 |
| Titel |
Inferring internal properties of Earth's core dynamics and their evolution from surface observations and a numerical geodynamo model |
| VerfasserIn |
J. Aubert, A. Fournier |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1023-5809
|
| Digitales Dokument |
URL |
| Erschienen |
In: Nonlinear Processes in Geophysics ; 18, no. 5 ; Nr. 18, no. 5 (2011-10-12), S.657-674 |
| Datensatznummer |
250013974
|
| Publikation (Nr.) |
 copernicus.org/npg-18-657-2011.pdf |
|
|
|
|
|
| Zusammenfassung |
| Over the past decades, direct three-dimensional numerical modelling has been
successfully used to reproduce the main features of the geodynamo. Here we
report on efforts to solve the associated inverse problem, aiming at
inferring the underlying properties of the system from the sole knowledge of
surface observations and the first principle dynamical equations describing
the convective dynamo. To this end we rely on twin experiments. A reference
model time sequence is first produced and used to generate synthetic data,
restricted here to the large-scale component of the magnetic field and its
rate of change at the outer boundary. Starting from a different initial
condition, a second sequence is next run and attempts are made to recover the
internal magnetic, velocity and buoyancy anomaly fields from the sparse
surficial data. In order to reduce the vast underdetermination of this
problem, we use stochastic inversion, a linear estimation method determining
the most likely internal state compatible with the observations and some
prior knowledge, and we also implement a sequential evolution algorithm in
order to invert time-dependent surface observations. The prior is the
multivariate statistics of the numerical model, which are directly computed
from a large number of snapshots stored during a preliminary direct run. The
statistics display strong correlation between different harmonic degrees of
the surface observations and internal fields, provided they share the same
harmonic order, a natural consequence of the linear coupling of the governing
dynamical equations and of the leading influence of the Coriolis force.
Synthetic experiments performed with a weakly nonlinear model yield an
excellent quantitative retrieval of the internal structure. In contrast, the
use of a strongly nonlinear (and more realistic) model results in less
accurate static estimations, which in turn fail to constrain the unobserved
small scales in the time integration of the evolution scheme. Evaluating the
quality of forecasts of the system evolution against the reference solution,
we show that our scheme can improve predictions based on linear
extrapolations on forecast horizons shorter than the system e-folding time.
Still, in the perspective of forthcoming data assimilation activities, our
study underlines the need of advanced estimation techniques able to cope with
the moderate to strong nonlinearities present in the geodynamo. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|