Geology Reference
In-Depth Information
contamination effects and presented in the Hierarchical
Data Format (HDF).
Zwally et al.
[1983] produced the
first atlas of sea ice in the Antarctica from ESMR data. It
was interesting to see for the first time the sea ice field that
encases the continent. One of the most unforeseen discov-
eries from this data set was a large “hole” of open water in
the middle of the sea ice cover during the winter of 1974.
That was the first observation of the phenomenon that
became later known as polynya (section 2.6.2).
ESMR was the precursor to the more widely used
microwave sensor; namely the Scanning Multichannel
Microwave Radiometer (SMMR), which was launched on
Nimbus‐7 and operated for 9 years from 1978 to 1987.
Due to power limitations, SMMR collected data every
other day. The sensor measured dual‐polarized microwave
emission at 6.63, 10.69, 18.0, 21.0 and 37.0 GHz. This
allowed for the mapping of ice concentration and the dis-
tinction between FY and MY ice types. The radiometers
on SSMR and all of the subsequent passive microwave
systems have been conical. The first algorithms to retrieve
sea ice concentration were developed to use SMMR data.
Among them are NORSEX [
Svendsen et al.,
1983], NASA
team [
Cavalieri et al.,
1984], and Bootstrap [
Comiso
, 1986].
The record of brightness temperature and the derived ice
concentration from SMMR using the NASA team algo-
rithm are made available on a 25 × 25 km grid in polar ste-
reographic projection via FTP from the NASA Goddard
Space Flight Centre (GSFC) [
Gloersen et al.,
1990].
Following the SMMR, the SSM/I was launched aboard
the Defence Meteorological Satellite Program (DMSP).
The sensor was onboard a series of satellite platforms
that started in 1978: F8, F10, F11, F13, F14, and F15. In
addition to a single vertical polarization channel operat-
ing at 22.2 GHz, the SSM/I carried radiometers that oper-
ated in dual polarization mode at 19.3, 37.0, and 85.5 GHz.
Data are sampled over 104.2° from end to end per scan
line while the scanner is looking backward with incidence
angle 53.1°. The spatial sampling rate for the first three
channels was 25 km and for the 85 GHz channel 12.5 km.
More details about the sensor are presented in
Hollinger
et al.
[1987], and an evaluation of its performance (stabil-
ity of the gain, radiometric calibration, co‐registration,
electronic noise and sensitivity, etc.) is presented in
Hollinger et al.
[1990]. SSM/I has been used to create the
longest record of sea ice extent and concentrations for the
polar regions during its 31 year life span before it failed in
February 2009. That is the legacy of SSM/I. Sea ice prod-
ucts from SSM/I are available from NSIDC.
By intercalibrating data from different passive micro-
wave sensors on different satellites, researchers could com-
pile an even longer record of sea ice in the polar regions to
study the variability and identify trend of ice distribution
and extent.
Cavalieri et al.
[2003] presents an analysis of
30 years of Arctic sea ice from passive microwave data.
They used data from the ESMR (December 1972-March
1977), SMMR and SSM/I (October 1978-June 1988), and
SSM/I alone (June 1987-December 2002). Operational ice
charts from the U.S. National Ice Center (NIC) were also
used to fill gaps. Other records of passive microwave data
that revealed seasonal, regional, and interannual variabil-
ity of Arctic ice are presented in
Parkinson et al.
[1999],
Parkinson and Cavalieri
[2002],
Zwally et al.
[2002a], and
LeDrew et al.
[1992].
The next generation of the SSM/I was the Special
Sensor Microwave Imager/Sounder (SSMIS) onboard
the same DMSP platform series F16, F17, and F18. The
first sensor was launched in October 2003. The data are
meant to provide a best estimate of current ice and snow
conditions based on available algorithms and informa-
tion. The data need to be intercalibrated for consistency.
Products can be accessed from the NSIDC website
http://
nsidc.org/data/nise1.html.
SSMIS also includes higher
frequency sounding channels.
The passive microwave sensing found more uses in
sea ice applications with the launch of the Advanced
Microwave Scanning Radiometer—Earth Observing
System (AMSR‐E) on NASA's Aqua platform, beginning
in 2001. This sensor was developed and provided by the
Japan Aerospace Exploration Agency (JAXA) in collabo-
ration with U.S. scientists. It measured dual polarized
brightness temperatures at 6.9, 10.7, 18.7, 23.8, 36.5, and
89.0 GHz. Its spatial resolution was better than SSM/I
(e.g., 6 × 4 km from the 89.0 GHz channel compared to
15 × 13 km from the 85.5 GHz channel of SSM/I; and 14 ×
8 km from the 36.5 GHz channel compared to 37 × 28 km
of SSM/I). The sensor was used extensively in sea ice and
snow research and monitoring until it failed in October
2011. In addition to sea ice concentration, AMSR‐E was
used to determine sea ice thickness (section 10.4) and sur-
face temperature (section 10.5).
On 18 May, 2012 JAXA launched the Advanced
Microwave Scanning Radiometer 2 (AMSR‐2) onboard
the Global Change Observation Mission (GCOM‐C1) sat-
ellite. It has provided continuation of observations after
the failure of its predecessor AMSR‐E in October 2011.
Its radiometer also uses six different frequency bands
ranging from 7 to 89 GHz. The footprint dimensions of
the 18.7 and 89.0 GHz channels are 14 km × 22 km and
3 km × 5 km, respectively. This is an enhanced resolution
compared with AMSR‐E channels. At a rotation rate of
the antenna once per 1.5 s, the swath of the imaged area is
1450 km. The first composite image of the Arctic ice was
acquired on 3 July, 2012.
In general, passive microwave observations have fairly
coarse resolution (a few kilometers or tens of kilometers).
Therefore, they are more suitable for synoptic observations
that serve more the climatic applications. For tactical and
local‐scale ice information, which is required in specific
