Atmospheric pressure loading causes an elastic deformation of the solid Earth.
The magnitude of the effect can be up to 15-20 mm and it can vary by as
much as 10 mm within a 24 hour period. As a result, this physical phenomenon
affects the coordinates of geodetic sites (VLBI, GPS, SLR, DORIS etc) and
should be accounted for in all high precision geodetic analyses.
This web page will provide examples of time series of calculated atmospheric
pressure loading deformations as well as GPS vertical time series
with and without the loading accounted for.
Contents
Atmospheric pressure loading model
The atmospheric pressure loading is calculated by Tonie van dam
using NCEP pressure data. The surface pressure is convolved using
Green's functions to generate the deformation at each node of
a 2.5 x 2.5 deg grid (in local N, E, U at each node). Oceanic
regions are treated as an Inverted Barometer. The loading in
coastal regions is calculated by subdividing the 2.5 deg grid
down to a 2.5 km grid, testing whether each cell is predominently
water or land and then convolving using the appropriate Green's
function.
The 10-year mean pressure value is subtracted from each 6 hourly
value at each node. The variations from the mean pressure are used
to calculate the displacements; thus, the displacements are with
respect to a 10 year mean pressure value. There is the potential
here that a scale factor could be introduced into the geodetic
analysis if the mean pressure value yields a systemeatic shift
in the ITRF coordinates of a site. We are still investigating this ....
The diurnal and semi-diurmal "atmospheric tides" are first
removed from the NCEP pressure data following the procedures
of Ponte and Ray (2002). Therefore, the modelled atmospheric
pressure loading is only the non-tidal component. The periodic
deformation associated with the S1 and S2 tides is added into
the analysis of the GPS data (within the GAMIT software module
called "MODEL", specifically in subroutine etide.f), resulting
in a complete atmospheric pressure loading model. This is exactly
the same approach as described in Petrov and Boy (2004).
The atmospheric pressure loading was incorporated into the
GAMIT software version 10.2, and binary grid files are available
from 1990 to 2004 in the CM, CE and CF reference frames. At present,
these grid files are NOT non-tidal files - that is, they do
contain deformation associated with partially sampled S1 and S2 tides.
There will be an incremental update of the GAMIT software in the
next month or so (ie hopefully by the end of July) that will
include software and grid files to permit the S1 and S2 tides
to be applied fully in conjunction with the application of
non-tidal loading, thereby completing the whole model.
ATM Loading time series
Here are some example time series of the vertical displacement
caused by atmospheric pressure loading during 2001.
Click on each image to see a larger version.
The effect on geodetic analyses
Time series of atm loading, observed ground pressure, GPS analysis without and with
atm loading applied at the GPS site at Landing Bluff, Antarctica, Kootwijk in Europe
and Kerguelen Island in the Southern Ocean.
Publications
How 7-parameter transformations can affect station height and
geocenter motion estimates from space-geodetic observations
Tregoning, P. and T. van Dam
J. Geophys. Res., 110, B03408, dio:10.1029/2004JB003334, 2005.
pdf
Atmospheric pressure loading corrections applied to GPS data at the observation level
Tregoning, P. and T. van Dam
Geophys. Res. Lett., 32, L22310, doi:10.1029/2005GL024104, 2005.
pdf
GAMIT scripts/code
The atmospheric loading model has been written into the GAMIT
GPS software and was released in version 10.2. The grids that
Tonie van Dam has provided to date do not have the partial
S1 and S2 deformation removed, nor is there code to apply the
periodic deformation fully.
This code has now been written and is undergoing final testing
by Paul Tregoning at The Australian National University. I am
hoping to be able to release this as an incremental update within
a couple of months, along with a whole suite of new, non-tidal
grid files computed by Tonie.
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