Geoscience Reference
In-Depth Information
gravity waves can also produce ter-diurnal oscillations. Due to interactions of tidal
and gravity waves, the
S
3
(
)
p
tide is irregularly distributed over the globe (Aso
2003
).
2.1.3 APL Services
APL services that provide 6-hourly vertical and horizontal corrections for VLBI,
GNSS, SLR sites as well as for the nodes on global grids have been established by
several institutions. Each service applies different methods and data to calculate the
displacements. Here, we briefly describe three services that provide global models
of the displacements from 1980 onward.
•
University of Luxembourg
. The displacements have been derived using the
method originally outlined in van Dam and Wahr (
1987
) with slight modifica-
tions in determination of the ocean mask, reference pressure and removing the
erroneous atmospheric tides. In van Dam and Wahr (
1987
), a 2
5
◦
×
5
◦
land-sea
.
2
.
25
◦
land-sea mask. Units that
contain only land are assigned the surface pressure defined by the original NCEP
gridded file. Those with only water are assigned the modified IB pressure, defined
in van Dam and Wahr (
1987
). 2
25
◦
×
mask was used. Presently, they use a 0
.
0
.
5
◦
×
5
◦
grid units containing water and land are
.
2
.
25
◦
units, each assigned either the land value or the IB
value as appropriate. They use a reference pressure determined as a 20 years mean.
The pressure data are low pass filtered to remove the erroneous atmospheric tides
in the surface pressure data. The filtering means that the online data are always
3-4 days behind the actual date.
25
◦
×
subdivided into 0
.
0
.
•
Goddord Space Flight Center
. The method to calculate the displacements is thor-
oughly described by Petrov and Boy (
2004
). Non-tidal loading displacements are
determined based on surface pressure data from NCEP with a horizontal resolu-
tion of 2
5
◦
. The reference pressure is calculated by averaging 20 years of surface
pressure data. To determine tidal loading displacements, the pressure tide model
of Ponte and Ray (
2002
) is used. Petrov and Boy (
2004
) adopt the method of van
Dam and Wahr (
1987
) to determine the land-sea mask.
.
•
Vienna University of Technology
. We use surface pressure as derived from pres-
sure level data from operational analysis as well as re-analysis data sets from the
ECMWF with a horizontal resolution of 1
◦
. Tidal and non-tidal loading displace-
ments are calculated using the methods described in Sects.
2.1.1
and
2.1.2
.The
Global Reference Pressure (GRP) model (Schuh et al.
2009
) is used to calculate
reference pressure. The 6-hourly vertical and horizontal corrections are provided
for all VLBI sites as well as for the nodes on a global 1
◦
grid.
Figure
8
shows the displacements at Algonquin Park (ALGOPARK), Canada in
vertical and horizontal directions (unit mm) determined by the three APL services.
It is obvious that the displacements provided by the Luxembourg and the Petrov
and Boy services are very similar with very small differences. The Vienna service
produces slightly different results from those of the other two. The difference could
