Time-resolved satellite gravimetry has revolutionized understanding of mass transport in the Earth system. Since 2002, the Gravity Recovery and Climate Experiment (GRACE) has enabled monitoring of the terrestrial water cycle, ice sheet and glacier mass balance, sea level change and ocean bottom pressure variations and understanding responses to changes in the global climate system. Initially a pioneering experiment of geodesy, the time-variable observations have matured into reliable mass transport products, allowing assessment and forecast of a number of important climate trends and improve service applications such as the U.S. Drought Monitor. With the successful launch of the GRACE Follow-On mission, a multi decadal record of mass variability in the Earth system is within reach.

Contributions of GRACE to understanding climate change.

Abstract Time-variable gravity measurements from the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) missions have opened up a new avenue of opportunities for studying large-scale mass redistribution and transport in the Earth system. Over the past 19 years, GRACE/GRACE-FO time-variable gravity measurements have been widely used to study mass variations in different components of the Earth system, including the hydrosphere, ocean, cryosphere, and solid Earth, and significantly improved our understanding of long-term variability of the climate system. We carry out a comprehensive review of GRACE/GRACE-FO satellite gravimetry, time-variable gravity fields, data processing methods, and major applications in several different fields, including terrestrial water storage change, global ocean mass variation, ice sheets and glaciers mass balance, and deformation of the solid Earth. We discuss in detail several major challenges we need to face when using GRACE/GRACE-FO time-variable gravity measurements to study mass changes, and how we should address them. We also discuss the potential of satellite gravimetry in detecting gravitational changes that are believed to originate from the deep Earth. The extended record of GRACE/GRACE-FO gravity series, with expected continuous improvements in the coming years, will lead to a broader range of applications and improve our understanding of both climate change and the Earth system. 

/pdf/applications-and-challenges-of-grace-and-grace-follow-on-1cbg6d62.pdf

Applications and Challenges of GRACE and GRACE Follow-On Satellite Gravimetry

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2021JB022124

Error Assessment of GRACE and GRACE Follow-On Mass Change

Wind-generated waves strongly interact with sea ice and impact air-sea exchanges, operations at sea, and marine life. Unfortunately, the dissipation of wave energy is not well quantified and its possible effect on upper ocean mixing and ice drift are still mysterious. As the Arctic is opening up and wave energy increases, the limited amount of in situ observations is a clear limitation to our scientific understanding. Both radar and optical remote sensing has revealed the frequent presence of waves in ice, and could be used more systematically to investigate wave-ice interactions. Here we show that, in cloud-free conditions, Sentinel-2 images exhibit brightness modulations in ice-covered water, consistent with the presence of waves measured a few hours later by the ICESat-2 laser altimeter. We show that a full-focus SAR processing of Sentinel-3 radar altimeter data also reveals the presence and wavelengths of waves in sea ice, within minutes of Sentinel-2 imagery. The SWIM instrument on CFOSAT is another source of quantitative evidence for the direction and wavelengths of waves in ice, when ice conditions are spatially homogeneous. In the presence of sea ice, a quantitative wave height measurement method is not yet available for all-weather near-nadir radar instruments such as altimeters and SWIM. However, their systematic co-location with optical instruments on Sentinel-2 and ICESat-2, which are less frequently able to observe waves in sea ice, may provide the empirical transfer functions needed to interpret and calibrate the radar data, greatly expanding the available data on wave-ice interactions. 

/pdf/wind-wave-attenuation-in-arctic-sea-ice-a-discussion-of-1k2we02i.pdf

Wind‐wave attenuation in Arctic sea ice: A DISCUSSION OF REMOTE SENSING CAPABILITIES

The 2004 Sumatra, 2010 Maule, and 2011 Tohoku great earthquakes triggered tsunamis as large as a few decimeters over a few 100 km in the open ocean. The transient ocean mass redistribution propagating as tsunamis changed the Earth’s gravity field enough to perturb the GRACE satellites’ orbits at ~ 500 km above the surface. The on-board microwave ranging system detected inter-satellite acceleration anomalies of up to 1.0–4.0 nm/s2. There is good agreement between GRACE measurements and tsunami models for the three events. Complementarily to buoys, ocean bottom pressure sounders, and satellite altimeters, GRACE is sensitive to the long-wavelength spatial scale of tsunamis and provides an independent source of information for assessing alternate early earthquake and tsunami models. Our study demonstrates an innovative way of applying GRACE and GRACE Follow-On data to detect transient geophysical mass changes which cannot be observed by the conventional monthly Level-2 and mascon solutions.

/pdf/grace-gravitational-measurements-of-tsunamis-after-the-2004-4mnqtba5xb.pdf

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

Variation of the equatorial moments of inertia associated with a 6-year westward rotary motion in the Earth

This work is supported by Taiwan Ministry of Science and Technology grants 105-2811-M-001-031 and 106-2116-M-001-013 and by National Nature Science Foundation of China grants 41474019, 41504014, and 41704012.

/pdf/variations-of-the-argentine-gyre-observed-in-the-grace-time-4kq9esqxol.pdf

Variations of the Argentine Gyre Observed in the GRACE Time-Variable Gravity and Ocean Altimetry Measurements

We first report the results from analyzing the GRACE satellite data for the coseismic and long-term post-seismic changes in the Earth’s gravity field and geoid height induced by the 2004 Sumatra-Andaman earthquake (Mw 9.2) and the 2012 Indian-Ocean earthquake (Mw 8.6). The results suggest that the two earthquakes have changed the gravity field and geoid height irreversibly (as opposed to cyclically). We next use satellite ocean altimetry measurements to search for the geoid height change induced by the Sumatra-Andaman earthquake, employing an effective method of extracting coherent space-temporal signals, namely the empirical orthogonal functions (EOF) analysis. Our results demonstrate that the sea level variation in the studied (tropical) area during the studied (10-year) period is dominated by the strong steric changes related to ENSO, to the point that the earthquake-induced signals, even using EOF for the largest earthquakes, are obscured and thus undetectable by ocean altimetry. However, this also means that steric and non-steric changes can be observed separately from artificial satellites. Article history: Received 24 May 2018 Revised 2 October 2018 Accepted 24 October 2018

/pdf/gravity-and-geoid-changes-by-the-2004-and-2012-sumatra-3qwqgolxaz.pdf

Gravity and geoid changes by the 2004 and 2012 Sumatra earthquakes from satellite gravimetry and ocean altimetry

Author(s): Yu, Yao; Sandwell, David T; Gille, Sarah T; Boas, Ana Beatriz Villas | Abstract: SUMMARY The Ice, Cloud and land Elevation Satellite 2 (ICESat-2) laser altimetry mission, launched in September 2018, uses six parallel lidar tracks with very fine along-track resolution (15nm) to measure the topography of ice, land and ocean surfaces. Here we assess the ability of ICESat-2 ocean data to recover oceanographic signals ranging from surface gravity waves to the marine geoid. We focus on a region in the tropical Pacific and study photon height data in both the wavenumber and space domain. Results show that an ICESat-2 single track can recover the marine geoid at wavelengths g20nkm which is similar to the best radar altimeter data. The wavelength and propagation direction of surface gravity waves are sometimes well resolved by using a combination of the strong and weak beams, which are separated by 90nm. We find higher than expected power in the 3–20nkm wavelength band where geoid and ocean signals should be small. This artificial power is caused by the projection of 2-D surface waves with ∼300nm wavelengths into longer wavelengths (5–10nkm) because of the 1-D sampling along the narrow ICESat-2 profile. Thus ICESat-2 will not provide major improvements to the geoid recovery in most of the ocean.

/pdf/assessment-of-icesat-2-for-the-recovery-of-ocean-topography-12o6du12if.pdf

Assessment of ICESat-2 for the recovery of ocean topography

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Variation of the equatorial moments of inertia associated with a 6-year westward rotary motion in the Earth

Variations of the Argentine Gyre Observed in the GRACE Time-Variable Gravity and Ocean Altimetry Measurements

Gravity and geoid changes by the 2004 and 2012 Sumatra earthquakes from satellite gravimetry and ocean altimetry

Assessment of ICESat-2 for the recovery of ocean topography