GLOBAL GRAVITATIONAL CHANGES DUE TO ATMOSPHERIC MASS REDISTRIBUTION AS OBSERVED BY THE LAGEOS NODAL RESIDUAL

Variations in the even-degree zonal gravitational field will produce perturbations to the otherwise constant nodal precession rate in satellite orbits. The contribution of the atmospheric mass redistribution to this nodal 'excitation' on the laser-ranging geodetic satellite Lageos is computed using the ECMWF (European Centre for Medium-range Weather Forecasts) meteorological analysis data over the period 1985-1991, with and without assuming the inverted-barometer (IB) effect for the ocean. These excitation time series are then compared with the observed Lageos nodal residual caused by global gravitational variations. Three frequency bands are examined with the following results. (i) In the interannual band (longer than a year), an apparent similar to 14-month phase lead is found in the atmospheric excitation relative to the Lageos observation. (ii) At seasonal periods, the atmospheric annual signal agrees reasonably well with the observations; the agreement is further strengthened by incorporating continental hydrological contributions. The semi-annual signal compares poorly with observations, indicating the presence of other geophysical sources than the atmosphere. (iii) At the intraseasonal time-scale (shorter than a year) a wide-band correlation coefficient of 0.64 is found between the observed and the atmospheric excitations with the IB effect. The non-IB model yields a somewhat lower correlation of 0.59, but a better correspondence in amplitude. The correlations, together with the corresponding coherence spectra, clearly demonstrate a strong atmospheric contribution to the global gravitational variations.