Atmosphere-induced short period variations of Earth rotation

Short period variations of Earth rotation are subject to a small but important influence from atmospheric dynamics associated with diurnal and semi-diurnal radiational atmospheric tides. Sharp peaks in the high-frequency spectra of Earth Rotation Parameters (ERP) are engendered by exchanges of angular momentum between the atmosphere and the solid Earth, and also by non-tidal oceanic angular momentum (OAM) caused by the oceanic response to air pressure variations and wind stresses. However, preceding studies have failed to provide truly reliable values and a complete picture of those intriguing effects. The heart of project ASPIRE is thus to determine accurate estimates of the atmosphere-induced daily and sub-daily ERP variations and acquire a deep understanding of the underlying Earth-atmosphere-ocean angular momentum transfer triggered by atmospheric tides. For the purpose of this key task, two distinct but fundamentally equivalent methods namely, the diagnoses of fluctuations in angular momentum and the analysis of Earth-atmosphere-ocean interaction torques will be applied in parallel. The first, purely atmospheric part of project ASPIRE builds on state-of-the-art meteorological data from three of the world`s important weather and research centers. The particular innovation at this step will be to perform error and reliability considerations for atmospheric angular momentum (AAM) and torque terms based on an unprecedented three-hour resolution across all atmospheric models. Those efforts are supported by a thorough numerical validation of the analytical equivalence between the time derivative of AAM and the total atmospheric torque. Previously unverified at high frequencies, such an AAM balance may now be feasible in light of recent substantial advances in meteorological analysis, and will also be deeply instructive as it allows crosschecking the results obtained from AAM and the torque approach. The second major area of operation within project ASPIRE will complete the modeling of global geophysical fluids relevant for sub-daily Earth rotation studies by tackling the dynamic ocean response to atmospheric tides using the global numerical Ocean Model for Circulation and Tides (OMCT). Experiments based on the standard configuration of OMCT as well as on regionally refined grids will lead to an unparalleled effort of studying both non-tidal OAM values and the corresponding oceanic torques at three-hourly intervals. The balance of high- frequency OAM shall be critically evaluated. In a final task, the coupled atmosphere-ocean excitation values will be assessed on the evidence of high-accuracy observations of ERP from modern geodetic observing systems, after removal of the generally predictable ocean tidal effects. The scientific progress implied by these comprehensive efforts will enable the Earth rotation community to better understand the validity of the relevant sub-diurnal signals arising from global circulation models. Project ASPIRE will thus improve the interpretation of observed high-frequency ERP signals and, in a broader sense, foster our understanding of the Earth as a complex system.

Knowledge and modeling of irregularities in Earth rotation - comprising nutation, polar motion, and changes in length-of-day - is integral to modern geodesy and a prerequisite for any positioning task involving space-borne observations, like those from Global Navigation Satellite Systems (GNSS). At the same time, linking the observed variability in Earth rotation with its geophysical causes provides insight into the structure of Earth's interior and the interactions between the solid surface and its enveloping fluids. Among the spectrum of dynamics in the atmosphere and the oceans, diurnal and semi-diurnal oscillations of radiational (thermal) origin have long been considered as vexing terms for Earth rotation, for their space-geodetic evidence being in clear disparity with predictions from geophysical fluid models. Project ASPIRE has been set out to resolve this mismatch and elucidate the exact nature of atmospheric tidal effects in Earth rotation.Very Long Baseline Interferometry (VLBI) observations of a longstanding prograde annual nutation anomaly were, for the first time, reconciled with diurnal S1 (24 hours) excitation estimates from geophysical fluid models. This result owes, in parts, to the careful selection of atmospheric assimilation systems in the project, but even more so to the fidelity with which the hydrodynamic response to the pressure tide from the atmospheric model has been accounted for. The deduced atmosphere-ocean excitation terms will aid forthcoming nutation theories in their unambiguous a priori account of Earth's prograde annual motion in space. Threads of refined ocean modeling and the combined application of both angular momentum and torque approach were finally brought together. Although a comprehensive validation of the atmospheric angular momentum balance in all components was tackled in the early project phase, we then primarily dealt with the more pronounced diurnal changes in length-of-day (15-20 microarcseconds). These aspects - and the spurious nature of high-frequency estimates of atmospheric angular momentum in particular - are novel results in the discipline and bear the potential of improving the modeling results for polar motion and length-of-day on other time scales. As a core product for geodetic practice, we used a mixed torque/angular momentum approach to put forth length-of-day excitation quantities that are consistent with space geodetic S1 determinations and should thus be included in upcoming a priori models of sub-daily Earth rotation variations.