Aorde
3 The need for an improved gravity field model
reference surface for land - ice - ocean changes and interaction: GEOID
transition
Figure 1The dual role of the gravity field in Earth sciences
However, the gravity field is not only a mirror of the mass distribution inside the
Earth but may also serve as a reference surface of all topographic processes.
This becomes obvious when one recalls that the geoid, the equipotential or level
surface of the Earth's gravity field at mean sea level, represents the hypothetical
ocean surface at rest. So it is the surface, which heights are referred to, and it
determines in which direction water flows. Thus, it is the natural reference surface
for the topography of land and ice surfaces and their temporal variations as well
as for the topography of the oceans. The latter has, in conjunction with satellite
altimeter measurements, strong implications for ocean circulation studies, because
it allows to determine the geostrophic surface currents. These, in turn, form the
basis of calculations of near-surface heat transport, may be assimilated in general
circulation models, and can be used together with in situ data in calculations of
mass, heat, salt transport at depths. In particular the deep ocean circulation is a
key parameter in regulating the Earth's climate on longer time scales.
The gravity signal and the spatial pattern of geodynamic processes and geophysical
features determine the requirements we have to impose on gravity field models in
terms of accuracy and resolution. Figure 2 illustrates the required accuracy as a
function of horizontal resolution necessary in order to resolve the quoted
geodynamical and tectonic features. The dark dashed line indicates that the gravity
signal of most of the characteristic features of interest cannot be resolved yet. This
weakness in gravity field knowledge is related to the limitations of current observation
techniques, mainly terrestrial gravimetry, satellite altimetry, and conventional satellite
tracking. For instance, after more than 50 years of terrestrial gravimetry, surface
gravity data are very precise, but still highly incomplete, inhomogeneous with many
gaps (high mountain areas, shallow water areas, polar regions, lakes), and often
contaminated by systematic error. Satellite altimetry measures so to say the ocean
geoid but is much too approximate, since actually the real and not the idealized
ocean surface is measured; it deviates from the geoid at the meter level. Gravity
field modelling by satellite orbit analysis of many, mostly non-geodetic satellites
using various ground-based tracking techniques at many observatories, can only
resolve the long wavelength features, i.e., wavelengths of a few thousand kilometers
and longer. Therefore neither the accuracy nor the resolution of current geopotential
models can be expected to improve significantly by additional data from conventional
gravity field sensors.
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