Broadband assessment of degree-2 gravitational changes from GRACE and other estimates, 2002-2015

Space geodetic measurements, including the Gravity Recovery and Climate Experiment (GRACE), satellite laser ranging (SLR), and Earth rotation provide independent and increasingly accurate estimates of variations in Earth's gravity field Stokes coefficients C-21, S-21, and C-20. Mass redistribution predicted by climate models provides another independent estimate of air and water contributions to these degree-2 changes. SLR has been a successful technique in measuring these low-degree gravitational changes. Broadband comparisons of independent estimates of C-21, S-21, and C-20 from GRACE, SLR, Earth rotation, and climate models during the GRACE era from April 2002 to April 2015 show that the current GRACE release 5 solutions of C-21 and S-21 provided by the Center for Space Research (CSR) are greatly improved over earlier solutions and agree remarkably well with other estimates, especially on S-21 estimates. GRACE and Earth rotation S-21 agreement is exceptionally good across a very broad frequency band from intraseasonal, seasonal, to interannual and decadal periods. SLR C-20 estimates remain superior to GRACE and Earth rotation estimates, due to the large uncertainty in GRACE C-20 solutions and particularly high sensitivity of Earth rotation C-20 estimates to errors in the wind fields. With several estimates of C-21, S-21, and C-20 variations, it is possible to estimate broadband noise variance and noise power spectra in each, given reasonable assumptions about noise independence. The GRACE CSR release 5 solutions clearly outperform other estimates of C-21 and S-21 variations with the lowest noise levels over a broad band of frequencies.