On the Sea-Ice Conditions Observed by
SAR and AVHRR Imagery in the Ross Sea during Spring
Adrian Hauser1, Matthew
Lythe and Gerd Wendler2
International Centre for Antarctic Information and
Christchurch, New Zealand
1 Institut für Meteorologie und Klimaforschung,
Universität Karlsruhe, Germany
2 Geophysical Institute, University of Alaska, Fairbanks,
SAR (Synthetic Aperture Radar) imagery from ERS 2 was
obtained from the Terra Nova region in the Ross Sea in
spring of 1996. Such active microwave images have a high
resolution (14 m) and penetrate clouds and darkness.
However, the interpretation of the data is not simple, as
besides ice thickness, surface roughness, salinity and
water content influence the backscatter.
Statistical procedures were developed, which reduced
the speckle noise, but obtained the boundaries between
different ice types. More than 95 % of the sea ice could
be classified in one of four established sea ice
categories based on the backscatter spectrum after 10
iterations. Some of the findings are:
- The size of the Terra Nova Polynya depended strongly
on the strength of the off-shore winds, which were
measured by Italian Automatic Weather Stations (AWS). It
appear that its presents and size is more dependent on
atmospheric than on oceanographic forcing.
- The Terra Nova Polynya is frequently not ice free,
but covered, at least in part, by thin, rough sea ice.
This could be deduced both, from thermal IR AVHRR as from
- The strong katabatic wind flow down the Priestley
and Reeves Glaciers, combined in even stronger winds on
the Nansen Ice Shelf. Offshore the Nansen Ice Shelf, IR
images displayed an area of relatively warm temperatures,
previously reported on by the Italian AWS stations.
- While large changes in the ice types with time were
observed in front of the Nansen Ice Shelf and areas
further off the coast, sea ice conditions changed little
in over a month for protected areas, for example between
the Campbell Ice Tongue and Cape Washington. Here, and in
other coastal areas, the ice appears to be landfast.
- Occasionally, the ice classification derived from
the SAR images was not conclusive, as the
intercomparision with AVHHR visible and IR data showed.
For example, thin, newly formed sea ice produced a
similar backscatter as floating, freshwater