"The radiosonde data prepared by my collaborators made it possible to directly study the environmental conditions under which shallow and deep clouds occurred," Jingfeng Wang of MIT told environmentalresearchweb. "In fact, we have refuted several initial thoughts on the possible explanations of the observed cloud patterns as the research progressed."

Wang and colleagues believe they have assembled by far the most direct evidence that local atmospheric circulations driven by the inhomogeneous land cover due to deforestation in the Amazon basin are responsible for the distributions of clouds. "A common explanation would be that more active shallow clouds over the deforested area were caused by the boundary layer turbulence, which is more intensive due to stronger surface heating over grassy surfaces than over dense intact forest," he said. "We found this is not true in the deforested Amazon."

Together with colleagues at MIT, the University of Michigan, the Instituto Nacional Presquisa Espaciais, Brazil, and the University of California, Irvine, Wang studied an area of rainforest in the Rondonia, Brazil using radiosonde data taken in 1994 as part of the Rondonian Boundary Layer Experiment (RBLE-3) under the Anglo-Brazilian Amazonian Climate Observation Study (ABRACOS). Cloud information was provided by ivisible and infrared images from two NOAA satellites – GOES-7 and GOES-8. The area of deforestation studied contained a typical “fishbone” pattern of strips of tree removal.

While partially deforested areas exhibited a less unstable atmosphere than the neighbouring dense forest, the team found that shallow clouds formed over them. The researchers believe this was most likely due to mesoscale circulations developing from the contrast between forested and unforested strips and acting as a lifting mechanism. Boundary layer turbulence appeared to play a secondary role. Over the forest, a lack of lifting mechanism suppressed convective activity even though the atmosphere was more unstable. Those shallow clouds that did develop over forested areas eventually became deep clouds.

"The Amazon rainforest has some resilience in response to human-related disturbances - deforestation, fire etc. – but only to a certain degree," said Wang. "The clouds-landcover interplay may provide a negative feedback mechanism to restore the lost forest as long as the forest over a large domain is not completely removed. Whether the Amazon ecosystem is able to recover from the deforestation, in my opinion, depends on the size and shape of the land-cover caused by the deforestation."

Wang reckons this study is a step towards finding the threshold for deforestation beyond which the "green-ocean" of the Amazon – so-called because of the similarity in behaviour of the atmosphere above it to that above the oceans – will collapse.

Now the researchers plan to study other areas in the Amazon where deforestation has created much more uniform land cover to find out whether mesoscale circulations still play a major role in cloud climatology.

The researchers reported their work in PNAS.