Loading of the Earth's crust due to the redistribution of global air masses and associated pressure changes can displace the positions of geodetic sites by more than 1 cm both vertically and horizontally. The displacements are mainly dominated by the effects of synoptic scale systems with wavelengths of 1000 to 2000 km and periods of approximately two weeks. As can be seen from the figure below, peak-to-peak vertical displacements of 10 to 20 mm are common at mid-latitude regions, where large amplitude pressure systems can be found.

Deformation of the Earth's surface due to atmosphere pressure loading for 1 August 2008, 6 h UTC. Units: [mm]. |

Atmosphere pressure loading (APL) effects on geodetic sites have been observed in high-precision space geodetic data such as Global Positioning System data (van Dam et al., 1994) and Very Long Baseline Interferometry data (van Dam and Herring, 1994). As these observations are primarily used for geodynamic studies, i.e. plate tectonics, Earth rotation, post-glacial rebound, it is important to remove the displacement signals due to APL from the data. Thus, a rigorous computation of the displacements is imperative.

In the APL part of GGOS Atmosphere we focused on developing a standard procedure for the calculation of the displacements and on providing accurate correction values that would be suitable for routine analysis of space geodetic observations. In the course of this, we addressed the following aspects:

- Definition of the Green's functions and reference frames. A proper Earth model was selected and the Earth's loading response, which is best described by Load Love Numbers (LLN), was determined thereupon. The LLN values, which in turn are important in the definition of the Green's function, are specified both in the center of mass (CoM) and the center of Earth (CoE) reference frames.
- Computational aspects of the Green function convolution: An accurate method to calculate the mean value of the Green function integral proposed by Hirt et al. (2011) is applied. The method can avoid singularity problems.
- Inverted Barometer (IB) correction: The modified IB assumption proposed by van Dam & Herring (1994) was adopted.
- Definition of the land-sea mask: When the IB correction is applied, information of the land and oceanic parts are important in the calculation of the displacement. An accurate, steady-state land-sea mask was determined from ETOPO5 data.
- Modeling tidal and non-tidal pressure deformations: As we routinely use the 6-hourly fields of the ECMWF data, inadequately represented diurnal (S1) and semi-diurnal (S2) tidal atmospheric pressure effects have to be removed. Mean diurnal, semi-diurnal signals, and ter-diurnal (S3) global grids were developed using 3-hourly analysis/forecast pressure fields from 5 years of ECMWF operational delayed cut-off data. The results from this task are also important for the AGC part, since S1, S2, and S3 pressure tidal models are crucial for the determination of gravity corrections.

D.D. Wijaya, J. Böhm, M. Karbon, H. Krásná, H. Schuh. Atmospheric pressure loading, in J. Böhm and H. Schuh (eds): Atmospheric Effects in Space Geodesy, Springer Verlag, pp. 137-157, doi:10.1007/978-3-642-36932-2_4, 2013.

L. Petrov, J.-P. Boy. Study of the atmospheric pressure loading signal in very long baseline Interferometry observations, J. Geophys. Res., 109, B03405, doi:10.1029/2003JB002500, 2004.

T.M. Van Dam, T.A. Herring. Detection of atmospheric pressure loading using very long baseline Interferometry measurements, J. Geophys. Res., 99, 4505-4517, 1994.

T.M. Van Dam, G. Blewitt, M.B. Heflin. Atmospheric pressure loading effects on Global Positioning System coordinate determinations, J. Geophys. Res., 99, 23939-23950, 1994.

C. Hirt, W.E. Featherstone, S.J. Claessens. On the accurate numerical evaluation of geodetic convolution integrals, J. Geod., 85 (8), pp. 519-538, doi:10.1007/s00190-011-0451-5, 2011.