NATIONAL REPORT OF THE FEDERAL REPUBLIC OF ... - IAG Office

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1. Joint research activities
In order to organize joint research activities in “Earth
rotation and global dynamic processes” in Germany, since
the beginning of 2006 ten related sub-projects are supported
by the German research funding organization DFG (Deutsche
Forschungsgemeinschaft) in the frame of a research
unit (MÜLLER et al., 2005). Based on the general survey of
SCHUH et al. (2003) exposing the present state as well as
necessary milestones for future research work concerning
modelling, observation and analysis techniques, the main
objective of this coordinated project is a comprehensive
description and explanation of underlying physical phenomena
contributing to variations of earth rotation by taking
into account consistently the interactions and coupling
mechanisms of the various sub-systems of the earth. Such
an integral treatment of earth rotation based on existing and
new observational data became possible by comprising
experts of observation techniques, data processing and
analysis as well as in particular modelling. The research
unit with participating scientists and institutions from
geodesy, geophysics, meteorology, and oceanography will
provide significant contributions to international activities
and programs such as GGOS (Global Geodetic Observing
System) and GMES (Global Monitoring for Environment
and Security).
In close cooperation with the research unit an earth system
model for physically consistent simulations of atmospheric,
oceanic and hydrological induced variations of earth
rotation, deformation and gravity field is developed in a
research project supported by DFG with participating
German scientists from geodesy, meteorology and oceanography.
The dynamical system model couples numerical
models of the atmosphere, of ocean tides and circulation
as well as of continental discharge considering consistent
mass, energy and momentum fluxes between these nearsurface
subsystems of the earth in order to allow for
explanations and interpretations of geodetically observed
variations of global parameters of the earth.
2. Theory of earth rotation
2.1 General studies
A detailed overview of theoretical and observational
foundations of earth rotation studies, a review of the present
state of modelling and observation as well as a specification
Earth Rotation – Theory and Analysis
M. THOMAS 1 , M. SOFFEL 1 , H. DREWES 2
of needs for future research projects was given by SCHUH
et al. (2003).
ENGELS (2006) describes various formulations of the
momentum and angular momentum balance on the basis
of elements of continuum mechanics. He estimates the
impact of second order terms, referring to, e.g., the earth's
flattening and incremental inertial forces, on the equations
of polar motion and polar wandering derived from the
balance equation of angular momentum and discusses the
effects arising from neglecting these second order terms
on the angular velocity vector of a homogeneously layered,
spherical, viscoelastic and rotating earth affected by surface
mass loads. ENGELS (2006) comes to the conclusion that
the classical “spherical solution” exclusively differs from
his enhanced solution with respect to higher order terms
of the earth's flattening.
JOCHMANN (2003) studied the effect of assumed mass
redistributions on the Chandler period and found that large
variations of the Chandler period of several days detected
by several polar motion time series analyses are unlikely,
and that it is sufficient to assume an invariable period for
currently available time series.
SEITZ (2004) developed the non-linear gyroscopic Dynamic
Model for Earth rotation and Gravity (DyMEG) based on
a triaxial ellipsoid of inertia and driven by lunisolar torques
and consistent atmospheric and oceanic angular momenta
in order to investigate interactions between geophysically
and gravitationally induced polar motion and the earth's
free wobbles. DyMEG reproduces the period and damping
of the earth's free polar motion (Chandler wobble) from
rheological and geometrical parameters by solving the
Liouville equation numerically as an initial value problem.
Since spectral analyses of both atmospheric and oceanic
excitations gave no hint for increased power in the Chandler
frequency band, SEITZ et al. (2004) concluded that stochastic
signals in the climate dynamics as caused by the weather
and oceanic mass redistributions are a sufficient source to
maintain the amplitude of the earth's free wobble by
resonant interaction. Depending on the quality of the
excitations, the correlation between the numerical results
for polar motion from DyMEG and IERS data reach up to
99% (SEITZ, 2005; SEITZ et al, 2005). In order to assess the
dependence of the numerical solution on the initial values
and rheological or geometrical parameters like Love
numbers and the earth's principal moments of inertia, SEITZ
1 Maik Thomas / Michael H. Soffel: Institut für Planetare Geodäsie, Technische Universität Dresden, Helmholtzstr. 10, D-01062 Dresden,
Germany, Tel. +49 -351 - 4633 4200, Fax +49 -351 - 4633 7019, e-mail maik.thomas@tu-dresden.de / michael.soffel@tu-dresden.de
2 Hermann Drewes: Deutsches Geodätisches Forschungsinstitut (DGFI), Alfons-Goppel-Str. 11, D-80539 München, Germany,
Tel. +49 - 89 - 23031-1106, Fax +49 - 89 - 23031 1240, e-mail drewes@dgfi.badw.de