How much radiant energy do clouds trap within Earth's atmosphere versus how much do they reflect back out into space? Do aerosols affect the net radiative effects of clouds? Because there are so many different types of clouds, and because they form, move, change, and dissipate so rapidly over time and space, these are difficult questions to answer. Due to NASA satellite technology, cloud research has made great strides over the last several decades. Satellites have revealed that as environmental conditions change, so too do clouds and their roles within the climate system. Yet, without quantitative understanding of cloud physics, climate models cannot predict how cloud changes will affect Earth's total energy budget, and therefore cannot realistically predict future climate change. Here, we showcase a study of aerosol and cloud interactions over the Brazilian rainforest.
A team of physical scientists, led by Yoram Kaufman and Lorraine Remer, have been studying how aerosols influence clouds, which in turn would influence climate. When pollution aerosols mingle with certain kinds of clouds, the clouds' properties are changed. They become whiter, more reflective, and longer lasting, which in turn enhances their ability to shade and cool the surface below them. Dubbed the "indirect effect of aerosols," Kaufman and Remer attempted find out just how much brightening and prolonged lifetimes would enhanced clouds' ability to reflect sunlight back to space. The ideal tool for studying both aerosol and clouds effects is one with a wide spectral range, fine spatial resolution and cover large parts of the globe at least daily. The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA's Terra and Aqua satellites proved to be the best choice.
Examples were needed that could showcase the indirect effect. Clouds inside and outside of the pollution plume must be roughly at the same altitude, and be created via the same meteorological processes. Ilan Koren, a research scientist, found the perfect clouds over the Amazon basin - a smoke plume embedded within a layer of low-level cumulus clouds.
Click on image to see animation
Satellite images of the Amazon rainforest rarely show smoke and cumulus clouds
together. Smoke, mainly from agricultural fires, displaces the cumulus clouds that normally form
above the forest each afternoon. A uniform layer of scattered cumulus clouds is typically present,
along with some thunderstorms, over the Amazon rainforest. Click on this image of a day with little
smoke, to see the effect heavy smoke makes on cloud formation. (NASA images by Jesse Allen and Robert Simmon)
The Amazon Basin is an ideal location. Over the rainforest, widely scattered cumulus clouds develop at the top of the boundary layer nearly every day. Usually forming between one and three kilometers, thesecumulus clouds are all about the same size, and are uniformly distributed with spacing in between. By afternoon, , they cover most of the Amazon jungle during the dry season (April through October). Moreover, during the dry season, there are typically hundreds of intense fires burning across the region, producing thick plumes of smoke that may travel for hundreds of kilometers.
Interestingly, Koren found a clear relationship between the clouds and the smoke. Areas of smoke coincided with reduced cloudiness; where the smoke was heaviest, the cloud cover fraction dropped from about 40% to near zero. He wondered why the smoke and clouds didn't exist together. Did the smoke kill the clouds?
