Extending the Global Mass Change Data Record: GRACE Follow‐On Instrument and Science Data Performance

Abstract

Since June, 2018, the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) is extending the 15-year monthly mass change record of the GRACE mission, which ended in June 2017. The GRACE-FO instrument and flight system performance has improved over GRACE. Better attitude solutions and enhanced pointing performance result in reduced fuel consumption and gravity range rate post-fit residuals. One accelerometer requires additional calibrations due to unexpected measurement noise. The GRACE-FO gravity and mass change fields from June 2018 through December 2019 continue the GRACE record at an equivalent precision and spatiotemporal sampling. During this period, GRACE-FO observed large interannual terrestrial water variations associated with excess rainfall (Central US, Middle East), drought (Europe, Australia), and ice melt (Greenland). These observations are consistent with independent mass change estimates, providing high confidence that no intermission biases exist from GRACE to GRACE-FO, despite the 11-month gap. GRACE-FO has also successfully demonstrated satellite-to-satellite laser ranging interferometry. Plain Language Summary Mass change is a fundamental climate system indicator and provides an integrated global view of how Earth's water cycle and energy balance are evolving. The Gravity Recovery and Climate Experiment (GRACE) mission monitored mass changes every month from 2002 through 2017. Since June 2018, GRACE Follow-On (GRACE-FO) continues this data record, tracking and monitoring changes in ice sheets and glaciers, near-surface and underground water storage, as well as changes in sea level and ocean currents. GRACE-FO instruments have been successfully calibrated and are providing new monthly mass change observations at a consistent spatial resolution and data quality with GRACE. Since its launch, GRACE-FO has measured record land water storage changes in 2018 and 2019 in response to extreme heat waves and droughts over Europe and Australia, as well as to extreme rainfall events over the United States and Middle East. In the summer of 2019, GRACE-FO measured record-level Greenland mass loss rates. A novel laser ranging interferometer was successfully demonstrated on GRACE-FO, laying the groundwork for improved future satellite gravity observations.

Quantifying the Central European Droughts in 2018 and 2019 With GRACE Follow‐On

“…GRACE applications in hydrology were manifold, such as quantifying the contributions of the continental ice sheets to sea level rise (Velicogna & Wahr, 2006), groundwater changes (Frappart & Ramillien, 2018), water storage capacity and flood potential (Reager & Famiglietti, 2009), or drought effects, for example, in California's Central Valley (Famiglietti et al., 2011). The monitoring of water mass anomalies from space initiated with GRACE is being continued by the GRACE‐FO mission (Landerer et al., 2020) launched in May 2018.…”

GRACE gravitational measurements of tsunamis after the 2004, 2010, and 2011 great earthquakes

“…Our study demonstrated a new application of GRACE microwave inter-satellite tracking data to study gravitational changes from transient (high-frequency) geophysical processes with tsunamis as an example. Equipped with an improved KBR system and laser ranging interferometer (Landerer et al 2020), the GRACE Follow-on satellites will reduce the detection threshold for measuring gravity changes and may lead to more accurate detection of tsunamis as they propagate across major ocean basins, as well as other transient geophysical mass changes. The L1B processing strategy can be optimized so as to deliver low-latency GRACE Follow-on measurements.…”

Arctic Sea Level Budget Assessment during the GRACE/Argo Time Period

“…GRACE, with two satellites flying at~500 km altitude separated by an along-track distance of~200 km in a near-polar orbit (inclination: 89 • ), was the only satellite mission designed to be directly sensitive to mass changes by means of gravity. GRACE ended scientific operations in June 2017 and was succeeded by the follow-on mission GRACE-FO one year later [30]. Spatiotemporal characteristics of the Earth's gravitational field, which varies spatially due to the rotation of the Earth, positions of topographic masses, and heterogeneous density distribution (here of particular interest: density and mass of the ocean's interior), affect the accelerations acting on the two satellites and thereby entail variations of their relative distance.…”

“…Furthermore, the monthly averaged atmosphere-ocean dealiasing product (GAD) [37,38] was restored, and mean GAD was subtracted for removal of mean atmospheric surface pressure in both solutions, respectively. While the mascon version is corrected for GIA after [39], the SH-type postprocessing involves an ensemble mean of the models by [30], ICE-6G_C (VM5a) [40] and [41]. Both solutions represent mass changes over the ocean integration kernel, expressed as monthly surface-density change, i.e., with respect to the mean of a common (temporal) base line, which corresponds to millimeters of equivalent water height at 1000 kg/m 3 density.…”