Along‐Orbit Analysis of GRACE Follow‐On Inter‐Satellite Laser Ranging Measurements for Sub‐Monthly Surface Mass Variations

Abstract

We examined the sensitivity of GRACE Follow‐On (GRACE‐FO) laser ranging interferometer (LRI) measurements to sub‐monthly time‐variable gravity (TVG) signals caused by transient, high‐frequency mass changes in the Earth system. GRACE‐FO LRI provides complementary inter‐satellite ranging measurements with higher precision over a wider range of frequencies than the baseline K‐band microwave ranging system. The common approach for studying mass variation relies on the inverted TVG or mascon solutions over a period of, for example, one month or 10 days which are adversely affected by temporal aliasing and/or smoothing. In this article, we present the alternative along‐orbit analysis methodology in terms of line‐of‐sight gravity difference (LGD) to fully exploit the higher precision LRI measurements for examination of sub‐monthly mass changes. The discrepancy between “instantaneous” LGD LRI observations and monthly‐mean LGD (from Level‐2 data) at satellite altitude indicates the sub‐monthly gravitational variability not captured by monthly‐mean solutions. In conjunction with the satellite ocean altimetry observations, high‐frequency non‐tidal atmosphere and ocean models, and hydrology models, we show that the LGD LRI observations detect the high‐frequency oceanic mass variability in the Argentine Basin and the Gulf of Carpentaria, and sub‐monthly variations in surface (river) water in the Amazon Basin. We demonstrate the benefits gained from repeat ground track analysis of GRACE‐FO LRI data in the case of the Amazon surface water flow. The along‐orbit analysis methodology based on LGD LRI time series presented here is especially suitable for quantifying temporal and spatial evolution of extreme, rapidly changing mass variations.

Atmospheric Contributions to Global Ocean Tides for Satellite Gravimetry

“…The evaluation of pre‐fit residuals reveal that implementing the new models for S 1 has a positive impact with respect to the background models previously applied in latest available release 06 of the GRACE and GRACE‐FO gravity fields published by GFZ. Results for the smaller tides are less conclusive, but further progress is expected as soon as the benefits of the new LRI instrument operated on GRACE‐FO as a technology demonstrator (Ghobadi‐Far et al., 2022) are fully utilized also in the gravity field estimation. It is further planned to remove signals from all 16 waves discussed in this paper from the next release (RL07) of Atmosphere and Ocean De‐Aliasing Level‐1B product (Shihora et al., 2022), so that those coefficients only represent non‐tidal variability and can be used in combination with the tidal models presented here.…”

“…It is interesting to note that for waves with beat frequencies around 3–6 cpd, S 3 has still the most energy but it is closely followed by its side lines T 3 and R 3 (T 3 , S 3 , and R 3 differ by only 1 cpy). All other remaining tides are substantially smaller, but nevertheless reach signal magnitudes that are considered relevant in view of the high sensitivity of the new Laser Ranging Interferometer (LRI) on GRACE‐FO (Ghobadi‐Far et al., 2022). Signals at all other possible frequencies including N 2 and L 2 are even well below the smallest constituent S 6 that is considered here, and we deem them as non‐significant.…”

Assimilation of GRACE Follow‐On Inter‐Satellite Laser Ranging Measurements Into Land Surface Models

“…On the other hand, in the LGD assimilation, the alternative transition matrix ( H ′) is computed by matrix multiplication of H with the Green's function matrix (see, e.g., Y. Chen et al., 2008; Ghobadi‐Far et al., 2022) as follows, $$${\mathbf{H}}^{\prime }=\mathbf{A}\mathbf{H},$$$ where A is the Green's function matrix. In the newly proposed assimilation case, model state variables are multiplied by H ′ instead of H .…”

“…d 1 i and d i 2 are the distance between the grid cell i and the GRACE‐FO satellite 1 and 2, respectively. This equation is further modified to account for the elastic loading effect (Ghobadi‐Far et al., 2022).…”

Non‐Tidal Background Modeling for Satellite Gravimetry Based on Operational ECWMF and ERA5 Reanalysis Data: AOD1B RL07

“…The simulated OBP is very similar in the tropical and subtropical open ocean with a global average of the standard deviation difference of just under 1 hPa. Notable discrepancies are found in coastal regions and enclosed seas such as the Gulf of Carpentaria, where the effects of SAL have a significant impact (Ghobadi‐Far et al., 2022). The most significant differences, however, are found in the southern ocean in the region of the ACC.…”