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

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.