There is an ongoing study of land use change, led my my close research colleague, Udaysankar Nair, that will add to our understanding of the role of landscape in weather and climate. The program is called BUFEX for The Bunny Fence Experiment. The New York Times published an article on this program by Sonal Noticewala on August 14 2007 entitled “At Australia’s Bunny Fence, Variable Cloudiness Prompts Climate Study“.
A University of Alabama at Huntsville press release summarizes this field campaign to collect real world data on this climate forcing. The July 31 2007 news article reads,
“What is happening along the bunny fence to keep rain off SW Australia’s farmland?
Something odd is happening to the weather along hundreds of miles of bunny fence in Southwestern Australia.
Two scientists from The University of Alabama in Huntsville are part of an international team that will spend August studying why most of the clouds that form in that region form over the native vegetation north and east of the fence line, instead of the farmland on the other side of the fence.
Even rain seems to keep on the bunny side: Rainfall over the farm area might have fallen as much as 20 percent since it was cleared for agriculture.
“There are multiple things going on, so we haven’t proven that the changes in clouds and rain are due to clearing native vegetation,” said Dr. Udaysankar Nair, a research scientist in UA Huntsville’s Earth System Science Center and one of the study team’s leaders.
The leading theory is that the dark-colored native plants absorb more sunlight and release more heat into the atmosphere than the lighter-colored farmland. The rising heat may carry both energy and water vapor into the upper atmosphere, where they can combine to form clouds.*
There is some evidence to support that theory: Satellite and aircraft observations often show clouds forming over the native vegetation and abruptly ending at the fence, especially during the spring and summer.
‘Even when the fields are covered with crops on one side, it isn’t as dark,’ Nair said. “There isn’t enough thermal ‘kick’ to form clouds.”
This year’s research expedition is the third studying the impact of changing land use on cloud formation, rain and wind, with prior studies in December of 2005 and 2006 that looked at summer conditions after the harvest. This year the team will focus on the winter season while crops are growing. With funding from the National Science Foundation and the Australian Research Council, scientists from the U.S., Australia and Germany will use instruments on the ground and carried aloft by airplanes and balloons.
The data they gather is plugged into computer models of the atmosphere to help determine why the weather is changing in that region.
While the energy budget difference between the two sides of the fence will be studied, other factors related to land use might also be influencing cloud cover and rain, said Nair, including changes in local circulation patterns and how many small particles are being lifted from the surface into the atmosphere.
Differences in temperatures over the native scrubland and the farm region might also be changing wind patterns, with warm convective upwelling over the hot, dry scrub pulling cooler, somewhat damper air in from across the fence – what the team refers to as the ‘corn breeze.’
These temperature differences might also be altering the local sea breeze. The study’s early data shows the daily sea breeze arriving over the warmer native vegetation side of the fence earlier than over the cooler farm side.
When the team looked at clouds over the region, they found physical differences between clouds over the farm region and clouds over the native scrubland. These changes might be due to differences in the size of water droplets inside the clouds.
‘The size of the cloud droplets depends on aerosols ‹ the particles of dust, soot and salt suspended in the atmosphere,’ Nair said. ‘Aerosols have an important role in forming the nuclei for water vapor to attach to, forming droplets and clouds. But there is a trip point. If you have small aerosol particles or too many particles the chance of rain goes down.’
‘Because of that, cloud cover doesn’t always translate into more rain. With fewer aerosols you might get fewer clouds but more rain.’
‘When we made a small sample of aerosol measurements, we found that on the agriculture side there were more aerosols and more fine aerosols. Our initial results indicate the possibility that there are so many particles and so small that they form a lot of small droplets, which are less conducive to rain.’
The team found that during late spring and summer, dust devils ‹ heat-powered vortices that can stretch more than 50 meters high ‹ might be contributing to the aerosol differences by giving aerosol particles a boost into warm, rising air. While dust devils form on both sides of the fence, they are more likely to form on the farm side of the fence where there is less ground cover and the ground is ‘smoother.’
Other possible sources of fine aerosols are areas where salt is being leached into the soil by a rising water table, including new salt flats growing on the farm side of the fence. When farmers cleared 13 million hectares for cultivation, they cut down the deep-rooted native trees. The roots of these trees reached into and pulled water from the water table. When the trees were destroyed the water table began to rise, bringing dissolved salts to the surface.
Not only do the salts destroy crops, they are also a ready source of fine grain aerosols.
‘Since these salts are being brought to the surface as a byproduct of land use change,’ Nair said, ‘increased aerosol production coupled with more efficient dust devil formation over the agricultural area shows one of the complex ways land use change might influence the atmosphere.’
‘This is the opposite of what we experience in the Southeastern U.S., where forests transpire water that cools the air above them. Plants that evolved in the dry Australian climate, such as eucalyptus trees, release very little of their precious water into the air. Much of the soil in Southwestern Australia is sandy and bright, so without vegetation for cover it is bright and reflective. This reflective soil reflects sunlight back into space rather than absorbing it and converting it to infrared heat.”