GravIS: User-Friendly Mass Anomaly Products from Satellite Gravimetry for Climate Change Research

# GravIS: Mass Anomaly Products from Satellite Gravimetry

Accurately quantifying global mass changes at the Earth’s surface is crucial for understanding the dynamics of the climate system and its evolution. Satellite gravimetry, particularly through the Gravity Recovery and Climate Experiment (GRACE) and its successor GRACE Follow-On (GRACE-FO), is currently the only remote sensing technique capable of tracking large-scale mass variations, providing valuable monitoring opportunities across various geoscientific disciplines.

To improve accessibility to GRACE and GRACE-FO results beyond the geodetic community, the Helmholtz Centre for Geosciences (GFZ) developed the Gravity Information Service (GravIS) portal. This user-friendly platform allows researchers to access mass anomaly products tailored for hydrology, glaciology, and oceanography applications. The mass change data is continuously updated with new monthly gravity field models, available in both gridded representations and as time series for predefined regions. The GravIS web portal also visualizes and describes these products, highlighting their significance for a variety of studies in the geosciences.

Together with GFZ’s complementary information portal, Global Water Storage, GravIS enhances the dissemination of satellite gravimetry knowledge in both scientific and societal contexts, emphasizing the importance of the GRACE and GRACE-FO missions in understanding climate system changes.

The GravIS products are categorized into several datasets according to their specific applications, accessible via links in the data description.

Introduction

Understanding the redistribution of mass globally is vital for analyzing geophysical processes connected to the climate system and its changes. Such mass changes lead to spatial and temporal variations in the Earth’s gravity field, which can be detected using satellite gravimetry. From 2002 to 2017, the GRACE mission generated high-temporal-resolution global gravity field models of exceptional quality. Its successor, GRACE-FO, launched in 2018, continues this work, providing monthly gravity field estimates with an effective spatial resolution of approximately 300 km for over 20 years. These monthly gravity field solutions enable the inference of mass changes at the Earth’s surface, including variations in the atmosphere, terrestrial hydrosphere, cryosphere, and oceans. Moreover, GRACE and GRACE-FO can detect signals from solid Earth phenomena, such as megathrust earthquakes and their related co- and post-seismic deformations, along with long-term mass redistributions in the upper mantle due to glacial isostatic adjustment (GIA). Consequently, both missions significantly contribute to a deeper understanding of geophysical processes within the Earth’s system and its long-term climate change impacts.

Publications related to GRACE and GRACE-FO have continuously increased over the two-decade mission lifespan, with a notable rise in hydrological studies in recent years.

Available GravIS Mass Anomaly Products and Their Processing Methods

GravIS provides a range of products linked to the GRACE and GRACE-FO missions, distinguishing between datasets in the spectral domain (Level-2B) and spatial domain (Level-3). A unique feature of GravIS is its dedicated Level-3 products for three distinct geographical domains: continental ice sheets, non-glaciated land surfaces, and oceans. All Level-3 products are offered as gridded datasets and average values over predefined regions.

Spectral Representation: Post-Processed Gravity Field Models (Level-2B Products)

Level-2B processing starts with monthly GRACE and GRACE-FO Level-2 products, which represent the Earth’s gravity field in the spectral domain through spherical harmonic (SH) coefficients. Two independent processing chains generate GravIS products: one based on the latest GRACE and GRACE-FO Level-2 products processed at GFZ, and the other using the latest release from the Combination Service for Time-variable Gravity fields (COST-G).

To derive SH coefficients containing only signals caused by mass redistribution, various corrections and reductions are applied to the Level-2 solutions. The processing sequence includes subtracting a long-term mean gravity field from the monthly models, filtering SH coefficients using an anisotropic filter, replacing specific low-degree SH coefficients, and accounting for geocenter variations and tidal aliasing errors.

Anisotropic Filtering

Due to the observation geometry, GRACE and GRACE-FO gravity field solutions exhibit highly anisotropic error characteristics. Filtering is essential for decorrelating systematic errors and optimally separating signal from noise. The VDK filter, adapted from the decorrelation method by Kusche et al. (2009), is used at GravIS to smooth the Level-2B products effectively.

Replacement of Specific Low-Degree SH Coefficients

Monthly estimates of the SH coefficient of degree 2 and order 0 (C20) are affected by spurious systematic effects. Thus, native C20 coefficients are replaced with more reliable estimates based on satellite laser ranging observations. Additionally, the C30 coefficient, critical for estimating Antarctic ice-mass changes, is also addressed during processing.

Spatial Representation: Mass Anomaly Grids and Regional Averages (Level-3 Products)

GRACE and GRACE-FO gravity fields provide unique insights into mass transport processes across various Earth system components. GravIS groups mass anomalies into three regimes: ice-mass changes in Antarctica and Greenland, terrestrial water storage variations over continents, and barystatic sea-level change and bottom pressure variations in the oceans.

Ice-Mass Changes in Antarctica and Greenland

GravIS offers gridded ice-mass change products for both the Antarctic Ice Sheet (AIS) and the Greenland Ice Sheet (GrIS). These products utilize unfiltered GravIS Level-2B coefficients, providing detailed spatial patterns of mass change rates and integrated time series for major drainage basins.

Terrestrial Water Storage Variations

Temporal changes in the Earth’s gravity field are primarily interpreted as variations in terrestrially stored water masses. GravIS provides gridded and regionally averaged products obtained from Level-2B coefficients, offering insights into TWS anomalies across global river basins.

Barystatic Sea-Level Change and Ocean Bottom Pressure Variations

GravIS also provides gridded and regionally averaged products for ocean bottom pressure (OBP) variations, which are crucial for understanding mass changes in ocean basins.

The GravIS Web Portal

The GravIS web portal was developed to meet two main needs: providing a basic description of user-friendly products and enabling interactive exploration of various datasets. The portal includes dedicated subpages for different geographical domains, offering visualizations and time series plots of regional averages.

Application of GravIS Products

Satellite gravimetry has evolved into a reliable observing system for monitoring mass transport, critical for various applications, such as drought monitoring and understanding groundwater dynamics. GravIS supports these applications by providing regularly updated mass anomaly products.

Limitations of Mass Anomaly Products Based on Satellite Gravimetry

While satellite gravimetry offers unique insights into mass changes, it also has limitations. Mass change estimates are more accurate when averaged over larger regions, as averaging reduces uncertainties and mitigates spatial leakage, especially in coastal areas.

Code and Data Availability

GravIS data sets, including Level-2B and Level-3 products, are dynamically updated as new data from GRACE-FO becomes available. A Python toolbox for computing regional TWS time series and their uncertainties is also publicly accessible.

Conclusions and Outlook

GravIS provides user-friendly mass anomaly products derived from GRACE and GRACE-FO satellite data, which are crucial for studies related to mass redistribution processes in the Earth’s system. Regular updates and easy accessibility are expected to stimulate further applications of satellite gravimetry across various scientific fields.

In the future, GravIS aims to introduce new products derived from satellite gravimetry, including prototypes for groundwater storage variations and near-real-time products for rapid assessments of floods and droughts.

Through GravIS and the Global Water Storage portal, GFZ intends to enhance the recognition and utilization of GRACE and GRACE-FO data in the geosciences, ensuring continuity in providing mass anomaly products even beyond the mission lifetime of GRACE-FO.