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

Abstract

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 ΔC21, ΔS21, and ΔC20. 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 ΔC21, ΔS21, and ΔC20 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 ΔC21 and ΔS21 provided by the Center for Space Research (CSR) are greatly improved over earlier solutions and agree remarkably well with other estimates, especially on ΔS21 estimates. GRACE and Earth rotation ΔS21 agreement is exceptionally good across a very broad frequency band from intraseasonal, seasonal, to interannual and decadal periods. SLR ΔC20 estimates remain superior to GRACE and Earth rotation estimates, due to the large uncertainty in GRACE ΔC20 solutions and particularly high sensitivity of Earth rotation ΔC20 estimates to errors in the wind fields. With several estimates of ΔC21, ΔS21, and ΔC20 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 ΔC21 and ΔS21 variations with the lowest noise levels over a broad band of frequencies.

Assessment of degree-2 order-1 gravitational changes from GRACE and GRACE Follow-on, Earth rotation, satellite laser ranging, and models

“…Previous studies indicate that EOP-estimated ΔC 21 and ΔS 21 agree well with other geodetic observations (Bloßfeld et al 2015;Chen et al 2016;Göttl et al 2018). The major challenge of estimating ΔC 21 and ΔS 21 variations from PM observations is the need for independent quantifications of motion-term excitations using atmospheric and oceanic models.…”

Optimizing estimates of annual variations and trends in geocenter motion and J2 from a combination of GRACE data and geophysical models

“…Those variations are directly related to variations of the C 20 Stokes coefficient ( ). The GRACE‐based C 20 coefficient is subject to large uncertainties [ Chen et al , ], presumably due to tide‐like aliases, and it is a common practice to replace it with estimates from other techniques, such as SLR. The GRACE‐OBP method solves the problem by replacing the GRACE‐based C 20 estimate with an alternative one that does not require an explicit observation of that coefficient.…”

Statistically optimal estimation of degree-1 and C20 coefficients based on GRACE data and an ocean bottom pressure model

“…Omission of the degree-1 contribution leads to significant errors in surface mass estimates (Wu et al 2012). Another problem of GRACE monthly solutions is that the C 20 coefficient is subject to large uncertainties (Chen et al 2016), presumably due to thermal-related systematic errors in the accelerometer data (Cheng & Ries 2017). Therefore, for the purpose of inferring surface mass anomalies, a GRACE user is advised to complement GRACE solutions with independently estimated degree-1 coefficients and replace the native GRACE C 20 coefficients with more accurate ones.…”