NASA Study Improves Ability to Predict Aerosols’ Effect on Cloud Cover

    WASHINGTON, Aug. 14 /EMWNews/ -- Using a novel

theoretical approach, researchers from NASA and other institutions have

identified the common thread that determines how aerosols from human

activity, like the particles from burning of vegetation and forests,

influence cloud cover and ultimately affect climate. The study improves

researchers' ability to predict whether aerosols will increase or decrease

cloud cover.



    "We connected the dots to draw a critical conclusion, and found

evidence over the Amazon that traces the direct path of the effect of human

activity on climate change by way of human-caused aerosols," said study

co-author Lorraine Remer, a physical scientist at NASA's Goddard Space

Flight Center in Greenbelt, Md. "During the dry season in the Amazon, the

only aerosols of any magnitude are from smoke emerging from human-initiated

fires."



    It is well documented from previous studies that aerosols play an

essential role in how clouds develop. With this knowledge, a team comprised

of Remer, Ilan Koren of the Weizmann Institute in Rehovot, Israel and J.

Vanderlei Martins of the University of Maryland Baltimore County set out to

explore one of the least understood but most significant aspects of climate

change caused by human activity: the connection between a change in the

amount of human-caused aerosols and change in the structure of clouds.

Findings from their study will be published tomorrow in the journal

Science.



    "Scientists have observed instances where increases or decreases in the

amount of these tiny particles have increased and decreased cloud cover in

different places and times," said Remer. "We saw an example of this

ourselves: increased aerosols over the Amazon produced less cloud cover.

Over the Atlantic Ocean, however, increased aerosols actually produced more

cloud cover. We wanted to know what the link was between these different

outcomes from varying amounts and types of aerosols. This paper gives us a

clear picture of what is occurring."



    The team developed an analytical model, or line of thinking that

combined knowledge of cloud development, satellite observations and

mathematical calculations of aerosol concentration and cloud properties in

an effort to explain how the two opposing effects of aerosols on clouds can

influence cloud coverage and life cycle.



    "This result helps us understand aerosols' effect on a cloud's mass and

lifetime - how long it will provide cloud cover, how deep the clouds will

be, and when and where it will rain," said Remer. "This improved

understanding leads to prediction and prediction can help us plan and

perhaps prevent some of the potential consequences of putting aerosols from

human activity into the atmosphere."



    To test their model, Remer's team used aerosol and cloud observations

from NASA's Terra satellite of the Amazon during the 2005 dry season The

season offers stable weather conditions and an abundance of human-caused

aerosols from fires, set to clear new land and burn through old pastures to

prepare the land for the next crop season.



    Aerosols are the tiny particles that make up smoke, dust, and ocean

spray. Traveling on wind currents, aerosols move from their source and into

the atmosphere, where they become individually encased by water and turn

into the droplets that combine to create clouds.



    Cloud microphysics makes clear that the larger the number of aerosol

particles suspended in air the less water in the atmosphere is available

for condensation on each individual particle. Under these conditions, a

cloud will have a much larger number of small droplets. The smaller the

droplets, the longer it will take for a cloud to rain. Aerosol-rich clouds

like this spread out by winds, produce less rainfall, and last longer,

creating more cloud cover.



    However, aerosols also influence clouds through their ability to absorb

heat from the sun. The trapped heat causes the atmospheric layer to warm

up, and changes the environment in which the cloud develops. The overall

result is to make the environment less hospitable for cloud growth. Even

the smallest resulting changes in cloud cover can significantly warm or

cool the atmosphere and change when and where fresh water will be available

in the region.



    "As we'd expected in applying our model, increased smoke from the fires

created clouds rife with a more pronounced radiative effect - rich with

human-caused aerosols that absorbed sunlight, warmed the local atmosphere,

and blocked evaporation. This led to reduced cloud cover over the Amazon,"

said co-author Martins. "And it's encouraging to know the science behind

our model should stand no matter the region."