Determining the amount and type of sea ice in the polar oceans is crucial to
improving our knowledge and understanding of polar weather and long term climate
fluctuations. These views from two satellite remote sensing instruments; the
synthetic aperture radar (SAR) on board the RADARSAT satellite and the
Multi-angle Imaging SpectroRadiometer (MISR), illustrate different methods that
may be used to assess sea ice type. Sea ice in the Beaufort Sea off the north
coast of Alaska was classified and mapped in these concurrent images acquired
March 19, 2001 and mapped to the same geographic area.
To identify sea ice types, the National Oceanic and Atmospheric Administration
(NOAA) National Ice Center constructs ice charts using several data sources
including RADARSAT SAR images such as the one shown at left. SAR classifies sea
ice types primarily by how the surface and subsurface roughness influence radar
backscatter. In the SAR image, white lines delineate different sea ice zones as
identified by the National Ice Center. Regions of mostly multiyear ice (A) are
separated from regions with large amounts of first year and younger ice (B-D),
and the dashed white line at bottom marks the coastline. In general, sea ice
types that exhibit increased radar backscatter appear bright in SAR and are
identified as rougher, older ice types. Younger, smoother ice types appear dark
to SAR. Near the top of the SAR image, however, red arrows point to bright areas
in which large, crystalline "frost flowers" have formed on young, thin ice,
causing this young ice type to exhibit an increased radar backscatter. Frost
flowers are strongly backscattering at radar wavelengths (cm) due to both
surface roughness and the high salinity of frost flowers, which causes them
to be highly reflective to radar energy.
Surface roughness is also registered by MISR, although the roughness observed
is at a different spatial scale. Older, rougher ice areas are predominantly
backward scattering to the MISR cameras, whereas younger, smoother ice types are
predominantly forward scattering. The MISR map at right was generated using a
statistical classification routine (called ISODATA) and analyzed using ice
charts from the National Ice Center. Five classes of sea ice were found based
upon the classification of MISR angular data. These are described, based on
interpretation of the SAR image, by the image key. Very smooth ice areas that
are predominantly forward scattering are colored red. Frost flowers are largely
smooth to the MISR visible band sensor and are mapped as forward scattering.
Areas mapped as blue are predominantly backward scattering, and the other three
classes have statistically distinct angular signatures and fall within the
middle of the forward/backward scattering continuum. Some areas that may be
first year or younger ice between the multi year ice floes are not discernible
to SAR, illustrating how MISR potentially can make a unique contribution to sea ice mapping.
The Multi-angle Imaging SpectroRadiometer observes the daylit Earth continuously
and every 9 days views the entire globe between 82 degrees north and 82 degrees
south latitude. This data product was generated from a portion of the imagery
acquired during Terra orbit 6663. The MISR image has been cropped to include an
area that is 200 kilometers wide, and utilizes data from blocks 30 to 33 within
World Reference System-2 path 71.
Image credit: NASA/GSFC/LaRC/JPL, MISR Team; RADARSAT image courtesy NOAA
Satellite Active Archive. Figure courtesy IEEE, previously published
in: A. Nolin et al., IEEE Transactions on Geoscience and Remote
Sensing, July 2002, v. 40, pp. 1605-1615.
Text acknowledgment: Clare Averill (Acro Service Corporation/Jet Propulsion
Laboratory), Florence Fetterer (National Snow and Ice Data Center) and
Anne W. Nolin (Oregon State University).
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