Presently, the longest, most consistent and continuous sea ice record
available is from the multi-frequency passive microwave satellite data
which began in 1978. The first multi-frequency passive microwave
satellite sensor was the NASA Nimbus 7 Scanning Multichannel Microwave
Radiometer (SMMR: 10/78 to 08/87) which has been followed by the
Defense Meteorological Satellite Program Special Sensor Microwave
Imagers (SSM/I F8: 07/87 to 12/91; SSM/I F11: 12/91 to 09/95; and SSM/I
F13: 05/95 to present).
The monthly images shown in the animations were derived from the SMMR
and SSM/I satellite data with Goddard Space Flight Center Bootstrap
algorithm (Comiso, 1983; Comiso, 1995), while the daily 1999 data were
derived from the GSFC NASA Team algorithm (Cavalieri et al., 1984;
Gloersen and Cavalieri, 1986). Both datasets were provided by the EOS
Distributed Active Archive Center (DAAC) at the National Snow and Ice
Data Center (NSIDC), University of
Colorado, Boulder, CO.
The GSFC Bootstrap and NASA Team passive microwave algorithms do a
very consistent job in tracking the ice edge - which is typically identified
as the 15% sea ice concentration contour. Sea ice concentration
retrievals within the ice edge, however, are less consistent due to
changing surface conditions caused by passing weather systems and/or
air temperature fluctuations. In general, sea ice concentrations below
about 40% are typically associated with marginal ice zone conditions,
while concentrations between 40% to 70% are typically open pack
conditions, and concentrations over about 70% are usually close pack
conditions. These rough categories should be kept in mind when
observing the sea ice concentration variability demonstrated in the
animations. For example, if one is considering doing field operations
on the pack ice (e.g. floe transects, ice coring, under ice water
collection, penguin or seal tagging, etc), areas that show consistent
coverage of concentrations greater than 70% would be most ideal;
otherwise, those areas that show high variability will be less reliable
for providing a firm platform for ice studies.
The following photos were taken during NBP99-06 Palmer LTER sea
ice cruise (late June to early July, 1999). They provide a visual
reference for the broad concentration categories described above.