12 October 2011
Source: Planet Earth
The circulation of warm and cool air at the forest's edge produces a 'vegetation breeze' – much like a sea breeze. This air movement creates rain clouds at the boundary but stops clouds forming over the cooler forest.
Because this happens at the boundary between forest and open areas, the pattern of land clearance influences the strength of the effect, so these findings have particular implications for rainforest management.
Scientists already knew about the effect of these temperature differences on local weather patterns from satellite observations, but no one had looked in detail at what might happen to rainfall as a result.
'There has been lots of work looking at deforestation at larger scales, but you have to understand the implications for rainfall of these smaller-scale processes to properly understand the long-term impacts of deforestation,' said Dr Garcia-Carreras, lead author of the report published in Geophysical Research Letters.
The researchers used a Met Office computer model to simulate rainfall over the different types of land. They ran the simulations many times, varying the amount of heat exchanged from the surface to the atmosphere, and the extent of the forested and cleared areas.
They found that rainfall over the remaining forested areas could be less than half what would be expected if there was no deforestation, while it was between four and six times higher over the deforested areas.
This happens because the vegetation breezes generate clouds which move air upwards. This updraft produces a downward air movement around it, which suppresses cloud formation – and therefore rain.
'The downward movement is relatively gentle, but it can suppress cloud formation as much as 80km from the vegetation boundary,' said Dr Garcia-Carreras.
The full implications of these variations aren't yet clear; it's possible that reduced rainfall over the already relatively dry West African rainforests could hasten the decline of the forest, creating a 'negative feedback' loop.
'African rainforests already have the lowest rainfall of any rainforest ecosystem on Earth, which could make them particularly sensitive to changes in local weather patterns,' said Dr Garcia-Carreras.
The effect on rainfall was clear for all the vegetation models the researchers ran, but they emphasise that peak rainfall is hard to predict because it is affected by the size of the temperature differences, the size of the vegetation patches and the distance between patches.
So areas with more and smaller areas of deforestation – such as the 'fishbone' pattern of deforestation common in the Amazon – are likely suffer a much stronger suppression of rainfall than regions with fewer but larger deforested patches.
The research was funded by NERC as part of the African Monsoon Multidisciplinary Analysis (AMMA) campaign.