Study Finds Rain Falls Mainly on Dry Plains

Thursday, 13 September 2012 ABC/AFP

Afternoon rain is more likely to fall over regions that have drier soils than those containing moisture, according to a study published by the journal Nature.

While the researchers suggest some computer models for global warming may be over-estimating the risk of drought, one expert says the issue isn't so cut and dry.

Previous research has suggest that dry soils have less moisture to suck up into the atmosphere, which reduces rainfall and thus causes even greater aridity.

But when researchers, led by Chris Taylor from Britain's Centre for Ecology and Hydrology looked at images from weather satellites which track the development of storm clouds across the globe they found this wasn't the case.

They found that afternoon storms were likely to occur when soils were parched - not over soils that were moist.

The apparent reason: drier soils create stronger warm winds called thermals, which boost the chance of rain.

"Both heat and moisture are critical ingredients for rain clouds to build up during the afternoon," Taylor explains.

"On sunny days the land heats the air, creating thermals which reach several kilometres up into the atmosphere. If the soil is dry, the thermals are stronger, and our new research shows that this makes rain more likely."

The data trawl covered six continents, looking at surface soil moisture and rainfall patterns on daily and three-hourly time steps, with a resolution of 50 to 100 kilometres, over a decade.

"It's tempting to assume that moist soils lead to higher evaporation, which in turn stimulates more precipitation," says Wouter Dorigo of the Vienna University of Technology, a co-author.

"This would imply that there is a positive feedback loop: moist soils lead to even more rain, whereas dry regions tend to remain dry... (But) these data show that convective precipitation is more likely over drier soils."

Gaps in study

But Dr Andy Pitman, Director of the ARC Centre of Excellence of Climate System Science in Sydney, isn't convinced by the researcher's conclusions.

"If you look at where they found little correlation over the tropical forest regions, that isn't anything to do with a lack of a feedback between soil moisture and rainfall. That is because you can't see the soil moisture from satellites. They are masked by very dense vegetation," says Pitman.

He says that even in areas where satellites can remotely sense soil moisture it can only measure the first few millimetres of the soil.

Pitman also says the study doesn't take into account the role of plants in "pumping" water into the atmosphere.

"In areas of with significant vegetation, water comes mainly through the vegetation and the roots go down into the soil from several centimetres to many metres. The method the authors used does not capture the role of vegetation, or the role of deeper soil moisture."

Pitman adds that the severity of drought in regions such as Australia, has more to do with ocean-related phenomena such as El Nino and the Indian Ocean Dipole.

"It isn't the evaporation of moisture from soils which influences the persistence of Australian droughts that last many years - it is the oceans."

Source: ABC Science News