Atmosphere-Induced Short Period Variations of Earth Rotation (ASPIRE)

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By utilizing high-resolution atmospheric data from numerical weather prediction models provided by different international modelling centres, we will re-assess the relevant excitation mechanisms of the variable Earth rotation on diurnal and sub-diurnal periods. In addition to operational atmospheric data-sets, re-analysis time-series with at least 3-hourly resolution will be incorporated. Our analyses will be primarily based on data from the European Centre for Medium-Range Weather Forecasts (ECMWF), augmented by the U.S. weather models from the National Center for Environmental Prediction (NCEP) and the National Aeronautics and Space Agency (NASA). To take explicitly the dynamic response of the oceans to sub-daily atmospheric variations into account, we will perform additional experiments with the global numerical Ocean Model for Circulation and Tides (OMCT), thereby taking into consideration global model data-sets of all geophysical fluids that are relevant for diurnal variations of the Earth rotation.Besides establishing individual angular momentum variations for the sub-systems considered, we will in particular analyse torques acting at different horizontal and lateral boundaries between atmosphere, oceans, and the solid Earth. We thereby aim at an improved understanding of the dominating physical transfer processes of angular momentum as represented by the models considered, and at the same time attempt a thorough numerical validation of the analytically shown equivalence between the angular momentum and the torque approach. Although the main focus of this project will be on atmospheric processes, the experiments planned with OMCT will provide an estimate on the oceanic contributions to the angular momentum budget for the time-scales considered, thereby allowing to finally compare modelling results with observed non-stationary variations of Earth rotation on diurnal and sub-diurnal periods.