Zonal tidal variations in UT1 observed by VLBI (Invited)

The deformations of the Earth caused by the zonal part of the tidal potential induce fluctuations in the Earth rotation rate and thus in universal time (UT1) and length of day (LOD) with periods from ~5 days to 18.6 years. UT1 variations were derived from 25 years of VLBI observations, using the VLBI software package OCCAM. The atmospheric influence on UT1 was computed from atmospheric angular momentum functions (AAMF) provided from NCEP data and for comparison purposes from AAMF derived from ECMWF data as well. The tidal terms with periods up to 35 days were estimated in a least squares adjustment introducing the remaining UT1 variations as pseudo-observations. The resulting amplitudes were compared with the values of the model recommended in the IERS Conventions 2003 and with a combination of other currently available models according to Gross (2009). Our study revealed significant discrepancies between the observed variations and variations predicted by the conventional model in the fortnightly tidal band, which can be considerably reduced when the combined model suggested in Gross (2009) is applied.