Assessing the role of corrections included in the GRACE/GRACE-FO data in determination of hydrological and cryospheric signal in polar motion excitation

<p>Assessing the impact of continental hydrosphere and cryosphere on polar motion (PM) variations is one of the crucial tasks in contemporary geodesy. The pole coordinates, as one of the Earth Orientation Parameters, are needed to define the relationship between the celestial and terrestrial reference frames. Therefore, the variations in PM should be monitored and interpreted in order to assess the role of geophysical processes in this phenomenon.</p><p>The role of hydrological and cryospheric signals in PM is usually examined by computing hydrological excitation (hydrological angular momentum, HAM) and cryospheric excitation (cryospheric angular momentum, CAM) of  PM, commonly treated together as HAM/CAM.</p><p>The Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) missions deliver temporal variations of the gravity field resulting from changes in global mass redistribution. The so-called GRACE/GRACE-FO Level-3 (L3) data delivers changes in terrestrial water storage (TWS) that can be used for computation of HAM/CAM.</p><p>For best possible representation of TWS, a number of corrections are introduced in the L3 data by computing centres. Such corrections are, among others, glacial isostatic adjustment (GIA) correction, geocenter correction and C<sub>20</sub> coefficient correction.</p><p>The main goal of this study is to examine the impact of corrections included in GRACE/GRACE-FO data on HAM/CAM determined. More specifically, we test their influence on HAM/CAM trends, seasonal changes and non-seasonal variations. We also examine the change in compliance between HAM/CAM and hydrological plus cryospheric signal in geodetically observed excitation when the corrections are used. To achieve our goals, we use GRACE and GRACE-FO L3 datasets provided by Jet Propulsion Laboratory (JPL), Center for Space Research (CSR), and Goddard Space Flight Center (GSFC).</p>

Comparison between polar motion excitation functions estimated from recent geophysical models and observations

<p>Continental hydrological loading by land water, snow, and ice is a process that influences the Earth’s inertia tensor and is very important for full understanding of the excitation of polar motion. In this study, the hydrological contribution to decadal, inter-annual and multi-annual suppress polar motion derived from different GRACE (Gravity Recovery and Climate Experiment) solutions as well as from SLR (Satellite Laser Ranging) and some climate models from CMIP6 project data is discussed here.</p><p>The main aim of this study is to show the influence of different representations of hydrological angular momentum (HAM) coming from different GRACE (mas concentration solutions - mascons, Terrestrial Water Storage changes, and Stokes Coefficients), SLR, and climate models solutions on agreement between Geodetic Angular Momentum (GAM) and geophysical excitations of polar motion been a sum of Atmospheric, Oceanic and Hydrological Angular Momentum (AAM+OAM+HAM) in different spectral bands.</p><p>To do that, the geodetic and geophysical excitation functions are transformed into time-scale domain using the discrete wavelet transform based on the Complex Morlet wavelet functions. Next, the time series (GAM vs. geophysical ones) are compared in terms of semblance filtering, on the basis of their phase, as a function of frequency, and amplitude information of their cross-wavelet power.</p><p>Here, we would like to present the consistency between full polar motion excitations and geophysical fluids,  that are the sum of AAM  (pressure + wind), OAM  (bottom pressure +  currents), and HAM contributions. This analysis could let us indicate, which hydrological representation of different HAM solutions cause the biggest errors in the geodetic budget.</p>

Hydrological excitation of polar motion determined from CMIP6 climate models

<p>Climate models provide important information to understand how the climate has changed in the past and how it can evolve in the future. Such models simulate in detail the physics, chemistry and biology of the atmosphere, oceans and land hydrosphere. Climate models are developed and constantly updated by a number of modelling groups around the world. A large number of models makes it necessary to store them in one place, so that they can be easily accessed and compared. The objective of the Coupled Model Intercomparison Project phase 6 (CMIP6) is to make the multi-model output publicly available in a standardized format. This framework aims to improve our understanding of climate changes resulting from both natural factors and changes in radiative forcing. The CMIP6 models are useful in many scientific applications regarding evolution of processes occurring in the atmosphere, ocean and continental hydrosphere.</p><p>In this study, we use the chosen climate models to assess the role of land hydrosphere changes in polar motion. The mass variations of land water storage impacts the Earth’s inertia tensor and causes disturbances of the pole motion. Such temporal variations of polar motion due to continental hydrosphere are described with hydrological angular momentum (HAM). Here, we use soil moisture and snow water equivalent variables, which are delivered by CMIP6 simulations, to compute time series of HAM. We then analyse HAM variability in a wide variety of oscillations, taking into account trends, seasonal, short-term non-seasonal and long-term non-seasonal changes. We consider past changes in HAM but also analyse its future evolution. This will allow to determine how future changes in the terrestrial hydrosphere will affect the movement of the pole. The consistency between HAM obtained from various CMIP6 models is assessed as well.</p>

Comparing and evaluating GRACE and GRACE-FO mascon data in the study of polar motion excitation

<p>The Gravity Recovery and Climate Experiment (GRACE) mission has provided global observations of temporal variations in mass redistribution at the surface and within the Earth for the period 2002–2017. Such measurements are commonly exploited to interpret polar motion changes due to variations in the Earth’s surficial fluids, especially in the continental hydrosphere. Such impacts are usually examined by computing the so-called hydrological polar motion excitation (Hydrological Angular Momentum, HAM). The great success of the GRACE mission and the scientific robustness of its data contributed to the launch of its successor, GRACE Follow-On (GRACE-FO), which begun in May 2018 and continues to the present.</p> <p>This study compares the estimates of HAM computed from GRACE and GRACE-FO mascon data provided by three data centers: Jet Propulsion Laboratory (JPL), Center for Space Research (CSR), and Goddard Space Flight Center (GSFC). The analysis of HAM is performed for different spectral bands. A validation of different HAM estimates is conducted here using precise geodetic measurements of the pole coordinates and geophysical models (so-called geodetic residuals or GAO).</p> <p>Comparison of HAM computed from different mascon data sources indicates high consistency between the solutions provided by JPL and CSR, and low consistency between the GSFC solution and other data. The reason for this may be that the strategy used for GSFC mascons computation is different than methodology exploited by CSR and JPL teams. This study also indicates that HAM computed using CSR and JPL solutions are characterized by the highest consistency with GAO in all considered spectral bands.</p>

Prograde and Retrograde Terms of Gravimetric Polar Motion Excitation Estimates from the GRACE Monthly Gravity Field Models

From 2002 to 2017, the Gravity Recovery and Climate Experiment (GRACE) mission’s twin satellites measured variations in the mass redistribution of Earth’s superficial fluids, which disturb polar motion (PM). In this study, the PM excitation estimates were computed from two recent releases of GRACE monthly gravity field models, RL05 and RL06, and converted into prograde and retrograde circular terms by applying the complex Fourier transform. This is the first such analysis of circular parts in GRACE-based excitations. The obtained series were validated by comparison with the residuals of observed polar motion excitation (geodetic angular momentum (GAM)–atmospheric angular momentum (AAM)–oceanic angular momentum (OAM) (GAO)) determined from precise geodetic measurements of the pole coordinates. We examined temporal variations of hydrological excitation function series (or hydrological angular momentum, HAM) in four spectral bands: seasonal, non-seasonal, non-seasonal short-term, and non-seasonal long-term. The general conclusions arising from the conducted analyses of prograde and retrograde terms were consistent with the findings from the equatorial components of PM excitation studies drawn in previous research. In particular, we showed that the new GRACE RL06 data increased the consistency between different solutions and improved the agreement between GRACE-based excitation series and reference data. The level of agreement between HAM and GAO was dependent on the oscillation considered and was higher for long-term than short-term variations. For most of the oscillations considered, the highest agreement with GAO was obtained for CSR RL06 and ITSG-Grace2018 solutions. This study revealed that both prograde and retrograde circular terms of PM excitation can be determined by GRACE with similar levels of accuracy. The findings from this study may help in choosing the most appropriate GRACE solution for PM investigations and can be useful in future improvements to GRACE data processing.

Determining and Evaluating the Hydrological Signal in Polar Motion Excitation from Gravity Field Models Obtained from Kinematic Orbits of LEO Satellites

This study evaluates the gravity field solutions based on high-low satellite-to-satellite tracking (hl-SST) of low-Earth-orbit (LEO) satellites: GRACE, Swarm, TerraSAR-X, TanDEM-X, MetOp-A, MetOp-B, and Jason 2, by converting them into hydrological polar motion excitation functions (or hydrological angular momentum (HAM)). The resulting HAM series are compared with the residuals of observed polar motion excitation (geodetic residuals, GAO) derived from precise geodetic measurements, and the HAM obtained from the GRACE ITSG 2018 solution. The findings indicate a large impact of orbital altitude and inclination on the accuracy of derived HAM. The HAM series obtained from Swarm data are found to be the most consistent with GAO. Visible differences are found in HAM obtained from GRACE and Swarm orbits and provided by different processing centres. The main reasons for such differences are likely to be different processing approaches and background models. The findings of this study provide important information on alternative data sets that may be used to provide continuous polar motion excitation observations, of which the Swarm solution provided by the Astronomical Institute, Czech Academy of Sciences, is the most accurate. However, further analysis is needed to determine which processing algorithms are most appropriate to obtain the best correspondence with GAO.

Hydrological Excitations of Polar Motion Derived from Different Variables of Fgoals − g2 Climate Model

The hydrological contribution to decadal, inter-annual and multi-annual suppress polar motion derived from climate model as well as from GRACE (Gravity Recovery and Climate Experiment) data is discussed here for the period 2002.3-2016.0. The data set used here are Earth Orientation Parameters Combined 04 (EOP C04), Flexible Global Ocean-Atmosphere-Land System Model: Grid-point Version 2 (FGOAL-g2) and Global Land Data Assimilation System (GLDAS) climate models and GRACE CSR RL05 data for polar motion, hydrological and gravimetric excitation, respectively. Several Hydrological Angular Momentum (HAM) functions are calculated here from the selected variables: precipitation, evaporation, runoff, soil moisture, accumulated snow of the FGOALS and GLDAS climate models as well as from the global mass change fields from GRACE data provided by the International Earth Rotation and Reference System Service (IERS) Global Geophysical Fluids Center (GGFC). The contribution of different HAM excitation functions to achieve the full agreement between geodetic observations and geophysical excitation functions of polar motion is studied here.