G3.3

Non-tidal Earth rotation variations at intraseasonal periods are almost exclusively driven by mass redistributions within the atmosphere and ocean. Our capacity to model these signals has advanced over the past decades, but differences between the observed and modeled portions of the planetary angular momentum budget are still as large as 1–5 cm when expressed as axis displacement at the Earth's surface. A likely source for these significant errors is the ocean, poorly sampled with observations and thus not amenable to the sequential data assimilation machinery developed for the atmosphere. Moreover, the recent delineation of basin-wide ocean mass exchanges associated with the Madden-Julian Oscillation (MJO) in a high-resolution baroclinic model emphasizes that a revisit of standard forward modeling choices (e.g., grid spacings of ∼100 km) may be in order to better describe rapid, large-scale oceanic mass motions. In this contribution, I provide a brief overview of recent progress in the field and suggest that dynamically consistent model-data syntheses, as practiced by the consortium on Estimating the Circulation and Climate of the Ocean (ECCO), are a viable route to mitigate deficiencies in present oceanic angular momentum (OAM) series on intraseasonal (but also longer) time scales. As ocean state estimates continue to be refined by the central ECCO production, I assess the benefits of higher model resolution with OAM series from an eddy-permitting (1/6°) forward simulation, descending from the current ECCO release in its discrete setup. The resulting mass and motion terms indeed provide smaller Earth rotation residuals than other available OAM estimates, possibly due to the model resolving important topographic interactions and the dynamic response to MJO in the 30–80-day band. However, these improvements come at disproportionally large computational costs, and iteratively fitting an eddy-permitting general circulation model to oceanographic observations may still be prohibitive in the near future. Instead, efforts should be devoted to extending the present coarser-resolution ECCO framework to new data constraints and shorter adjustment intervals. Of particular interest in the context of Earth rotation are non-standard daily GRACE gravity field solutions, which contain realistic information on oceanic mass-field variability below the nominal GRACE Nyquist period of 60 days.

How to cite: Schindelegger, M.: Modeling rapid Earth rotation variations – where are we going next?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1616, https://doi.org/10.5194/egusphere-egu21-1616, 2021.

One hour single baseline VLBI sessions, so-called Intensives, are routinely observed to derive UT1-UTC with a short latency. The selection of baselines for VLBI Intensive sessions and their application for the determination of UT1-UTC is a complex task. Thus far, it has been understood that long east-west extensions are critical for the accuracy of UT1-UTC. In this presentation, we show, that the answer is not as simple as that. 

We run Monte-Carlo simulations for a global 10° grid of artificial station locations and discuss the suitability of the individual baselines for UT1-UTC estimation based on the formal error of dUT1. The antennas are located at latitudes of -80° to 80° and longitudes of 0° to 180° and are assumed to have the same properties than the WETTZ13S telescope. The nine stations at longitude 0° on the northern hemisphere are defined as reference stations. In total, 2898 possible baselines between the reference stations and other artificial stations are investigated over one year based on monthly schedules to minimize potential seasonal variations. Thus, with this study, it is possible to derive a complete picture of which baselines are most suitable for dUT1 estimates. 

In general, the findings show optimal global geometries concerning Intensives. For example, we can confirm that the IVS-INT1 baseline including the stations Kokee and Wettzell is among the best ones available. Furthermore, we show that north-south baselines are also sensitive to dUT1 as long as their orientations are not parallel to the Earth rotation axis. Moreover, we highlight that east-west baselines on the equator are not suitable for estimating dUT1 due to the lack of variety in right-ascension of the visible sources. Additionally, we highlight, that very long baselines are problematic due to the highly restricted mutual visibility.

How to cite: Kern, L., Schartner, M., Soja, B., Nothnagel, A., and Böhm, J.: On the geometry of baselines suitable for UT1 estimation with VLBI Intensive sessions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-917, https://doi.org/10.5194/egusphere-egu21-917, 2021.

Variations in the Earth's rotation can be examined in the low-frequency and high-frequency temporal scales. The low-frequency variations are dominated by the annual and Chandler wobbles, while the high-frequency variations are primarily caused by tidal effects and mass redistributions within the system Earth. Depending on the purpose, the Earth Rotation Parameters (ERPs) can be estimated in different time resolutions using space-geodetic techniques, especially using GNSS. However, the residual signals between different space geodetic techniques or satellite constellations indicate system-specific differences, which have to be correctly identified.

This research provides the daily, and sub-daily series of Earth Rotation Parameters (ERPs) estimated using GPS, GLONASS, and Galileo observations. We test different sampling intervals of estimated ERPs from 1h to 24h. The GNSS-based sub-daily estimates have been compared with the external models of variations in ERPs induced by the ocean tides from the IERS 2010 Conventions, a new model by Desai-Sibois, and the VLBI-based model by Gipson.

Any system-specific ERPs are affected by the orbital and draconitic signals. The orbital signals are visible in all system-specific ERPs at the periods that arise from the resonance between the Earth's rotation and the satellite revolution period, e.g., 8.87h, 34.22h, 3.4 days, 10 days for Galileo; 7.66h, 21.29h, 3.9 days, 7.9 days for GLONASS; 7.98h (S3 tidal term), 11.97h (S2 tidal term), 23.93h (S1 tidal term) for GPS. In the Galileo and GLONASS solutions, the artificial non-tidal signals' amplitudes can reach up to 30 µas. The GPS-derived sub-daily ERPs suffer from the overlapping periods of the diurnal and semidiurnal tidal terms and the harmonics of the GPS revolution period. After recovery of 38 sub-daily tidal terms, the Galileo-based model is more consistent with the external models than the GPS-based model, especially in the prograde diurnal band. The results confirmed that the Desai–Sibois model is more consistent with GNSS observations than the currently recommended model by the IERS 2010 Conventions. Moreover, GPS-based length-of-day (LoD) is systematically biased with respect to the IERS-C04-14 values with a mean offset of −22.4 µs/day, because of the deep resonance 2:1 between the satellite revolution period and the Earth rotation. The Galileo-based and GLONASS-based solutions are almost entirely free of this issue. Against the individual system-specific solutions, the multi-GNSS solution is not affected by most of the system-specific artifacts. Thus, multi-GNSS solutions are clearly beneficial for the estimation of both daily and sub-daily ERPs.

How to cite: Zajdel, R., Sośnica, K., Bury, G., and Kazmierski, K.: Estimating variations in Earth rotation using GPS, GLONASS, and Galileo, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2646, https://doi.org/10.5194/egusphere-egu21-2646, 2021.

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.

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.

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.

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 C₂₀ coefficient correction.

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).

How to cite: Śliwińska, J., Wińska, M., and Nastula, J.: Assessing the role of corrections included in the GRACE/GRACE-FO data in determination of hydrological and cryospheric signal in polar motion excitation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5426, https://doi.org/10.5194/egusphere-egu21-5426, 2021.

Angular momentum is fundamental to the structure and variability of the atmosphere and hence regional weather and climate. Total atmospheric angular momentum (AAM) is also directly related to the rotation rate of the Earth and hence the length of day. However, the long-range predictability of fluctuations in the length of day, atmospheric angular momentum and the implications for climate prediction are unknown. Here we show that fluctuations in AAM and the length of day are predictable out to more than a year ahead and that this provides an atmospheric source of long-range predictability of surface climate. Using ensemble forecasts from a dynamical climate model we demonstrate predictable signals in the atmospheric angular momentum field that propagate slowly and coherently polewards into the northern and southern hemisphere due to wave-mean flow interaction within the atmosphere. These predictable signals are also shown to precede changes in extratropical surface climate via the North Atlantic Oscillation. These results provide a novel source of long-range predictability of climate from within the atmosphere, greatly extend the lead time for length of day predictions and link geodesy with climate variability.

How to cite: Scaife, A., Hermanson, L., van Niekerk, A., Baldwin, M., Belcher, S., Bett, P., Comer, R., Dunstone, N., Geen, R., Hardiman, S., Ineson, S., Knight, J., Nie, Y., Ren, H., and Doug, S.: Long-Range Predictability of the Length of Day and Extratropical Climate., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-191, https://doi.org/10.5194/egusphere-egu21-191, 2021.

One special component of the Coupled Model Intercomparison Project phase 6 (CMIP6) is the so-called ScenarioMIP. Different Earth system variables are provided within this project originating from numerous models and model runs operated by research centers around the globe. The models simulate future climate, based on alternative scenarios of future greenhouse gas emissions and land use changes linked to socioeconomic factors. The simulations, which cover the 21^st century, use different forcings that are defined from a combination of possible future pathways of societal development, the Shared Socioeconomic Pathways (SSPs), and the Representative Concentration Pathways (RCPs), identified by what radiative forcing level might exist in 2100. In this study, we focus on the analysis of multi-model projections of zonal wind fields, stemming from historical simulations and from the four tier 1 alternate scenarios SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5. We investigate the integrated effect of variations in the atmosphere on the axial component of the Earth rotation vector, quantified as length of day (LOD) variations. Generally, larger emissions lead to some stronger zonal wind patterns in much of the upper atmosphere.  The long-term variability and trends in LOD are examined w.r.t. a multi-model-mean and compared with the respective variations in the projected global temperature to study the potential impact of global warming on the Earth rotation speed.

How to cite: Böhm, S. and Salstein, D.: Axial atmospheric Earth rotation excitation predicted from CMIP6 model simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5601, https://doi.org/10.5194/egusphere-egu21-5601, 2021.

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.

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.

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.

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.

How to cite: Wińska, M., Śliwińska, J., and Nastula, J.: Comparison between polar motion excitation functions estimated from recent geophysical models and observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9490, https://doi.org/10.5194/egusphere-egu21-9490, 2021.

The Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) provides the geodetic infrastructure needed to monitor the Earth system.. The understanding of forced temporal variations of celestial pole motion (CPM) could bring us significantly closer to meeting the GGOS goals (i.e. 1 mm accuracy and 0.1 mm/year stability on global scales in terms of the ITRF defining parameters). Besides astronomical forcing, CPM excitation depends on the processes in the fluid core and the core-mantle boundary. The same processes are responsible for the variations of the geomagnetic field (GMF). This study investigates the interconnection between the celestial pole offset (CPO) and effective geophysical processes that contribute to the Earth's rotational variation. We use the CPO time series obtained from very long baseline interferometry (VLBI) observations together with the latest GMF data such as geomagnetic jerk and magnetic dipole moment, and a state-of-the-art geomagnetic field model to explore the correlation between CPM and GMF. 
Our results confirm the findings of previous studies, revealing that  substantial free core nutation (FCN) disturbance occurred at the epochs close to the GMJ events. The results also reveal some common features in the FCN and GMF variation, which show the potential to improve knowledge regarding the GMF's contribution to the Earth's rotation.

How to cite: Modiri, S., Heinkelmann, R., Belda, S., Hoseini, M., Korte, M., Malkin, Z., Ferrándiz, J. M., and Schuh, H.: A First Assessment of the interconnection between celestial pole offset and geomagnetic field variations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7235, https://doi.org/10.5194/egusphere-egu21-7235, 2021.

In 2020 new estimations of nutation amplitudes or precession parameters have been published or presented at main meetings. The derivation of corrections to improve the current precession-nutation models was encouraged by Resolution 5 of the 2019 General Assembly of the International Association of Geodesy (IAG). Besides, the GGOS/IERS Unified Analysis Workshop held in October 2019 recommended that effort to be prioritized among the tasks of the current IAU/IAG Joint Working Group on Improving Theories and Models of the Earth’s rotation (JWG ITMER).

This presentation is intended to present comparisons of some of those new semi-empirical and semi-theorical precession-nutation models developed by different authors from either VLBI solutions of individual analysis centers or combinations of them. The models recently introduced by the authors that were reported at the AGU 2020 Fall Meeting are included in this assessment.

How to cite: Ferrándiz, J. M., Juárez, M. A., Belda, S., Baenas, T., Modiri, S., Heinkelmann, R., Escapa, A., and Schuh, H.: Assessing recently improved precession-nutation models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10180, https://doi.org/10.5194/egusphere-egu21-10180, 2021.

IAU2000 (Mathews et al. 2002) incorporates some second order terms in the sense of perturbation theories in its formulation. In particular, the second order Poisson amplitudes independent of the Earth structure. They are borrowed from the rigid Earth theory REN2000 by Souchay et al. (1999). Their inclusion, however, is inconsistent (Escapa et al. 2020) since they are convolved with the MHB2000 transfer function, rendering them Earth dependent.

In that IAU2000 scheme, second order contributions depending on the Earth structure are totally ignored, as it is the case in the rigid Earth theory (Souchay et al. 1999). That structure dependent terms affect both a part of Poisson second order amplitudes and all the Oppolzer ones. Getino et al. (2021) have shown that the numerical contribution of the ignored Poisson terms is not negligible. In addition, the dependence of the respective amplitudes on the fluid core present quite different features from those of first order terms.

These facts pose some significant problems in the application of IAU2000 transfer function and the estimation of basic Earth parameters when second order terms are included, which are discussed in this communication.

How to cite: Escapa, A., Getino, J., Ferrándiz, J. M., and Baenas, T.: Implications of second order nutation terms in IAU2000 framework, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14236, https://doi.org/10.5194/egusphere-egu21-14236, 2021.