Exploiting the SLR+GRACE/GRACE-FO Hybrid Solutions to Determine Gravimetric Excitations of Polar Motion

In this study, we investigate the excitation of polar motion (PM) caused by changes in the distribution of hydrosphere mass. This phenomenon is expressed through hydrological angular momentum (HAM) series, which can be estimated using global models of the continental hydrosphere, Earth's gravity field variations, and numerical climate models.

The Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) missions have provided crucial data for understanding the time-variable gravity field affected by changes in global mass distribution. These missions also reveal how mass variations in the continental hydrosphere and cryosphere affect PM. However, it has been identified that GRACE/GRACE-FO solutions have certain limitations when determining geopotential coefficients of lower degrees. As a remedy, it has become standard practice to replace the C₂₀ values obtained from GRACE/GRACE-FO solutions with corresponding estimates from Satellite Laser Ranging (SLR). Similar substitutions are also recommended for the C₂₁ and S₂₁ coefficients.

In our study, we employ hybrid SLR+GRACE/GRACE-FO time-variable gravity solutions, made available by the Institute of Geodesy at Graz University of Technology (ITSG) and Centre National D'Etudes Spatiales (CNES), to determine hydrological/gravimetric angular momentum (HAM). To evaluate the SLR+GRACE/GRACE-FO-based HAM series, we compare them with the hydrological signal in observed PM excitation, referred to as geodetic residuals (or GAO). Additionally, we compare these series with HAM derived from GRACE/GRACE-FO data, focusing on seasonal and non-seasonal variations.

Our findings indicate that the excitation functions derived from the hybrid solution closely align with the GAO. In fact, they are similar or even better consistent with GAO than the series derived from the GRACE/GRACE-FO data.