Diurnal and subdiurnal effects of the atmosphere on the Earth rotation and geocenter motion

The diurnal cycle in the atmospheric angular momentum (AAM) and in the wind and surface pressure fields is studied with a realistic atmospheric general circulation model (GCM) in which the AAM budget is very well closed. For this, we used a 1 year simulation. From a geodetic point of view, we find that this model predicts AAM variations at diurnal timescale which produce a polar motion near 0.2 milliarc second. Additionally, at the same period, the model predicts a geocenter motion of the order of a millimeter. These results are compared with those obtained with the National Centers for Environmental Prediction/National Center for Atmospheric Research and the European Centre for Medium-Range Weather Forecasts operational analysis data sets. As the AAM budget is not exactly closed in those two data sets, large quantitative differences with the GCM are found. These results witness that there are problems in using AAM values from the major weather prediction center to estimate the AAM and torques variation at diumal and subdiurnal timescales. We have also computed, for the three models, the spherical harmonics decomposition of the diurnal and semidiurnal surface pressure signals. The results show large differences from one model to another, which advices carefulness when correcting gravity missions (as Gravity Recovery and Climate Experiment (GRACE), for instance) from the high-frequency effect of the atmosphere on the orbit, using operational analysis.