect of the climate system. “We’ve been seeing it in the models for several years, but not in the real world,” Harris said.

That changed with a detailed study of the Sahel, the semi-arid band of territory that stretches across Africa just south of the Sahara Desert. Using satellite observations of both soil moisture and precipitation, the scientists found that the models were right. “We were quite surprised,” Harris said. “We were able to detect a clear signal that rainfall was happening more over drier soils than wet.”

They expanded the study to every continent, save Australia, and saw the same effect. In retrospect, Harris said, it makes sense. “To get rainfall, you need fuel and you need a trigger. The fuel is moisture in the atmosphere, and it’s true that there’s less of that hovering above arid soil. But you also need a trigger.”

The trigger here is the strong updrafts that result as dry soil warms during the day and heats the air above it. The updrafts, in turn, carry what moisture the air contains up to altitudes where it condenses into raindrops. Wet soil doesn’t heat as much, so the updrafts tend to be weaker.

Harris noted that the bottom line includes a number of caveats. One is that this effect only applies to rainstorms that pop up on warm afternoons, and “not,” he said, “rainfall associated with frontal systems, or with topographic features such as mountain ranges.” The Asian monsoon, for example, wouldn’t be subject to the dry-soil phenomenon.

Another caveat: “We’re able to detect a signal that rainfall is happening, but not how much rain is actually falling,” Harris said.

This may explain the apparent paradox of how the U.S. could be going through the worst drought in more than 50 years — a drought characterized by unusually low soil moisture.

Harris said that while rainstorms may start over dry areas, they don’t necessarily stay there. “Over time, yes, you do get rain over wet places,” he said. “But most of the time, it starts on the dry side of the line.”

Overall, the research doesn’t contradict the general consensus among climate scientists that global warming will make wet areas wetter and dry areas drier. It could, however, mean that current projections about where seasonal droughts will take place and how long they’ll last will need some revising.