Lustrumboek "The 5th Element"
Current investigations into the tidal problem focus on the situation in coastal
zone such as the South China Sea region where non-linear interaction with bottom
topography is a topic of study. In these regions the ocean tide signal is less well
known because of the increase in the spatial structure of the shallow water
equations. In this case it helps to start cooperations with other organisations such
as Delft Hydraulics, (het waterloopkundig laboratorium), since coastal tide gauge
data needs to be incorporated in the solutions. To summarize the situation: ocean
tide models developed by Delft geodesists are now the standard in remote
sensinging applications and are even applied in civil engineering applications
allowing an improved solution in the shallow water regions like the South China
sea area.
6.2 The air that we breath
The change of air pressure changes and wind effects are another source of
temporal gravity. The hint was given by the use of meteorological models that
'mP.'®ment the so-called inverted barometer equation, a simplified 1 cm per
millibar local air pressure to sea level variation response equation which is used
in the satellite altimeter data reduction process. Despite the simplicity of this
equation it turns out to be a quite effective way of reducing a major source of
variance from sea level changes recorded by satellite altimeter systems.
The structures in the air pressure distribution are by themselves a first source of
scientific research. Based upon experience one would expect that meteorological
effects are related to seasonal variations, and that most of the sort term variations
are random. The latter is not true. Hypothesized and conformed are coherent
global atmospheric gravity inertial waves that occur at periods of 5 days to two
weeks. Moreover in the tidal frequency band there exist atmospheric wave effects
that are caused by radiational effects. The case study of Mathijs Schouten was
related to the twice daily atmospheric wave effect which is visible in automated
barometer data recorded at the former Kootwijk laboratory. It is an important
signal that needs the correct treatment in the altimeter tide model development.
Deviations from the IB equation are known and can be verified by a careful
analysis of tide gauge data and global hydrodynamic models of the oceans. The
model that includes more physical reality would also involve wind effects that
are, as we all know, related to air pressure variations. There is for instance an
effect known as Ekman pumping whereby sea level locally changes as wind patterns
circulate over the area. Statistically this effect can be confirmed by means of the
contemporary altimeter data. Lucy Mathers, a postdoc originally from the
Proudman Oceanographic Laboratory is currently working on the IB problem.
The relation with gravity research of all the above is perhaps that self gravity
effects of air mass movements appear as temporal gravity effects of which it is
suggested that they can be seen in the geoid. Sandra Verhagen's master thesis
study recently confirmed that significant geoid effects can be expected. Geoid
changes of the order of several millimeters can be expected in the measurement
domain of a gravity mission apparently because the atmospheric mass anomaly
changes are sufficient. It is a candidate signal to be removed in the data reduction
procedures used for gravity missions.
6.3 The future
Understanding the future of the Earth static and temporal gravity field will undoubtly
bring more interesting problems on the table. The subject is proven to be multi
disciplinary and closely linked to many other areas in geophyisics. Whether the
expectations raised by a gravity mission can be met is still the question which
largely depends on the actual performance of the gradiometer in orbit. One
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