Measured and modelled absolute gravity changes in Greenland
Introduction
The Earth's response to changes in ice mass, known as GIA, can be one or a sum of an elastic signal and a viscous signal. A gravimeter can detect the GIA signal, however it detects also the change in mass as a change in the direct attraction. In present day glaciated areas, all signals can be present, whereas in former glaciated areas, such as Scandinavia and North America, only the viscous signal is present. Although other forces can also produce an elastic signal (e.g. the atmosphere or ocean tides), the focus in this study is on the signal produced by changes in ice masses. The direct attraction from the ice and ocean is also studied through modelling.
There is an interest in knowing the GIA signal since it can lead to erroneous mass balance estimates when using satellite data. Satellite data are biased by the presence of the GIA signal since satellites cannot distinguish between the signal from present day mass changes and the signal due to mass changes from, e.g., the last glacial maximum (LGM) (see Barletta et al., 2008; Shepherd et al., 2012). The signals are not only of interest for studies of mass balance, since studies of geodynamics and the Earth's rheology can also benefit from knowledge on GIA signals.
To study the GIA global positioning system (GPS), tide gauge and AG are the methods used. In the GNET project, part of the Polar Earth Observing Network (POLENET), a number of permanent GPS stations are deployed on the Greenlandic bedrock surrounding the ice sheet. This project started in 2007, and at present 57 stations are deployed. Some results of the GNET project have been presented in Khan et al. (2010) and Bevis et al. (2012).
In 2009, DTU Space started to conduct AG measurements at selected GNET stations with the purpose of initiating future time series of gravity change. Precise measurements of gravity change in remote places is a novel research area that has become possible with the advent of accurate portable absolute gravimeters.
The studies by De Linage et al. (2007) and Mémin et al. (2011) aimed to separate the viscous and elastic signals using GPS and AG data. Since we do not have sufficient AG data to initiate such a study, we investigate the modelled gravity signal and compare the results with our preliminary AG measurements.
Modelling the gravity signal is complex since it consists of many parameters and requires assumptions about the Earth in order to make the computations feasible. Different ice histories with different temporal resolutions are used. The ice history going back to the LGM is modelled since very limited data are available to constrain the ice history 21,000 years ago. Satellite missions such as ICESat and CryoSat have delivered valuable data about changes in ice volume during the last decade. These satellite data provide new possibilities for constructing models of present day ice mass change.
The uncertainties in the GIA signal originate from the uncertainties in the Earth model and the ice sheet's thickness and extent through time. Uncertainty estimates of the different signals will be calculated.
Section snippets
Measurements
The first AG measurements in Greenland were conducted with a JILAg meter in 1988 when three sites were established (Timmen et al., 2008). FG5 measurements were conducted at Kulusuk and Kellyville in the period from 1995 to 2000 (Wahr et al., 2008). Since 2009, DTU has conducted gravity measurements in Greenland by using an A10 (serial number 019) absolute gravimeter from Micro-g LaCoste. The manufacturer's specifications for this instrument are an accuracy and a repeatability of 10 μGal (1 μGal = 10
Modelling
For a gravimeter standing on the Earth's surface, the measurement consists of several signals, three of which are considered here: the viscous, the elastic and the direct attraction. The last two signals relate to present day ice mass changes, while the first occurs due to changes over several thousand years (since the LGM in this study). The viscous and elastic signals occur due to the rheology of the mantle and elasticity of the Earth, respectively, whereas the direct attraction results from
Discussion
For most sites, the main signal occurs due to the direct attraction either from the ice, as at HEL2, KBUG and ISOR, or from the ocean, as at LYNS and THU3. At KULU, the source of the measured gravity change still needs to be determined. When considering the direct attraction, there is agreement between the measured and modelled gravity changes. For HEL2 and KBUG, the modelling results show a decrease in the ice melt with the majority of the melt happening below and above the observer,
Summary and conclusion
We used satellite-derived ice mass models to estimate the present day elastic signal and the direct attraction as seen by a gravimeter on the surface of the Earth. In addition, tide gauge data and a tide model were used to estimate the direct attraction from the ocean. We presented the preliminary results of our AG measurements in Greenland and showed that we can operate the instrument in remote places and collect useful data. However, we acknowledge that more data are needed to improve the
Acknowledgments
The authors would like to thank Giorgio Spada for providing the code to model the viscous and elastic signals and for assistance with any questions regarding his code. Thanks to Louise Sandberg Sørensen and Karina Nielsen for providing the ICESat ice models. We would also like to thank our colleagues at Bundesamt für Kartographie und Geodäsie (BKG) for their help in the 2010 season. The APLO service is provided by the Goddard VLBI group, data of which are available on the web at //gemini.gsfc.nasa.gov/aplo
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