NASA - National Aeronautics and Space Administration

Above: 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. Click image to see larger picture Credit: NASA/USGS

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.

Above: Ice-free water is shown 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. Click image to see larger picture. Credit NASA/USGS

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.

Above: 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

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.

More information, movies and high resolution images see: NASA Goddard Space Flight Center
Krishna Ramanujan
NASA Goddard Space Flight Center