Andrew Yee[_1_]
May 21st 08, 06:38 PM
Media Relations
Pacific Northwest National Laboratory
Contact:
Mary Beckman, (509) 375-3688
Release date: April 17, 2008
Seeing clearly despite the clouds
A new approach dramatically improves the accuracy of atmospheric aerosol
measurements on cloudy days
RICHLAND, Wash. -- Satellites taking atmospheric measurements might now be
able to see blue skies as clearly as optimists do. Researchers have found a
way to reduce cloud-induced glare when satellites measure blue skies on
cloudy days, by as much as ten-fold in some cases. The result might lead to
more accurate estimates of the amount of sunlight penetrating the
atmosphere. Because clouds represent one of the largest areas of
uncertainty, eventually this could lead to improved climate models.
Sunlight bouncing off clouds blinds satellites trying to determine how much
the blue sky between is actually reflecting. Researchers at the Department
of Energy's Pacific Northwest National Laboratory have found that using an
indirect measurement of that reflected light can bring the measurements of
cloud-bordered blue skies to within about 10 percent of what other
instruments indicate, the researchers report March 28 in Geophysical
Research Letters.
"When researchers try to apply satellite technology originally developed for
clear skies to partly cloudy conditions, they find additional light
reflected from clouds," says PNNL atmospheric scientist Evgueni Kassianov.
"We can't use the same technology we use for clear skies for complex cloudy
skies."
Blue skies might seem empty, but they are full of naked-to-the-eye particles
called aerosols, which are made up of water and bits of matter. These
aerosols reflect sunlight. The more aerosols, the more sunlight is reflected
back to the satellite. But on cloudy days, clouds bounce sunlight all around
and make nearby aerosols seem brighter than they really are. Previous
research has shown that clouds can brighten aerosols even up to three
kilometers (almost two miles) away.
Atmospheric scientists convert the brightness of those aerosols into a value
called the aerosol optical depth. This is roughly how far light can
penetrate the air, sometimes thought of as visibility: skies with few
aerosols appear clear and skies with many appear hazy. Previous work has
shown that nearby clouds can increase the brightness of blue skies by 10 to
15 percent, underestimating the visibility by 140 percent.
To address the problem, Kassianov and his fellow PNNL researcher Mikhail
Ovtchinnikov took advantage of the fact that clouds largely reflect the same
amount of light regardless of the wavelength of light. Aerosols, on the
other hand, reflect sunlight at different wavelengths to differing degrees.
So, the two tested whether using ratios of sunlight reflectance at different
wavelengths might allow the extra reflectance from clouds to drop out of
their atmospheric images.
The idea worked. The researchers constructed two images of a patch of cloudy
sky using cumbersome computational methods that can subtract out estimated
cloud-induced glare. One was based on total reflected light, and the other
was based on ratios of how much light was reflected at two different
wavelengths. The ratio image provided a view of the cloudy sky with much
better contrast than the reflected light one, indicating that ratios better
delineate blue sky from cloud.
But a pretty picture won't help anyone recreate aerosols in a computer
model. So the team devised an innovative way to convert the ratios back into
aerosol optical depths. The team created a database that related wavelength
ratios, particles sizes, number of particles and aerosol optical depth. From
this database, two ratios would allow them to determine their aerosol
properties of interest.
The two then determined the accuracy of the ratio method. They selected data
from a typical summer day in the southern Great Plains, gathered via the
DOE's Atmospheric Radiation Measurement Climate Research Facility (ACRF) in
Oklahoma. Using the ratio method, they retrieved the aerosol optical depth
at three wavelengths (470, 660 and 870 nanometers) and compared this to the
original data. The ratio method estimated aerosol optical depth under partly
cloudy conditions with an error of only about 10 percent.
If the results hold up with additional testing, Kassianov says this approach
could be applied to data being collected by NASA's Earth Observing System in
skies near clouds.
"Researchers use different models and try to incorporate aerosol effects.
The models are so inaccurate that we don't know how much aerosols change
cloud properties," he says. "This tool could potentially increase the
accuracy of our climate models."
# # #
Reference: E. I. Kassianov, M. Ovtchinnikov, On reflectance ratios and
aerosol optical depth retrieval in the presence of cumulus clouds, Geophys
Res Lett, March 28, 2008, doi:10.1029/2008GL033231.
This work was supported by the Department of Energy's Office of Biological &
Environmental Research and the National Aeronautics and Space
Administration.
The William R. Wiley Environmental Molecular Sciences Laboratory is a
national scientific user facility sponsored by the Department of Energy's
Office of Biological and Environmental Research and located at Pacific
Northwest National Laboratory.
PNNL is a DOE Office of Science national laboratory that solves complex
problems in energy, national security and the environment, and advances
scientific frontiers in the chemical, biological, materials, environmental
and computational sciences. PNNL employs 4,000 staff, has a $760 million
annual budget, and has been managed by Ohio-based Battelle since the lab's
inception in 1965.
IMAGE CAPTION:
[http://picturethis.pnl.gov/picturet.nsf/by+id/DRAE-7DJVV2]
Clearly cloudy: Using the ratio method (right) clears up the satellite view
(left) of a partly cloudy sky.
Pacific Northwest National Laboratory
Contact:
Mary Beckman, (509) 375-3688
Release date: April 17, 2008
Seeing clearly despite the clouds
A new approach dramatically improves the accuracy of atmospheric aerosol
measurements on cloudy days
RICHLAND, Wash. -- Satellites taking atmospheric measurements might now be
able to see blue skies as clearly as optimists do. Researchers have found a
way to reduce cloud-induced glare when satellites measure blue skies on
cloudy days, by as much as ten-fold in some cases. The result might lead to
more accurate estimates of the amount of sunlight penetrating the
atmosphere. Because clouds represent one of the largest areas of
uncertainty, eventually this could lead to improved climate models.
Sunlight bouncing off clouds blinds satellites trying to determine how much
the blue sky between is actually reflecting. Researchers at the Department
of Energy's Pacific Northwest National Laboratory have found that using an
indirect measurement of that reflected light can bring the measurements of
cloud-bordered blue skies to within about 10 percent of what other
instruments indicate, the researchers report March 28 in Geophysical
Research Letters.
"When researchers try to apply satellite technology originally developed for
clear skies to partly cloudy conditions, they find additional light
reflected from clouds," says PNNL atmospheric scientist Evgueni Kassianov.
"We can't use the same technology we use for clear skies for complex cloudy
skies."
Blue skies might seem empty, but they are full of naked-to-the-eye particles
called aerosols, which are made up of water and bits of matter. These
aerosols reflect sunlight. The more aerosols, the more sunlight is reflected
back to the satellite. But on cloudy days, clouds bounce sunlight all around
and make nearby aerosols seem brighter than they really are. Previous
research has shown that clouds can brighten aerosols even up to three
kilometers (almost two miles) away.
Atmospheric scientists convert the brightness of those aerosols into a value
called the aerosol optical depth. This is roughly how far light can
penetrate the air, sometimes thought of as visibility: skies with few
aerosols appear clear and skies with many appear hazy. Previous work has
shown that nearby clouds can increase the brightness of blue skies by 10 to
15 percent, underestimating the visibility by 140 percent.
To address the problem, Kassianov and his fellow PNNL researcher Mikhail
Ovtchinnikov took advantage of the fact that clouds largely reflect the same
amount of light regardless of the wavelength of light. Aerosols, on the
other hand, reflect sunlight at different wavelengths to differing degrees.
So, the two tested whether using ratios of sunlight reflectance at different
wavelengths might allow the extra reflectance from clouds to drop out of
their atmospheric images.
The idea worked. The researchers constructed two images of a patch of cloudy
sky using cumbersome computational methods that can subtract out estimated
cloud-induced glare. One was based on total reflected light, and the other
was based on ratios of how much light was reflected at two different
wavelengths. The ratio image provided a view of the cloudy sky with much
better contrast than the reflected light one, indicating that ratios better
delineate blue sky from cloud.
But a pretty picture won't help anyone recreate aerosols in a computer
model. So the team devised an innovative way to convert the ratios back into
aerosol optical depths. The team created a database that related wavelength
ratios, particles sizes, number of particles and aerosol optical depth. From
this database, two ratios would allow them to determine their aerosol
properties of interest.
The two then determined the accuracy of the ratio method. They selected data
from a typical summer day in the southern Great Plains, gathered via the
DOE's Atmospheric Radiation Measurement Climate Research Facility (ACRF) in
Oklahoma. Using the ratio method, they retrieved the aerosol optical depth
at three wavelengths (470, 660 and 870 nanometers) and compared this to the
original data. The ratio method estimated aerosol optical depth under partly
cloudy conditions with an error of only about 10 percent.
If the results hold up with additional testing, Kassianov says this approach
could be applied to data being collected by NASA's Earth Observing System in
skies near clouds.
"Researchers use different models and try to incorporate aerosol effects.
The models are so inaccurate that we don't know how much aerosols change
cloud properties," he says. "This tool could potentially increase the
accuracy of our climate models."
# # #
Reference: E. I. Kassianov, M. Ovtchinnikov, On reflectance ratios and
aerosol optical depth retrieval in the presence of cumulus clouds, Geophys
Res Lett, March 28, 2008, doi:10.1029/2008GL033231.
This work was supported by the Department of Energy's Office of Biological &
Environmental Research and the National Aeronautics and Space
Administration.
The William R. Wiley Environmental Molecular Sciences Laboratory is a
national scientific user facility sponsored by the Department of Energy's
Office of Biological and Environmental Research and located at Pacific
Northwest National Laboratory.
PNNL is a DOE Office of Science national laboratory that solves complex
problems in energy, national security and the environment, and advances
scientific frontiers in the chemical, biological, materials, environmental
and computational sciences. PNNL employs 4,000 staff, has a $760 million
annual budget, and has been managed by Ohio-based Battelle since the lab's
inception in 1965.
IMAGE CAPTION:
[http://picturethis.pnl.gov/picturet.nsf/by+id/DRAE-7DJVV2]
Clearly cloudy: Using the ratio method (right) clears up the satellite view
(left) of a partly cloudy sky.