December 2012

In a study that overturns conventional thinking about the effect of soil wetness on rainfall, it has been found that afternoon rain is more likely over drier soils. The study, by Mr. Christopher M. Taylor, a meteorologist working for the UK Centre for Ecology and Hydrology, Oxon, U.K., and others, published in a recent issue of the journal Nature , spanned various areas in six continents. Focusing on the development of precipitation events during the afternoon, when sensitivity of convection to land conditions is expected to be maximised, a statistical analysis was done to evaluate the unexpectedness of rainfall occurring over the various regions across the globe. This unexpectedness was quantified wherever studied and expressed as a percentile value. The percentile is the likelihood whether soil wetness affects rainfall one way or another.

A low percentile value (say 10 per cent or less) of the means that rain is being observed more frequently over dry soil than one would expect if there were no soil effect. Conversely a high value (over 90 per cent) means that rain is more frequent over wet soil than one would expect with no soil effect. Explaining the rationale behind this analysis, Mr. Taylor, in an email to this correspondent, noted: “we have analysed our satellite images to pick out rain events which develop during the afternoon. We then looked at the soil moisture in that region on the morning before it rained, and compared the soil wetness in the place where it rained with a nearby (50-100km) place where it didn’t rain. “If soil wetness had no impact on rain then there would be a 50 per cent chance that in the place where it rained, the soil was wetter than where it didn’t rain. There would be equal probability of whether it would rain or not.” So some statistical analysis on the data was done to work out how likely it was that rain would be observed over drier soil. The study found many parts of the world had low percentiles (less than 10 per cent and often less than 1 per cent) and almost none with high percentiles (greater than 90 per cent).

Such clusters of low percentiles were found in semi-arid regions, most notably North Africa, but also in eastern Australia, Central Asia and Southern Africa. “Heat is as critical as moisture for rain clouds to build up during the afternoon. 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,” notes Mr. Taylor. These findings are in contrast to climate models which tend to indicate a positive feedback. A positive feedback means that if the soil is wet to start off with, it will get wetter because there will be more rain there. Conversely, and importantly in the context of drought prediction, once the soil gets dry, a positive feedback would favour continued lack of rain, prolonging a drought.

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