New data could improve monsoon prediction

13 June 2011 by Adele Walker

A new study of storms could improve rainfall prediction in dry regions, where drought and short growing seasons affect local populations badly.

Storm clouds over the savannah

Storm brewing over the savannah

An international team, led by the Centre for Ecology & Hydrology (CEH), used satellite observations of the Sahel region of West Africa to demonstrate that brief changes in soil moisture over areas of just tens of kilometres can affect storm generation.

Adding their results to climate models should improve rainfall predictions for these vulnerable areas, where the timing of the monsoon can mean the difference between success and failure for a whole season's crops.

The movement of water from the land to the atmosphere (evapotranspiration) can affect temperature and humidity in the lower atmosphere to produce storm clouds.

Scientists know that fixed landscape features - like lakes and forests - can influence this process. But other, less predictable, conditions affect evapotranspiration too. For example, when it rains, the top few centimetres of soil become soaked and distinct from the dry soils around them (known as the soil-moisture gradient).

"But this moisture evaporates quickly, perhaps over just a few hours," explains Dr Phil Harris, one of the authors of the report published in Nature Geoscience. "And it might affect just a few square kilometres."

An existing storm will strengthen over a wet region as there is more 'fuel'.

- Dr Phil Harris, CEH

Climate models have already shown the important relationship between the evapotranspiration of soil moisture and rainfall in semi-arid regions. But there are large variations between different models' predictions, and until now it hasn't been clear exactly what processes are behind the relationship or how strong the effect will be.

So the researchers set out to understand how transient soil-moisture gradients influence storm generation. Until now this has been hard to do, because observations of these short-lived features relied on an airborne survey being in exactly the right place at exactly the right time.

So the team used high-resolution satellite images - taken every 15 minutes at a scale of a few kilometres - to study storm generation on every day of the 2006-2010 wet seasons. In all they analysed 3765 storms across a region of around 2·5million km2, to see how often, when and where convection (cloud formation) was triggered.

The data revealed that changes in soil-surface moisture were strongly linked to the generation of storm clouds. In fact the researchers found that the likelihood of storm clouds being generated doubled over strong soil-moisture gradients - where the soil changes from dry to wet over a short area - compared to areas where soil moisture is the same.

"Because this dataset is much larger than those from previous studies and covers several years we're much more confident of the results," says Harris.

The researchers conclude that differences in soil moisture on scales of just tens of kilometres have a pronounced impact on rainfall in the Sahel.

"An existing storm will strengthen over a wet region as there is more 'fuel'," Harris explains.

The researchers think 1 in 8 of storms is affected by the land surface in this way.

"Now that we can quantify this process, and give the models the right conditions to work with, they are more likely to initiate the storms in the right places," says Harris.

The monsoon can arrive abruptly in the Sahel and the relative timing of planting and the starts of the wet season can mean the difference between a good crop and no crop at all. Better predictive modelling could make a huge difference to the people for whom the rains can be a matter of life or death.

'Frequency of Sahelian storm initiation enhanced over mesoscale soil-moisture patterns' - Christopher M Taylor, Amanda Gounou, Françoise Guichard, Phil P Harris, Richard J Ellis, Fleur Coubreux, Martin De Kauwe. Nature Geoscience, doi: 10.1038/NGEO1173.