Washington, May 28 : Scientists have used data from instruments in a constellation of NASA satellites to see deep inside clouds, which would help to shed new light on the link between clouds, pollution and rainfall.
Clouds have typically posed a problem to scientists using satellites to observe the lowest part of the atmosphere, where humans live and breathe, because they block the satellite's ability to capture a clear, unobstructed view of Earth's surface.
Now, a closer look into the clouds is possible due to the new satellites that comprise the Afternoon Constellation, or A-Train.
Jonathan Jiang of NASA's Jet Propulsion Laboratory, Pasadena, California, and colleagues used these A-Train sensors to find that South American clouds infused with airborne pollution - classified as "polluted clouds" - tend to produce less rain than their "clean" counterparts during the region's dry season.
Discovery of the link between rain and pollution was possible due to near-simultaneous measurements from multiple satellites making up the string of satellites in the A-Train.
"Typically, it is very hard to get a sense of how important the effect of pollution on clouds is," said Anne Douglass, deputy project scientist at Goddard for NASA's Aura satellite.
"With the A-Train, we can see the clouds every day and we're getting confirmation on a global scale that we have an issue here," he added.
Jiang's team used the Microwave Limb Sounder on the A-Train's Aura satellite to measure the level of carbon monoxide in clouds.
The presence of carbon monoxide implies the presence of smoke and other aerosols, which usually come from the same emission source, such a power plant or agricultural fire.
With the ability to distinguish between polluted and clean clouds, the team next used Aqua's Moderate Resolution Imaging Spectroradiometer to study how ice particle sizes change when aerosol pollution is present in the clouds.
The team also used NASA's Tropical Rainfall Measuring Mission satellite to measure the amount of precipitation falling from the polluted and clean clouds.
All three measurements together show the relationship between pollution, clouds and precipitation.
The team found that polluted clouds suppressed rainfall during the June-to-October dry season in South America, which is also a period of increased agricultural burning. During that period, it was more difficult for the measurably smaller ice particles in aerosol polluted clouds to grow large enough to fall as rain.
This trend turned up seasonal and regional differences, however, and aerosol pollution was found, on average, to be less of a factor during the wet monsoon seasons in South America and in South Asia.
Other physical effects, such as large-scale dynamics and rainy conditions that clear the air of aerosol particles, might also be at play, the researchers suggest.