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Nearly
total neglect of the non-tidal atmospheric and oceanic mass
redistribution is the current practice in orbit solutions. These
mass movements not only change the geopotential in a complicated fashion
but also act erratically. While the dominant sources of environmental
mass redistribution occur with seasonal and annual periods, estimates
of shorter period atmospheric mass distributions alone have been shown
to produce 3-5 cm level signals in 5-day orbits of Starlette [ Pavlis et
al., 1992]. This analysis was based on the evaluation of the
gravitational changes implied by the European Center for
Medium-range Weather Forecast (ECMWF) global atmospheric circulation
model [ Chao and Au, 1991; Eanes and Watkins, 1991].
Steinberg et al., [1994] have used global ocean circulation
model output to gauge the impact of ocean mass redistribution on the
temporally varying gravity field. Chao et al., [1995] and
Gross and Chao, [1990] have also assessed other sources of
gravitational changes. Before forward modeling of environmental mass
redistribution from ocean/atmosphere global circulation models can be
adopted in orbit solutions, they must be verified. SLR is likely to
continue playing a critical role in these assessments. To date, direct
assessment of non-tidal changes in the geopotential has been restricted
to variations in a few zonal harmonics based on orbital evolution studies
on the Lageos satellite. Nerem et al., [1993b] compared estimated
monthly values for the J
and J
harmonics with those predicted
by the ECMWF atmospheric circulation model and found compelling
agreement for J
. Similar results were obtained by Gegout and
Cazenave [1993] and Chao and Eanes, [1995]. Zonal coefficient rates
have been estimated from the SLR data in studies performed by Cheng
et al., [1990, 1991] and Schutz et al., [1993]. The importance of
having accurate values for these zonal rates to constrain post-glacial
rebound models is described in Mitrovica and Peltier [1993]
and for ice mass balance models in Trupin [1993].
Satellite solutions place bounds on the aggregate mass
redistribution ongoing within the Earth's systems. The most
challenging aspect of studying temporal gravitational variations will be
to attempt to separate the contributions from individual
geophysical processes, given the estimate of their overall effect from
these satellite determinations. This is an ill-posed, geophysical
inversion type of problem, since a particular mass distribution cannot
be determined uniquely from the gravitational potential it generates.
Next: 3.1.3. Apparent geocenter
Up: 3.1. Static and
Previous: 3.1.1. Tidal modeling.
U.S. National Report to IUGG, 1991-1994
Rev. Geophys. Vol. 33
Suppl., © 1995 American Geophysical Union