Analysis of the polar motion excitation function based on ITSG daily models

Understanding the polar motion (PM) changes is one of the major tasks in geodesy. In order to understand this changes cause by internal forces, it is necessary to analyse the so called PM excitation functions of geophysical fluids layers, namely atmosphere, ocean, and land hydrology with cryosphere. The impact of atmosphere and oceans is pretty well understood, but the impact of land hydrology is not well known 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 are possible, among other things, 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 compare the equatorial components of the PM excitation (χ1 and χ2) calculated using the ITSG (The Institute of Geodesy at Graz University of Technology) daily gravity field models (ITSG-Grace2014, ITSG-Grace2016, ITSG-Grace2018) with other daily models and reference data, in order to assess their accuracy. The comparison of successive versions of ITSG solutions (2014, 2016, 2018) will allow for the assessment of the improvement in the accuracy of the excitation functions determined on their basis. Their potential for use in future research will be evaluated. These models are an object of study due to the fact that they have a daily temporal resolution, unlike most models that offer a monthly resolution.

The ITSG-Grace models used in the study were created by researchers at Technische Universität Graz using data obtained from the GRACE (Gravity Recovery and Climate Experiment) and GRACE-FO (Gravity Recovery And Climate Experiment-Follow-On) missions. The equatorial components of the PM excitation were calculated from the relationship between χ1, χ2 and degree-2, order-1 Stokes coefficients (∆C21, ∆S21) available in the ITSG-Grace models. In this study, HAM series from the LSDM (Hydrological Land Surface Discharge Model) delivered by GFZ (GeoForschungsZentrum in Potsdam) and GAO geodetic residuals, being a differences between geodetic angular momentum (GAM) and a sum of atmospheric and oceanic excitaions, provided by Observatoire de Paris were used for comparison with PM excitation series determined from ITSG-Grace models.

In order to compare the PM excitation determined from ITSG-Grace models with series obtained using other models, trends as well as seasonal and non-seasonal variations were determined. Correlations between the series, amplitude agreement and root mean square errors were calculated, on the basis of which their accuracy and relations with other models were assessed.