Analysis of the ITSG-Grace daily models in the determination of polar motion excitation function

The polar motion (PM) changes comprehension is one of the major tasks in geodesy. In order to learn about the changes in PM caused by internal forces, it is necessary to perform detailed analyses of the so-called PM excitation functions of geophysical fluids layers - atmosphere, ocean and terrestrial hydrosphere with cryosphere. The impact of land hydrosphere is not well understood and the study of the Hydrological Angular Momentum (HAM) is still the main research topic in finding the agreement between observed geodetic changes in PM and geophysical ones. The study of different HAM excitations is possible through the use and analysis of temporal gravity models, which are constantly being developed by numerous research centres around the world.

The main aim of this study is to check the usefulness of ITSG daily gravity field models in determination of PM excitation at sub-seasonal time scales. To do so, we compare the equatorial components (χ1 and χ2) of the HAM calculated using the ITSG (The Institute of Geodesy at Graz University of Technology) daily gravity field models (ITSG-Grace2016, ITSG-Grace2018) with hydrological signal in the PM excitation (geodetic residuals) and HAM from LSDM (Hydrological Land Surface Discharge Model). Data from ITSG are the object of our work because they have a daily temporal resolution and allow us to determine oscillations with higher frequencies than the more commonly used monthly data. The period of our work was narrowed down to the years of the GRACE mission with the highest density of data, in order to avoid the impact of data gaps that can disrupt our short-term analyses. We limit our study to the period between 2004 and 2011 due to meaningful gaps in GRACE observations before and after this period.

The equatorial components of the PM excitation were calculated from the relationship between χ1, χ2 and the Stokes coefficients of degree-2, order-1 (∆C21, ∆S21) provided in the ITSG-Grace models. In this work, the HAM and SLAM (Sea-level Angular Momentum) series from the LSDM provided by GFZ (GeoForschungsZentrum in Potsdam) and the geodetic residuals, which are the differences between the geodetic angular momentum (GAM) and the sum of atmospheric and oceanic excitation, provided by Observatoire de Paris, were used for comparison with the PM excitation series determined from the ITSG-Grace models. In order to evaluate PM excitations obtained from ITSG-Grace models with series from other models, we determined short-period amplitude spectra and calculated correlation and root mean-square errors (RMSE). Our analyses confirm the existence of a sub-monthly signal in the HAM series determined from ITSG daily data. We observed a similar signal in LSDM-based HAM but with notably weaker amplitudes.