Reconstructing GRACE-type time-variable gravity from the Swarm satellites

H Maja P Richter,Anna Klos,Elena Rangelova,Christina Lück,Michael G Sideris,Jürgen Kusche

doi:10.1038/s41598-020-80752-w

Abstract

The Gravity Recovery and Climate Experiment (GRACE) mission has enabled mass changes and transports in the hydrosphere, cryosphere and oceans to be quantified with unprecedented resolution. However, while this legacy is currently being continued with the GRACE Follow-On (GRACE-FO) mission there is a gap of 11 months between the end of GRACE and the start of GRACE-FO which must be addressed. Here we bridge the gap by combining time-variable, low-resolution gravity models derived from European Space Agency’s Swarm satellites with the dominating spatial modes of mass variability obtained from GRACE. We show that the noise inherent in unconstrained Swarm gravity solutions is greatly reduced, that basin averages can have root mean square errors reduced to the order of text {cm} of equivalent water height, and that useful information can be retrieved for basins as small as 1000 times 1000,hbox {km}. It is found that Swarm data contains sufficient information to inform the leading three global mass modes found in GRACE at the least. By comparing monthly reconstructed maps to GRACE data from December 2013 to June 2017, we suggest the uncertainty of these maps to be 2{-}3,text {cm} of equivalent water height.

Highlights

The Gravity Recovery and Climate Experiment (GRACE) mission has enabled mass changes and transports in the hydrosphere, cryosphere and oceans to be quantified with unprecedented resolution
The high accuracy of the GRACE solutions is due to the ultra-precise inter-satellite ranging system, while with satellite laser ranging (SLR) and Swarm, the gravity field solutions can only be retrieved from the tracking of the spacecraft orbit perturbations, inevitably resulting in lower spatial resolution
We find that the Swarm time series have the potential to fill the gap between GRACE and GRACE-FO

Summary

60 Ga: CHLM

From 0.14 m (monthly Swarm-only solution) to 0.05 m when we use the reconstructed solution (or 0.05 m for Swarm reconstructionresidual ). The Swarm-only and Swarm-reconstructed solutions agree at the level of RMSE 0.09 m for d/o 12 and RMSE 0.18 m for d/o 40 after the end of GRACE, and they seem to extend the time series without noticeable outliers (except for monthly d/o 40 data). We derived linear trends and annual amplitudes from daily GPS displacements, GPS displacements averaged to monthly time scale, and displacements estimated for GRACE, Swarm-only, Swarm-reconstructed and Swarm-reconstructedresidual data. We find that both Swarm-reconstructed and GRACE-predicted displacements reproduce well inter-annual signals observed by GPS, while the fits for trends and at the annual timescale are moderate.

Conclusions

Findings

Methods