Thorough investigation of Earth rotation irregularities plays a key role in advancing the accuracy of space geodetic products, which have become truly indispensable to global positioning tasks. Moreover, dissecting the observed variability of the rotation axis in terms of its geophysical origin promotes insight into Earth system dynamics. A considerable part of the modern-day interest in those effects pertains to short periods, encompassing also a minute but measurable influence of diurnal and semidiurnal atmospheric tides on polar motion and changes in length of day (LOD). The physical character of this forcing is known to be twofold — the atmosphere exchanges angular momentum with the solid Earth both directly through boundary layer stresses and indirectly through the oceans — but existing quantifications of the elicited rotational perturbations are still at discord and thus a source of uncertainty in the analysis and interpretation of space geodetic data.

Project ASPIRE is conceived as a comprehensive treatise on the nature and scope of atmosphere-induced (semi)diurnal polar motion signals and changes in LOD from the viewpoint of geophysical modeling. The central idea is to determine 3-hourly estimates of both atmospheric angular momentum (AAM) and Earth-atmosphere torques (see figure) from a carefully selected pool of meteorological analysis-forecast data of three state-of-the-art atmospheric assimilation systems:

- ERA-Interim, the current reanalysis of the European Centre for Medium-Range Weather Forecasts,
- MERRA (Modern Era-Retrospective Analysis for Research and Applications) produced by NASA's GMAO (Global Modeling and Assimilation Office), and
- The National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR).

A mutual comparison of individual (semi)diurnal excitation quantities (pressure and wind AAM on the one side, ellipsoidal, mountain, and friction torques on the other side) among all models indicates their reliability for short period Earth rotation studies. This inquiry is augmented by an in-depth analysis of the required balance between the time derivative of AAM and the total atmospheric torque. These investigations form the foundation for the project's goal to deduce credible estimates of the associated excitation effects and eventually advance the agreement between modeled and observed Earth rotation variations.

Being a joint project together with Section 1.3 Earth System Modelling of Geoforschungszentrum Potsdam, the second area of operation within ASPIRE addresses the dynamic ocean response to atmospheric tides using the global numerical Ocean Model for Circulation and Tides (OMCT, Dobslaw and Thomas, 2005). Experiments with regionally refined grids will provide the foundation on which oceanic angular momentum at 3-hourly intervals and the corresponding torques will be intercompared. The combined atmosphere-ocean excitation values will be assessed on the evidence of Earth rotation solutions from geodetic observing systems.

M. Schindelegger. Atmosphere-induced short period variations of Earth rotation, Geowissenschaftliche Mitteilungen, Schriftenreihe der Studienrichtung Vermessung und Geoinformation, Wien, Heft. Nr. 96, 2014.

H. Dobslaw, M. Thomas. Atmospheric induced oceanic tides from ECMWF forecasts, Geophys. Res. Lett., Vol. 32, L10615, doi:10.1029/2005GL022990, 2005.