Bridging the Potential Gap in Polar Ice Sheet Melt Estimates between GRACE and GRACE Follow-On Using SLR/DORIS Data

The GRACE mission has provided invaluable insight into polar ice sheet mass change since 2002. However, the GRACE mission might cease functioning before the August 2017 launch of its successor mission GRACE Follow-On. To mitigate against this potential lack of data, we reconstruct a single set of monthly, global gravity fields using conventional tracking data that would be uninterrupted over the potential gap. These fields would therefore provide continuous coverage of the polar ice sheet mass changes. We combined the conventional tracking data (SLR/DORIS) and GRACE fields via an Empirical Orthogonal Function (EOF) analysis, whereby the conventional tracking data temporal modes are obtained by fitting the SLR/DORIS Stokes coefficients to the GRACE spatial modes via linear least-squares. Combining those temporal modes with GRACE spatial modes yields the reconstructed global gravity fields. The validity of these reconstructed fields is characterized via Monte-Carlo simulations. In these simulations, randomly generated data gaps replace some of the GRACE fields. The mass change curve associated with each simulation is then compared to the original reconstruction (the "truth"). We see that the differences between the truth and the simulations are within the noise of the signal, except for the simulations with the longest gaps. For those, there are non-negligible differences in the trend. However, major short-term mass changes in Greenland are still captured. For example, even without data over the 2013 year (during the hiatus in Greenland ice sheet melt), the reconstructed mass change still captured this specific event. Similarly, the 2009 snowfall anomaly in East Antarctica is also detected in a reconstructed field, despite using simulated GRACE fields with a gap over that 2009 time frame.