How can Altimetry be used for Glaciology?
There now exists radar altimeter measurements from 1978 through the present
of the Greenland and Antarctic Ice sheets, the spatial and temporal coverage
is summarized in this table.
The NASA Pathfinder program is endeavoring to
reference data from all missions into a common datum using a common set of
atmospheric, instrument, and earth dynamic corrections where possible.
This will give scientists a time-series of consistent data over 20 years to
investigate seasonal and interannual variations of the ice-sheet surface
balance, which is caused by variations in snowfall and ice melting
( Zwally et al. 1989, Lingle et al.
1994 )
In addition, precise measurements of elevation
changes can be used to estimate total mass balance of the ice sheets,
which had not been obtainable to sufficient accuracy with other techniques.
The crossover data gives elevations for different tracks at the same location,
are best used for ice sheet change studies.
The multi-mission data can also be combined to give dense topographic
coverage crucial for a variety of glaciological studies, from numerical
modeling of the ice dynamics to monitoring of key glaciological features:
ice drainage basins, ice-shelf grounding lines, ice divides, subglacial
lakes, ice shelf fronts, and ice rises.
Topographic grids (Level 4) using
multi-mission data are crucial to delineating the key glaciological features
of the ice sheets mentioned above. For more detailed elevation
information the georeferenced databases (Level 3)
that contain the full-rate along track data can be utilized.
The ice elevation data set (Level 2 - Ice Data Record)
consists of the full rate altimetry with location, time, surface height
and all pertinent corrections. It is mainly used by scientists who are
trying to improve the algorithms to calculate the corrections or who require
knowledge of what effect the corrections have on the data. Most researchers
do not require this much detail and would be better served by the
georeference data bases which contain
the precise surface height after all corrections have been applied at the full
data rate.
The altimeter waveform return gives
a measure of the surface roughness on scales of sub-meter to tens of
kilometers and indicates if surface penetration and/or volume scattering
is present. These results are used to investigate regional, interdecadal
and interannual changes in the ice sheet and sea ice surface characteristics
(Yi and Bentley 1994, Davis and Zwally 1993).
The returns are also used to identify sea ice boundaries and extent. These
are available on the Level 1 - Waveform Data Records.
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