October 2, 2003
Scientists Use Satellite to "Pond-er" Melted Arctic Ice
NASA researchers and other scientists used a satellite combined with
aircraft video to create a new technique for detecting ponds of water
on top of Arctic sea ice. Until now, it was not possible to accurately
monitor these ponds on ice from space.
Water that forms on sea ice during the summer, called a melt pond, absorbs
the Sun's energy rather than reflecting it back to space the way ice
does. The balance between reflected and absorbed energy has a large effect
on Arctic and global climate. When more ponds of water form on the Arctic
sea ice cover in early summer, more heat is absorbed, causing the Arctic?s
sea ice cover to melt faster during the summer. Knowledge of when and
where these melt ponds form will help scientists calculate the balance
of energy in the Arctic and improve their knowledge and projections of
climate both regionally and globally.
By using detailed aircraft video of Arctic surfaces and comparing those
with coarser satellite imagery, the researchers were able to recognize
rough features in the satellite data that corresponded to ponds on ice,
ocean water, and un-melted sea ice. Now, they are able to use a satellite
to monitor sea ice, without the aid of the aircraft video. Satellites
offer the advantages of frequent regular flyovers that cover vast areas
all at once.
"Our new technique offers the possibility of determining when and mapping
where these melt ponds form and would greatly aid our understanding of
the Arctic heat balance," said co-author Donald Cavalieri, a senior research
scientist at NASA's Goddard Space Flight Center (GSFC), Greenbelt, Md.
An article describing the new technique appeared in a recent issue of
the journal, Remote Sensing of Environment.
During spring and summer, these melt ponds cause existing sea ice to
melt faster and greatly reduce the ice's ability to reflect sunlight.
This can create a positive feedback, where an increasing number of melt
ponds absorbs more heat and causes sea ice cover to melt even faster.
During the warmer months, melt ponds can cover up to 50 percent of the
Arctic sea ice area. There may be a relationship between the fraction
of melt ponds and the amount of sea ice cover at summer?s end. Researchers
know from satellite records covering the last 30-years that the Arctic
sea ice cover at summer?s end has been decreasing rapidly. This new technique
may help them determine whether there has also been an increase in the
number of melt ponds over this period.
This new technique to detect melt pond coverage uses NASA's Enhanced
Thematic Mapper Plus (ETM+) instrument on the Landsat 7 satellite, developed
with the aid of much higher resolution video imagery from a NASA supported
aircraft experiment during the summer of 2000.
"This result is an excellent example of how the coordinated use of satellites
and research aircraft are used to develop new techniques for observing
the Earth," Cavalieri added.
By using video footage from an aircraft flight at an altitude of almost
one and a half kilometers, the researchers were able to compare that
higher resolution footage with Landsat 7 images passing over the same
path above Baffin Bay in the Arctic on the same day. They then compared
the Landsat imagery with the aircraft video.
While Landsat 7 shows less detail, it covers vast areas all at once.
The aircraft video, on the other hand, allows researchers to view a 1.5
meter area in detail.
By classifying 13 high resolution images from the aircraft into areas
of ocean, ice with ponds, and pond-free ice and then comparing these
areas with the different wavelength bands of Landsat, the researchers
were able to develop a new method to calculate the extent of open water,
melt ponds, and sea ice over large areas using Landsat data by itself.
"Previously there were no systematic measurements of melt ponds, but
this technique with the Landsat creates the possibility of determining
when and where these melt ponds form," Cavalieri said. "It helps us understand
the heat balance, which ties into the global climate system."
Lead author Thorsten Markus, who is currently conducting research at
sea, is also a scientist at NASA GSFC. Other co-authors include Mark
Tschudi, National Center for Atmospheric Research, Boulder, Colo., and
Alvaro Ivanoff, Science Systems and Applications, Inc., Lanham, Md.
The study was funded by NASA. NASA's Earth Science Enterprise is dedicated
to understanding the Earth as an integrated system and applying Earth
System Science to improve prediction of climate, weather, and natural
hazards using the unique vantage point of space.
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Baffin Bay Region, June 27, 2000
The larger background is an Advanced Very High Resolution Radiometer
(AVHRR) scene of the Baffin Bay region on June 27, 2000. The inset in
the lower right corner shows a gray scale Landsat 7 image for the same
day; aircraft tracks are in yellow. Credit: NASA
High-Resolution
image
Landsat Image with Ice and Water Classifications, Baffin Bay Region,
June 27, 2000
This is the same image from Landsat 7 as the gray scale inset in Figure
1, only here the surface types have been classified by color, using the
new satellite-based technique for measuring melt ponds. The image shows
ice-free water as black, sea ice with pond water on it (or ponded ice)
as blue, and sea ice without ponds on it (unponded ice) as white. Credit:
NASA High-Resolution
image
Melt Ponds on Sea Ice, June 27, 2000
This image shows digital true-color images of Baffin Bay taken from a
Navy research aircraft participating in the Meltpond2000 experiment.
The light blue areas show ponds of water on the sea ice, white areas
are sea ice without ponds, and the darker areas depict ice-free water.
Credit: NASA High-Resolution image |