Abstract
In this paper, we robustly analyze the noise reduction methods for processing spherical harmonic (SH) coefficient data products collected by the Gravity Recovery and Climate Experiment (GRACE) satellite mission and devise a comprehensive GRACE Matlab Toolbox (GRAMAT) to estimate spatio-temporal mass variations over land and oceans. Functions in GRAMAT contain: (1) destriping of SH coefficients to remove “north-to-south” stripes, or geographically correlated high-frequency errors, and Gaussian smoothing, (2) spherical harmonic analysis and synthesis, (3) assessment and reduction of the leakage effect in GRACE-derived mass variations, and (4) harmonic analysis of regional time series of the mass variations and assessment of the uncertainty of the GRACE estimates. As a case study, we analyze the terrestrial water storage (TWS) variations in the Amazon River basin using the functions in GRAMAT. In addition to obvious seasonal TWS variations in the Amazon River basin, significant interannual TWS variations are detected by GRACE using the GRAMAT, which are consistent with precipitation anomalies in the region. We conclude that using GRAMAT and processing the GRACE level-2 data products, the global spatio-temporal mass variations can be efficiently and robustly estimated, which indicates the potential wide range of GRAMAT’s applications in hydrology, oceanography, cryosphere, solid Earth and geophysical disciplines to interpret large-scale mass redistribution and transport in the Earth system. We postulate that GRAMAT will also be an effective tool for the analysis of data from the upcoming GRACE-Follow-On mission.
Highlights
Launched in March 2002, the Gravity Recovery and Climate Experiment (GRACE) satellite mission has provided direct observations of the global gravity field and its temporal variations with an unprecedented accuracy (Tapley et al 2004)
We present a GRACE Matlab Toolbox (GRAMAT) to process GRACE level-2 data and estimate spatio-temporal mass variations
This open-source package is likely to be useful for the Earth science community, especially for hydrologists, who are prone to ignore the non-negligible signal distortion and errors during the GRACE level-2 data processing and use GRACE level-3 gridded products directly
Summary
Launched in March 2002, the Gravity Recovery and Climate Experiment (GRACE) satellite mission has provided direct observations of the global gravity field and its temporal variations with an unprecedented accuracy (Tapley et al 2004). To facilitate the application of GRACE data, post-processed gridded level-3 products are available on the official GRACE Tellus website (https://grace.jpl.nasa.gov/). Thegraceplotter.com/) from the Centre National d’Etudes Spatiales (CNES, France), the visualization tool from GFZ (http://icgem.gfz-potsdam.de/ICGEM/ICGEM.html), and the GRACE data analysis website from the University of Colorado Boulder, USA (http://geoid.colorado.edu/grace/ dataportal.html). For these gridded level-3 products and on-line tools, only the results with specific destriping methods and Gaussian filters are available. Users cannot assess the differences between various methods and select an optimal one for their case study These gridded products cannot provide an unbiased time series of mass variations in a user-specified region, because the signal leakage effect in the region is variant on a case-by-case basis. Users can implement the leakage reduction process, assess the uncertainty of GRACE mass estimates, and tentatively modify source codes in GRAMAT to develop their own post-processing methods
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