Degree-1 and C20 coefficients

Introduction

Geocenter Motion

We define the center of mass of the whole Earth system (CM) as geocenter. Its motion when viewed from the center of figure of the solid Earth surface (CF) is known as geocenter motion. The three Cartesian components of geocenter motion are one-to-one related to the time variations in the degree-1 geopotential coefficients. Geocenter motion involves tidal, non-tidal and secular components. Here, we aim to produce geocenter motion time-series that are free of tidal signals as well as non-tidal signal in atmosphere and ocean (compatible with GRACE monthly solutions).

Note that the Glacial Isostatic Adjustment (GIA) effect is modeled and removed beforehand from the GRACE input data. Consequently, the resulting geocenter motion time-series does not contain the GIA-related secular signal.

Changes in the Earth's Dynamic Oblateness

Changes in the Earth’s dynamic oblateness are also known as ΔJ2 ( ΔJ2= sqrt(5) * C20). Just like geocenter motion, they reflect large-scale mass redistribution within the Earth system.

Data Access

We provide degree-1 and C20 time-series produced with two methods: (i) the GRACE-OBP approach and (ii) the Combination approach.

(i) The GRACE-OBP  Approach

The GRACE-OBP approach was  first proposed by Swenson et al. (2008) to estimate geocenter motion from a combination of GRACE data and the output of an ocean bottom pressure model. We extended that approach to co-estimate the C20 coefficients simultaneously (Sun et al., 2016a). Also, the implementation parameters were tailored via an end-to-end simulation study to optimize the estimates of annual variations (sun et al., 2016b).

Degree-1 and C20 can be found here.
Degree-1 and
C20 where the GIA signal is restored can be found here.
Note that both provided time-series are free of non-tidal atmospheric and oceanic effects.
Data format description:
data files contain headers (indicated with a leading ‘#’); the data themselves are in five columns, which represent time, C10, C11, S11 and C20, respectively.

(ii) The Combination Approach

The combination approach is an original development of our research team (Sun et al., 2017). It also uses the GRACE data and an ocean bottom pressure model as input datasets. However, it further takes into account the error information of the two datasets. As a result, the obtained estimates are statistically-optimal. Furthermore, realistic formal uncertainties and correlations are also provided together with the degree-1 and C20 coefficients. The combination approach reduces to the GRACE-OBP method if one assumes that noise in GRACE data absent and noise in the OBP model is white.

Degree-1 and C20 as well as their error time-series can be found here.
The product where the GIA signal is restored can be found here
.
Note that both provided time-series are free of non-tidal atmospheric and oceanic effects.
Data format description:
data files contain headers (indicated with a leading ‘#’); The data following the headers are given in 15 columns, which are time, C10, C11, S11, C20, C10 C10 Var, C10 C11 CoVar, C10 S11 CoVar, C10 C20 CoVar, C11 C11 Var, C11 S11 CoVar, C11 C20 CoVar, S11 S11 Var, S11 C20 CoVar, C20 C20 Var, respectively.

Research Team

Yu Sun, Riccardo Riva and Pavel Ditmar

References

Yu Sun, Pavel Ditmar, and Riccardo Riva (2016a), Observed changes in the Earth's dynamic oblateness from GRACE data and geophysical models, J. Geod., 90(1), 81–89, doi:10.1007/s00190-015-0852-y.

Yu Sun., Riccardo Riva, and Pavel Ditmar (2016b), Optimizing estimates of annual variations and trends in geocenter motion and J2 from a combination of GRACE data and geophysical models, J. Geophys. Res. Solid Earth, 121(11), 8352–8370, doi:10.1002/2016JB013073.

Yu Sun, Pavel Ditmar, Riccardo Riva (2017), Statistically optimal estimation of degree-1 and C20 coefficients based on GRACE data and an ocean bottom pressure model, Geophysical Journal International, 210(3), 1305–1322, 2017. doi:10.1093/gji/ggx241.