Potentials and Deficiencies of GRACE-FO Line-of-Sight Gravity Observations

This study aims to evaluate the contribution of GRACE Follow-On (GRACE-FO) line-of-sight gravity difference (LGD) observations to the detection of Earth system mass variations. The current GRACE-FO gravity mission implements the low-low satellite-to-satellite tracking (ll-SST) technique. The conventionally derived mass variation products are not able to properly depict sub-monthly extreme weather events (EWEs) and natural hazards, given that data from the affected regions outside the occurrence periods of these short-term phenomena are also considered. However, the LGD connects the gravimetric observations to the intersatellite geometric measurements, allowing for an instantaneous observation of mass changes through intersatellite ranging.

An along-orbit analysis methodology is presented that employs instantaneous LGDs to detect sub-monthly mass variations caused by EWEs. Real intersatellite measurements of the laser ranging interferometer (LRI) on-board the two GRACE-FO satellites provided by Level-1B products are utilized as observations. The residual intersatellite range-acceleration is used as a first-order approximation of the instantaneous LRI-observed LGD, with an optimized band-pass filter (BPF) applied for signal improvement. This instantaneous LGD signal is derived by subtracting reference values computed from reduced-dynamic orbit data of the two satellites, and then corrected for non-tidal effects from atmosphere and ocean. Synthesized LGDs derived from gravity field products, such as GRACE-FO Level-2 time-variable gravity solutions, are compared with the instantaneous LGDs.

A geospatial-domain analysis is conducted using 3°×3° global monthly bin-maps that are generated with a bin-wise weighted average scheme. The two LGD components are therefore compared based on the 2D root mean squares (RMS), not only for selecting the optimal lower cut-off frequency of the BPF but also for validating the algorithm for estimating the instantaneous LGD signal. In addition, a case study of the extreme rainfall in eastern Australia in March 2021 highlights the effectiveness and advantage of the along-orbit method in capturing both monthly and sub-monthly mass variations. Key findings include the detection of terrestrial water storage changes due to the rainfall and the temporal progression of this event, emphasizing the potential of intersatellite LRI observations for near real-time monitoring of Earth system mass variations. It is also demonstrated that, as a tiny component, the residual instantaneous LGD is extremely sensitive to reference values under this processing scheme, which poses substantial challenges to the use of satellite orbits other than pure dynamic orbits for reference value computation in both LGD signal analysis and interpretation.