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An investigation of glaciological and meteorological parameters in the Lambert Glacier Basin using high precision GPS strategies


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Hurd, R 2002 , 'An investigation of glaciological and meteorological parameters in the Lambert Glacier Basin using high precision GPS strategies', PhD thesis, University of Tasmania.

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Changes in the Earth's global climate and sea level are two of the most important
issues facing scientists today. Antarctica plays a vital role in any consideration of sea
level change as the southern polar ice-sheet holds the majority of non-oceanic water
on the Earth's surface (IPCC, 1997). Changes in the volume of the Antarctic ice-sheet
are intricately linked to global climate change due to the strong interactions between
the components of the Earth's system in this region. However, in spite of its potential
impact on global climate and sea level change, the mass balance (net gain or loss in
volume) of the Antarctic ice sheet is poorly known. Thus, in an effort to understand
past and current climate and its susceptibility to change, scientists have embarked
upon programs to study and monitor many aspects of the Antarctic environment.
One scientifically important area of the Antarctic ice-sheet is the Lambert Glacier
Basin (LGB) in East Antarctica. The LGB is an ice drainage basin that covers
approximately 13% of the Antarctic ice cap and transports ice from this region
through a small section (approximately 1.7%) of the continental coastline. During the
late 1980s, several major Australian glaciology programs focussed on the LGB
region, collecting field measurements for mass balance studies. The commencement
of these programs coincided with the introduction of reliable, civilian quality Global
Positioning System (GPS) units for use in scientific research and this was therefore
selected as the primary means of collecting position data for the LGB fieldwork. One
glaciology program in particular used GPS instruments to determine repeat site
positions along a 2015 km traverse route, approximately following the 2500 m
surface contour of the LGB. This series of GPS surveys were undertaken between
1989 and 1995, a period which coincided with major changes in the GPS system,
advances in GPS technology, the introduction of permanent GPS networks and
extreme variations in the ionospheric cycle. Using the GAMIT/GLOBK software package (Herring, 1999; King and Bock, 1999),
all five seasons of LGB GPS data have been analysed. The primary results of this
analysis include determinations of ITRF97 positions for the LGB sites with
horizontal uncertainties (la) of approximately 3 mm and height uncertainties at the
majority of sites within 10 mm. From the repeat occupation of the LGB sites, surface
ice velocities with uncertainties (la) of approximately 0.01 m/yr were computed.
These high quality position and velocity estimates have been analysed in terms of
identifying topographic and ice flux characteristics of the LGB. The GPS derived results have also been compared with remote sensing (i.e., European Remote-sensing
Satellite altimeter Digital Elevation Models) and other available datasets in the LGB
region (i.e., UPS derived velocities from other studies). The velocity results from the
GPS processing were used to validate the mass balance modelling technique of
Testut (2000). This validation process has indicated that the modelling generally
produces balance velocities within 20% of the observed values and a flow pattern
close to that indicated by the GPS results. These comparisons between modelled and
observed values are within the expected error-bounds of the modelling process and
indicate that the LGB is close to a state of balance.
The velocity and topography of the LGB are important data for the analysis of ice
movement out of the basin, but in terms of mass balance studies, the amount of ice
introduced into the system is also vital information. Measuring atmospheric water
vapour is one way of obtaining information on the amount of moisture available for
addition into the ice systems. It is now well known that the delay of GPS signals by
the Earth's troposphere may be interpreted as a measurement of precipitable water
(PW) above the UPS site. In this study it was proposed that, in principle, PW
estimates for the LGB GPS sites could be determined using the GPS meteorology
technique even though expected values would be less than 2 mm. GPS-meteorology calculations were initially carried out on data from AmundsenScott
(South Pole), Mawson, Davis and Casey (East Coast of Antarctica) to
determine the absolute limits of the technique. The results of this analysis showed
constant, site-specific biases of approximately 2 mm for the GPS PW values when
compared to those determined from radiosonde observations for the same period
(winter 1998 and summer 1999). However, the precision (around the bias) of these
UPS PW results was approximately 0.6 mm indicating that signal to noise ratios
were at an acceptable level (> 3).
The positive evaluation of the UPS-meteorology technique at the permanent UPS
sites allowed the method to be applied to LGB UPS data from 1993/94 and 1994/95.
From the LGB PW results, a general decrease in PW over the eastern sector of the
traverse route was observed between summer 1993/94 and summer 1994/95. This
result is in agreement with the mean PW trends observed for these periods in timeseries
analysis of 11 years of radiosonde data at Davis and Mawson and strongly
suggests that the UPS results are a reflection of actual variation over this time period.
An important result from this first-ever observation of PW over the interior LGB is
the ability to validate modelling of atmospheric processes over inland Antarctica,
where ground-based measurements are incredibly scarce. The GPS PW values
determined in this study compare well to National Centre for Environmental
Prediction model values at the LGB elevations, with differences of less than 1 mm. The 11-year analysis of radiosonde data at Mawson and Davis was also an important
component in the study of PW over the LGB region as it displayed interesting
variations in the water vapour signal over various time-scales. An approximately 6
year cycle observed in the radiosonde PW appears to have a significant correlation
with the passage of the Antarctic Circumpolar Wave through the study region.
It is clear that high quality GPS data are now able to provide much more than just
positions. In this study, it has been demonstrated that GPS techniques provide a
valuable tool for generating a wide-range of information that are important for
studying the current state of the Antarctic and global environment. Through reobservation
processes, these GPS techniques have also been used to determine
parameters and detect changes within the study region at unprecedented levels of
precision and provide the foundations for long-term studies of the LGB.

Item Type: Thesis - PhD
Authors/Creators:Hurd, R
Keywords: Ice sheets, Glaciers, Global Positioning System
Copyright Holders: The Author
Copyright Information:

Copyright 2002 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Substantial parts of published papers appear in this work including

Chapter 4 and Section 5.1
Manson, R., M. King and R. Coleman (1998). GPS: Putting it on ice. 39th Australian Surveyors Conference, 8-13th November 1998, Launceston, Tasmania, Australia. pp 31-42.

Chapter 4, Section 5.1 and Section 5.4
Manson, R., R. Coleman, P. Morgan and M. King (2000). Ice Velocities of the Lambert Glacier from Static GPS observations. Earth, Planets and Space 52(11), pp 1031- 1036.

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