May 3, 2007
WIDESPREAD 'TWILIGHT ZONE'
DETECTED AROUND CLOUDS
There
seems to be something new
under the sun -- in the sky, specifically -- that could complicate
scientists'
efforts to get a fix on how much the world will warm in the future.
Greenhouse
gases are not the only things in the air that influence the temperature
of our
atmosphere. Clouds and small airborne particles called aerosols also
play an
important and complicated role. And now a new ingredient has been
discovered:
an extensive and previously unseen “twilight zone”
of particles that represents
a gradual transition from cloud droplets to dry particles.
In
a study published last month,
scientists from the Weizmann Institute, Rehovot, Israel, and NASA's
Goddard
Space Flight Center, Greenbelt, Md., document for the first time that
air
around clouds that was previously considered clear is actually filled
with
particles that are neither cloud droplets nor typical dry aerosols such
as dust
and air pollution. Worldwide, up to 60 percent of the atmosphere
labeled as
cloud-free in satellite observations is actually filled with this
twilight zone
of in-between particles, according to the study.
“With the highly sensitive Earth-observing instruments NASA
has used since
2000, we can distinguish aerosols and clouds in greater detail than
ever
before,” said Goddard's Lorraine Remer, a co-author on the
study. “But the area
around clouds has given us trouble. The instruments detected something
there,
but it didn’t match our understanding of what a cloud or an
aerosol looked
like. What we think we’re seeing is a transitional zone where
clouds are
beginning to form or are dying away, and where humidity causes dry
particles to
absorb water and get bigger.”
Precisely
accounting for
everything in the atmosphere that can influence changes in global
temperatures
is critical to scientists’ quest to accurately predict what
Earth’s climate
will be in the future. The latest report from the Intergovernmental
Panel on
Climate Change, which assessed the potential risks of human-induced
climate change,
notes that the overall effect of clouds and aerosols on the amount of
heat held
in the atmosphere is still uncertain. Finding a previously unknown
ingredient
in the mix further complicates an already complex picture, but it also
holds
out the promise of resolving some nagging problems in climate change
science.
"The effects of this zone are not included in most computer models that
estimate the impact of aerosols on climate," said lead author Ilan
Koren of the
Weizmann Institute “This could be one of the reasons why
current measurements
of this effect don’t match our model estimates.”
The study was published April
18 in the American Geophysical Union's
Geophysical Research Letters.
Atmospheric
scientists have been
aware of an indistinct “halo” of particles
immediately surrounding individual
clouds, which are sometimes visible to the naked eye. These are thought
to be
aerosols accumulating moisture and growing in size, or a cloud droplet
shrinking as it evaporates. But the newly detected twilight zone
extends far
beyond single clouds to fill an entire cloud field.
The research team first came across evidence for this transitional zone
in
satellite measurements of aerosols that looked "suspicious," according
to
Remer. "After working with several years of data from the Moderate
Resolution
Imaging Spectroradiometer (MODIS) on NASA’s Terra and Aqua
spacecraft, we
consistently saw what appeared to be elevated levels of aerosols near
clouds.
But we couldn’t be sure that the instrument wasn’t
actually detecting stray
light bouncing off of the clouds."
The region around clouds is difficult to accurately observe with
instruments
like MODIS because they operate like our eyes, collecting light
reflected by
objects below. Scientists interpret the different characteristics of
the light
received, matching them to known light patterns from different objects
such as
clouds. But clouds are notorious light scatterers, and the "glare"
from the edge of clouds makes it hard to clearly detect what is around
them. To
be on the safe side, scientists mapping worldwide aerosols with MODIS
avoid a
1-kilometer border around clouds.
To find out whether the apparent aerosol detection around clouds in the
satellite data was real, Koren and his colleagues, including the late
Yoram
Kaufman of NASA Goddard, turned to an independent observing system on
the
ground: the NASA-sponsored Aerosol Robotic Network. The automated
instruments
in this global network minimize scattered light effects as they track
the sun
and take readings of the amount and size of aerosols in a narrow column
of
atmosphere between the instrument and the sun. When the sun is blocked
by a
cloud, the instrument doesn't make one of its regularly scheduled
readings,
which provides an indirect measure of the presence of a cloud.
Combining thousands of observations from 15 sites around the world, the
researchers found that the amount of aerosol systematically increased
as clouds
got closer, as did the size of the particles. This held true regardless
of
whether the site was in a relatively clean setting or one where
aerosols from
air pollution or biomass burning were common.
"We found that the region affected by this cloud field 'twilight zone'
extends to tens of kilometers beyond the identified cloud edge," said
Koren. "This suggests that 30 to 60 percent of the atmosphere
previously
labeled as 'cloud-free' is actually affected by cloud-aerosol processes
that
reflect solar energy back into space."
Introducing this new factor could lead climate scientists to
recalculate their
best estimates of how Earth's atmosphere holds and reflects solar
energy -- the
key to accurately predicting the future of global warming. "Current
estimates of the effect of aerosols on global temperatures, which is
primarily
cooling, may be too small because the large contribution from this
transition
zone has been overlooked," Remer said. "If aerosols are offsetting
warming more than we thought, it's possible that warming could increase
more
than expected in the future if aerosols continue to decline, as has
been
reported recently."
This summer the scientists hope to get a closer look at the "twilight
zone" and the hard-to-detect particles inside it with new measurements
by
the Aerosol Robotic Network and NASA aircraft.
For more
information and images,
visit:
http://www.nasa.gov/centers/goddard/news/topstory/2007/twilightzone_particles.html
Aerosol
Robotic Network:
http://aeronet.gsfc.nasa.gov/
Global
Aerosol "Sunscreen" Has Likely Thinned:
http://www.nasa.gov/centers/goddard/news/topstory/2007/aerosol_dimming.html
NASA
Explains Puzzling Impact of Polluted Skies on Climate:
http://www.nasa.gov/vision/earth/environment/pollution_clouds.html
Aerosols
Worldwide 2006 (image):
http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17575
Writer:
Steve Cole, NASA Goddard Space Flight Center
##
Contact:
Lynn Chandler
NASA Goddard
Space Flight Center
301-286-2806
This text is
derived from:
http://www.nasa.gov/centers/goddard/news/topstory/2007/twilightzone_particles.html
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