Geoscience Reference
In-Depth Information
The amplitude spectra of the displacements in vertical and east directions (Fig.
4
)
shows significant narrow-band diurnal and semi-diurnal signals. Petrov and Boy
(
2004
) mentioned that strong wide-band annual and semi-annual signals and rel-
atively weak signal for period below 10 days, except strong peaks at the diurnal
and semi-diurnal bands, are typical for the displacements at low-latitude stations.
In the mid-latitude regions, peak-to-peak variations in the vertical direction occur
with a period of about 5-12 days that correspond to the circulations of high and low
pressure structures in this regions, partly due to baroclinic variability (Dell'Aquila
et al.
2005
). These timescales represent the limit of validity of the IB assumption for
describing the oceanic response to atmospheric pressure forcing.
The effects of APL have been observed in high-precision space geodetic data, i.e.,
VeryLongBaseline Interferometry (VLBI) (vanDamandWahr
1987
;MacMillan and
Gipson
1994
; Petrov and Boy
2004
; Böhm et al.
2007
), Global Navigation Satellite
Systems (GNSS) (van Dam and Herring
1994
;vanDametal.
1994
; Tregoning and
van Dam
2005
; Dach et al.
2011
), and Satellite Laser Ranging (SLR) (Bock et al.
2005
). These observational data are often used for geodynamic studies and can be
important to remove the displacement signals due toAPL, which otherwise propagate
into other parameters and effects like hydrological loading and tropospheric delay
estimation. For the purpose of correcting APL signals in space geodetic observations,
it is necessary to provide the model and corrections for routine data reduction. In the
following sections, different approaches to model APL corrections will be discussed.
2 Modeling Atmosphere Pressure Loading
The IERS (International EarthRotation andReference Systems Service) Conventions
2010 (Petit and Luzum
2010
) describe two possibilities to model the APL effects:
(i) a geophysical approach using convolution of the actual loading distribution over
the entire surface of the solid Earth, (ii) an empirical model which is based on the
actual deformations derived from geodetic observations taken at individual sites.
Both approaches will be described in this section.
2.1 Geophysical Approach
Farrell (
1972
) considered the elastic yield of the solid Earth to changing surface loads
and solved the point loading problem for a spherically symmetric, non-rotating, elas-
tic, and self-gravitating Earth with a liquid core, by devising the Green's functions,
which encompass the Earth's response, over spherical harmonic degrees. The essen-
tial step in the calculation of surface displacements due to loading comprises the
global convolution of the load influence, which is represented by the corresponding
Load Love Numbers (LLN) inside the Green's functions, see Eqs. (
4
) and (
5
).
