Cloud Cover: Researchers Improve Predictions Of
Cloud Formation For Better Global Climate Modeling
Atmospheric scientists have
developed simple, physics-based equations that address some of the limitations
of current methods for representing cloud formation in global climate models –
important because of increased aerosol pollution that gives clouds more cooling
power and affects precipitation.
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These researchers -- led by
the Georgia Institute of Technology -- have also developed a new instrument for
measuring the conditions and time needed for a particle to become a cloud
droplet. This will help scientists determine how various types of emissions
affect cloud formation.
The research is funded by the
National Science Foundation, NASA and the National
Oceanic and Atmospheric Administration (NOAA).
Georgia Tech Assistant
Professor Athanasios Nenes will present
an invited lecture on the work at the American Geophysical Union’s fall meeting
in
Clouds play a critical role
in climate, Nenes explained. Low, thick ones cool the
earth by reflecting solar radiation whereas high, thin clouds have warming
properties by trapping infrared radiation emitted by the earth.
Scientists have learned that
human activities influence cloud formation. Airborne particles released by
smokestacks, charcoal grills and car exhaust restrict the growth of cloud
droplets, causing condensing water to spread out among a larger number of
smaller droplets. Known as the “indirect aerosol effect,” this gives clouds
more surface area and reflectivity, which translates into greater cooling
power. The clouds may also have less chance of forming rain, which allows cloud
to remain longer for cooling.
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“Of all the components of
climate change, the aerosol indirect effect has the greatest potential cooling
effect, yet quantitative estimates are highly uncertain,” said Nenes, who holds dual appointments in the Georgia Tech School of Earth and Atmospheric Sciences
and the School of Chemical and Biomolecular Engineering. “We need to get more rigorous
and accurate representation of how particles modify cloud properties. Until the
aerosol indirect effect is well understood, society is incapable of assessing
its impact on future climate.”
Current computer climate
models can’t accurately predict cloud formation, which, in turn, hinders their
ability to forecast climate change from human activities. “Because of their
coarse resolution, computer models produce values on large spatial scales
(hundreds of kilometers) and can only represent large
cloud systems,” Nenes said.
Aerosol particles, however,
are extremely small and are measured in micrometers. This means predictive
models must address processes taking place on a very broad range of scale.
“Equations that describe cloud formation simply cannot be implemented in
climate models,” Nenes said. “We don’t have enough
computing power -- and probably won’t for another 50 years. Yet somehow we
still need to describe cloud formation accurately if we want to understand how
humans are affecting climate.”
Scientists have tried to
predict cloud formation through empirical “parameterization” – techniques that
rely on empirical information or correlations, such as comparing the number of
particles in the atmosphere with the number of cloud droplets. “Yet there’s no
real physical link, no causality between those two numbers,” Nenes said.
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To address both the lack of
computer power and shortcomings of existing parameterization, Nenes and his research team have developed simple,
physics-based equations that link aerosol particles and cloud droplets. Then
these offline equations can be scaled up to a global level, providing accurate
predictions literally thousands of times faster than more detailed models.
For example, by determining
an algebraic equation for maximum supersaturation
(the point in a cloud where all droplets that could form, have formed), it is
then possible to calculate how many cloud droplets can form. That droplet
number reveals the optical (reflective) properties of a cloud, as well as its
potential for forming rain.
This modeling
method has proven successful in two field tests. In situ aircraft data was
collected from cumulus clouds off the coast of
That was a pleasant surprise
for the research team, which included Georgia Tech postdoctoral scholar Nicholas
Meskhidze and graduate student Christos Fountoukis. “We never expected to capture the physics to
that degree,” Nenes explained. “We were hoping for a
50 percent accuracy rate.”
Another key challenge in
predicting climate change is to understand how aerosols’ chemistry affects
cloud formation. Each particle has a different potential for forming a cloud
drop, which depends on its composition, location and how long it has been in
the atmosphere. Up to now, people have measured and averaged properties over
long periods of time. “Yet particles are mixing and changing quickly,” Nenes said. “If you don’t factor in the chemical aging of
the aerosol, you can easily have a large error when predicting cloud droplet
number.”
Working with Gregory Roberts
at the Scripps Institution of Oceanography, Nenes developed a new type of cloud condensation nuclei
(CCN) counter. This instrument exposes different aerosol particles to a supersaturation, which enables researchers to determine: 1)
how many droplets form and 2) how long they take to form.
Providing fast, reliable
measurements, this CCN counter can be used either on the ground or in an
aircraft. “It gives us a much needed link for determining how different types
of emissions will affect clouds formation,” Nenes
explained.
Nenes and Roberts have patented
the CCN instrument, and a paper describing the technology will be published in
an upcoming issue of Aerosol Science and Technology.
The CCN counter is being
commercialized by Droplet
Measurement Technologies in
Both the new modeling method and CCN instrument have far-reaching
applications for predicting climate change and precipitation patterns.
The indirect aerosol effect
is counteracting greenhouse warming right now, but this will stop at some
point, Nenes explained: “One of our goals as
scientists is to figure out how long we’ll have this cooling effect so that we
can respond to changes. Being able to predict climate change can help countries
with sustainability – from agricultural planning to global emission policies.”
RESEARCH NEWS &
PUBLICATIONS OFFICE
MEDIA
RELATIONS CONTACTS: Jane Sanders (404-894-2214); E-mail: jane.sanders@edi.gatech.edu);
Fax (404-894-4545) or John Toon (404-894-6986);
E-mail: (john.toon@edi.gatech.edu).
TECHNICAL
CONTACT: Athanasios Nenes
(404-894-9225); E-mail: (nenes@eas.gatech.edu)
WRITER: T.J.
Becker