New data on aerosols and clouds
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David Schnare
Apr 6, 2009, 9:01:53 AM4/6/09
to geoengineering
Published online 3 April 2009 | Nature | doi:10.1038/news.2009.215
More than a silver lining
Light reflected by clouds can brighten air kilometres away.
Lucas Laursen
There's more to clouds than meets the eye.Punchstock
A study looking at why clouds make the air near them glow more
brightly suggests climate models may need to be revised.
Atmospheric scientists already account for the brighter air close to
clouds, thanks to a 2007 study1 by Ilan Koren and his colleagues at
the Weizmann Institute of Science in Rehovot, Israel. The team showed
that cloud droplets, attached to dust and smoke particles, float in a
halo kilometres wide around clouds, bouncing sunlight back out of the
atmosphere. Seen from a satellite, that means air close to clouds
looks brighter.
But Tamás Várnai and Alexander Marshak of NASA's Goddard Space Flight
Center in Greenbelt, Maryland, have found that a related factor has
been left out of the current generation of algorithms used to
interpret satellite images2.
Using observations made by a pair of NASA satellites, the researchers
identified evidence of an effect called three-dimensional radiative
interaction, in which sunlight is reflected horizontally by clouds and
scattered from nearby air molecules back out of the atmosphere.
Clouding the issue
The effect was a theoretical possibility, but nobody knew whether it
was strong enough to be worth accounting for in satellite image-
processing algorithms.
"We showed that actually this effect cannot be neglected and makes up
a significant fraction of the enhanced radiation" near clouds, Marshak
says.
Aerosols — small particles in the air — are thought to affect local
temperatures, and are one of the least understood parts of climate
models. They can come from natural processes such as volcanoes or
forest fires, but also from human pollution.
They can have a cooling effect by "scattering sunlight back out to
space", explains Koren. But they also "absorb some heat and can
therefore warm air locally and change circulation".
To detect aerosols, atmospheric scientists measure the brightness of
the unclouded pixels in their satellite image and subtract the amount
of light reflected from the surface of Earth, attributing much of the
remainder to aerosols.
Koren's findings prompted researchers to treat the region around
clouds as extensions of the clouds, with declining brightness over a
distance of many kilometres. Still, much of the brightness was
attributed to the presence of aerosols, with the rest attributed to
increased humidity and some instrumentation errors.
Blue-sky approach
But Várnai and Marshak found that air molecules actually have a much
larger contribution. They found that the light reflected from the
cloud's halo had a bluer tint than should have been the case if
sunlight was being reflected equally. The bluish tint implies that
only small atmospheric molecules, and not the larger aerosols, were
responsible for the light reflected back out of the atmosphere.
"They showed that the short light waves have significantly more three-
dimensional effects than the longer waves," adds Koren.
Leaving such three-dimensional radiative interactions out of the
equation might mean that scientists are overestimating the amount of
aerosols in the sky, throwing off their predictive models.
Várnai and Marshak "are addressing an issue that we have been stuck
with for many years", says Olivier Boucher, a climate scientist at the
UK Met Office. The findings "will have a direct impact on studies of
aerosol's indirect effects" in climate models, he adds. The assumption
that three-dimensional radiative interaction was negligible "has been
widely made by everyone and has not been questioned enough".
References
1. Koren, I., Remer, L. A., Kaufman, Y. J., Rudich, Y. & Martins, J.
V. Geophys. Res. Lett. 34, L08805 (2007). | Article |
http://www.agu.org/pubs/crossref/2007/2007GL029253.shtml
2. Várnai, T. & Marshak, A. Geophy. Res. Lett. 36, L06807 (2009). |
Article |
http://www.agu.org/pubs/crossref/2009/2008GL037089.shtml
More than a silver lining
Light reflected by clouds can brighten air kilometres away.
Lucas Laursen
There's more to clouds than meets the eye.Punchstock
A study looking at why clouds make the air near them glow more
brightly suggests climate models may need to be revised.
Atmospheric scientists already account for the brighter air close to
clouds, thanks to a 2007 study1 by Ilan Koren and his colleagues at
the Weizmann Institute of Science in Rehovot, Israel. The team showed
that cloud droplets, attached to dust and smoke particles, float in a
halo kilometres wide around clouds, bouncing sunlight back out of the
atmosphere. Seen from a satellite, that means air close to clouds
looks brighter.
But Tamás Várnai and Alexander Marshak of NASA's Goddard Space Flight
Center in Greenbelt, Maryland, have found that a related factor has
been left out of the current generation of algorithms used to
interpret satellite images2.
Using observations made by a pair of NASA satellites, the researchers
identified evidence of an effect called three-dimensional radiative
interaction, in which sunlight is reflected horizontally by clouds and
scattered from nearby air molecules back out of the atmosphere.
Clouding the issue
The effect was a theoretical possibility, but nobody knew whether it
was strong enough to be worth accounting for in satellite image-
processing algorithms.
"We showed that actually this effect cannot be neglected and makes up
a significant fraction of the enhanced radiation" near clouds, Marshak
says.
Aerosols — small particles in the air — are thought to affect local
temperatures, and are one of the least understood parts of climate
models. They can come from natural processes such as volcanoes or
forest fires, but also from human pollution.
They can have a cooling effect by "scattering sunlight back out to
space", explains Koren. But they also "absorb some heat and can
therefore warm air locally and change circulation".
To detect aerosols, atmospheric scientists measure the brightness of
the unclouded pixels in their satellite image and subtract the amount
of light reflected from the surface of Earth, attributing much of the
remainder to aerosols.
Koren's findings prompted researchers to treat the region around
clouds as extensions of the clouds, with declining brightness over a
distance of many kilometres. Still, much of the brightness was
attributed to the presence of aerosols, with the rest attributed to
increased humidity and some instrumentation errors.
Blue-sky approach
But Várnai and Marshak found that air molecules actually have a much
larger contribution. They found that the light reflected from the
cloud's halo had a bluer tint than should have been the case if
sunlight was being reflected equally. The bluish tint implies that
only small atmospheric molecules, and not the larger aerosols, were
responsible for the light reflected back out of the atmosphere.
"They showed that the short light waves have significantly more three-
dimensional effects than the longer waves," adds Koren.
Leaving such three-dimensional radiative interactions out of the
equation might mean that scientists are overestimating the amount of
aerosols in the sky, throwing off their predictive models.
Várnai and Marshak "are addressing an issue that we have been stuck
with for many years", says Olivier Boucher, a climate scientist at the
UK Met Office. The findings "will have a direct impact on studies of
aerosol's indirect effects" in climate models, he adds. The assumption
that three-dimensional radiative interaction was negligible "has been
widely made by everyone and has not been questioned enough".
References
1. Koren, I., Remer, L. A., Kaufman, Y. J., Rudich, Y. & Martins, J.
V. Geophys. Res. Lett. 34, L08805 (2007). | Article |
http://www.agu.org/pubs/crossref/2007/2007GL029253.shtml
2. Várnai, T. & Marshak, A. Geophy. Res. Lett. 36, L06807 (2009). |
Article |
http://www.agu.org/pubs/crossref/2009/2008GL037089.shtml
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