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 p erturbations to the otherwise constant nodal precession rate in satell ite orbits. The contribution of the atmospheric mass redistribution to this nodal 'excitation' on the laser-ranging geodetic satellite Lageo s is computed using the ECMWF (European Centre for Medium-range Weathe r Forecasts) meteorological analysis data over the period 1985-1991, w ith and without assuming the inverted-barometer (IB) effect for the oc ean. 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 i nterannual band (longer than a year), an apparent similar to 14-month phase lead is found in the atmospheric excitation relative to the Lage os observation. (ii) At seasonal periods, the atmospheric annual signa l agrees reasonably well with the observations; the agreement is furth er strengthened by incorporating continental hydrological contribution s. The semi-annual signal compares poorly with observations, indicatin g the presence of other geophysical sources than the atmosphere. (iii) At the intraseasonal time-scale (shorter than a year) a wide-band cor relation coefficient of 0.64 is found between the observed and the atm ospheric excitations with the IB effect. The non-IB model yields a som ewhat lower correlation of 0.59, but a better correspondence in amplit ude. The correlations, together with the corresponding coherence spect ra, clearly demonstrate a strong atmospheric contribution to the globa l gravitational variations.