Chapter 6. Center for the Study of Earth from Space (CSES), 1985-2002
Greenland Ice Sheet Detection
Steffen works with the Greenland ice sheet
melt characteristics that are critical to the assessment of ice sheet mass balance
and the interpretation of the mass balance observations. Because of the
positive albedo feedback associated with snow melt and the fact that wet
snow absorbs as much as three times more incident solar energy than dry
snow, ice sheet melt characteristics play a major role in the energy and mass
exchanges at the ice sheet surface. Moreover, surface melt can act to enhance
the flow of outlet glaciers through crevasse over-deepening and is believed to
have contributed to the very rapid thinning of a number of outlet glaciers in
eastern Greenland. Detection of surface melt at large spatial scales is most effectively
accomplished through the use of satellite microwave data, which
has a clear melt signature that arises from the transition from volume- to surface-
scattering during melt onset. As such, wet-snow emission approaches
black body behavior, and this change in emission characteristics is detectable
by most microwave sensors at frequencies in excess of 10 GHz. These
changes in emission characteristics have formed the basis of several passivemicrowave-
based melt assessment algorithms developed by Waleed Abdalati
(Ph.D. 1995). The algorithm is applicable to the Scanning
Multi-channel, Microwave Radiometer (SMMR) and Special Sensor Microwave/
Imager (SSM/I) instruments, which have provided near-continuous
satellite coverage since October 1978.
Currently, Steffen is working with NASA JPL scientist Dr. Son Nighiem
on the application of QuikSCAT data to monitor the melt extent on the
Greenland ice sheet. Detection of surface melt at large spatial scales is most
effectively accomplished through the use of satellite passive and active microwave
data (i.e., SSM/I and QuikSCAT). The QuikSCAT backscatter
data at a radiometric resolution of 7 kilometer x 25 kilometer has a high
temporal coverage of all of Greenland two times per day. The diurnal difference
of day and night overpasses of the QuikSCAT scatterometer data is
used to develop and implement an algorithm to determine melt pattern,
melt onset timing, and melt duration. It is apparent that the surface melt of
the Greenland ice sheet can be used as a climate proxy-value to study the
seasonal and interannual variability of near-surface air temperature and
other parameters that can be related to the mass balance. With this satellite
study the diurnal, seasonal, and interannual variability of melt extent on
the Greenland ice sheet can be assessed to study long-term trends and
changes of surface climatology and melt extent, and to relate these changes
to forcings such as large-scale synoptic variability and the Arctic Oscillation
(AO)/North Atlantic Oscillation (NAO).
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