As it passed over Antarctica on December 16, 2004, the Multi-angle Imaging
SpectroRadiometer (MISR) on NASA's Terra satellite captured this image
showing a wavy pattern in a field of white. At most other latitudes, such
wavy patterns would likely indicate stratus or stratocumulus clouds. MISR,
however, saw something different. By using information from several of its
multiple cameras (each of which views the Earth's surface from a different
angle), MISR was able to tell that what looked like a wavy cloud pattern
was actually a wavy pattern on the ice surface. One of MISR's cloud
classification products, the Angular Signature Cloud Mask (ASCM), correctly
identified the rippled area as being at the surface.
In this image pair, the view from MISR's most oblique backward-viewing
camera is on the left, and the color-coded image on the right shows the
results of the ASCM. The colors represent the level of certainty in the
classification. Areas that were classed as cloudy with high confidence are
white, and areas where the confidence was lower are yellow; dark blue shows
confidently clear areas, while light blue indicates clear with lower
confidence. The ASCM works particularly well at detecting clouds over snow
and ice, but also works well over ocean and land. The rippled area on the
surface which could have been mistaken for clouds are actually sastrugi --
long wavelike ridges of snow formed by the wind and found on the polar
plains. Usually sastrugi are only several centimeters high and several
meters apart, but large portions of East Antarctica are covered by
mega-sastrugi ice fields, with dune-like features as high as four meters
separated by two to five kilometers. The mega-sastrugi fields are a result
of unusual snow accumulation and redistribution processes influenced by the
prevailing winds and climate conditions. MISR imagery indicates that these
mega sastrugi were stationary features between 2002 and 2004.
Being able to distinguish clouds from snow or ice-covered surfaces is
important in order to adequately characterize the radiation balance of the
polar regions. However, detecting clouds using spaceborne detectors over
snow and ice surfaces is notoriously difficult, because the surface may
often be as bright and as cold as the overlying clouds, and because polar
atmospheric temperature inversions sometimes mean that clouds are warmer
than the underlying snow or ice surface. The Angular Signature Cloud Mask
(ASCM) was developed based on the Band-Differenced Angular Signature (BDAS)
approach, introduced by Di Girolamo and Davies (1994) and updated for MISR
application by Di Girolamo and Wilson (2003). BDAS uses both spectral and
angular changes in reflectivity to distinguish clouds from the background,
and the ASCM calculates the difference between the 446 and 866 nanometer
reflectances at MISR's two most oblique cameras that view forward-scattered
light. New land thresholds for the ASCM are planned for delivery later
this year.
The Multi-angle Imaging SpectroRadiometer observes the daylit Earth
continuously and every 9 days views the entire globe between 82° north and
82° south latitude. This image area covers about 277 kilometers by 421
kilometers in the interior of the East Antarctic ice sheet. These data
products were generated from a portion of the imagery acquired during Terra
orbit 26584 and utilize data from within blocks 159 to 161 within World
Reference System-2 path 63.
MISR was built and is managed by NASA's Jet Propulsion Laboratory,
Pasadena, CA, for NASA's Office of Earth Science, Washington, DC. The
Terra satellite is managed by NASA's Goddard Space Flight Center,
Greenbelt, MD. JPL is a division of the California Institute of Technology.