Abstract
Satellite gravimetry allows for determining large scale mass transport in the system Earth and to quantify ice mass change in polar regions. We provide, evaluate and compare a long time-series of monthly gravity field solutions derived either by satellite laser ranging (SLR) to geodetic satellites, by GPS and K-band observations of the GRACE mission, or by GPS observations of the three Swarm satellites. While GRACE provides gravity signal at the highest spatial resolution, SLR sheds light on mass transport in polar regions at larger scales also in the pre- and post-GRACE era. To bridge the gap between GRACE and GRACE Follow-On, we also derive monthly gravity fields using Swarm data and perform a combination with SLR. To correctly take all correlations into account, this combination is performed on the normal equation level. Validating the Swarm/SLR combination against GRACE during the overlapping period January 2015 to June 2016, the best fit is achieved when down-weighting Swarm compared to the weights determined by variance component estimation. While between 2014 and 2017 SLR alone slightly overestimates mass loss in Greenland compared to GRACE, the combined gravity fields match significantly better in the overlapping time period and the RMS of the differences is reduced by almost 100 Gt. After 2017, both SLR and Swarm indicate moderate mass gain in Greenland.
Highlights
The Gravity Recovery and Climate Experiment satellite mission (GRACE) [1] dedicated to the observation of temporal variations of the gravity field allows for the quantification of ice mass loss of glacier accumulations in polar and sub-polar regions (e.g., [2,3,4])
glacial isostatic adjustment (GIA), i.e., the relaxation of the crust in reaction to the large scale ice melt after the last ice age, counteracts ice mass loss and the actual ice mass loss is even larger than indicated by the figures, while snow mass depends on the season and largely cancels out in a multi-year mean
The observation coverage is densest near the poles due to the polar orbits of the GRACE satellites and the noise is lowest near the poles
Summary
The Gravity Recovery and Climate Experiment satellite mission (GRACE) [1] dedicated to the observation of temporal variations of the gravity field allows for the quantification of ice mass loss of glacier accumulations in polar and sub-polar regions (e.g., [2,3,4]). This high resolution information is limited to the life-time of the GRACE satellites (2002–2017) and of the GRACE-FO (Follow On) [5] mission that was launched in May 2018, but suffered a failure of the main instrument processing unit between July and October 2018. For a focus on variations in Earth oblateness, where GRACE results are unreliable [13], compare Cox and Chao [14] Cheng and Tapley [15] and Bloßfeld et al [16]
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