1. A proposal for a consistent model of air pressure loading as part of the International Terrestrial Reference System (ITRS) Conventions Plag, H.-P. (1), van Dam, T. (2), Blewitt, G. (1), Kierulf, H. P. (3) (1) Nevada Bureau of Mines and Geology and Seismological Laboratory, University of Nevada, Reno, Nevada, USA (2) European Center for geodynamics and Seismology, Luxembourg (3) Norwegian Mapping Authority, Hønefoss, Norway A contribution of the GGFC Special Bureau for Loading Overview: Introduction: The goal and the options Spatial and temporal characteristics of the signal Summary of conclusions Recommendation

2. The GGOS Vision Geometry, kinematics GPS, altimetry, INSAR, mobile SLR Remote sensing Leveling Tide gauges Reference frame VLBI, SLR, LLR, DORIS, PRARE, GPS Earth Rotation See: reference frame, VLBI, LLR, SLR,GPS, DORIS Classical: astronomy Future: terrestrial gyroscopes Gravitational field Orbit analysis Hi-lo & lo-lo SST Satellite Gradiometry Ship/air-borne gravimetry Absolute gravimetry Gravity-recording (modified from Rummel, 2000) Introduction Goal: Consistent treatment of surface loading signals in reference frame determination and in access to the frame Accuracy: 1E-9 or better Loading impacts: station motion, gravity field, Earth rotation.

3. Access to Reference Frame Problem: Common Mode Variations (CMV) degrade the accuracy of the access CMVs are due to: orbit, clocks and ERP errors unmodeled variations in the station coordinates: - ocean tidal loading - atmospheric loading - non-tidal ocean loading - hydrological loading (continental hydrosphere) - cryospheric loading non-linear motion at the reference sites (geodynamics) Traditionally through positioning relative to reference points Recently ad hoc through (highly accurate) satellite orbits and clocks as well as Earth rotation parameters

4. Current focus: Atmospheric loading Option 1: Inclusion of model predictions on the observation level in the analysis Option 2: Inclusion during the combination/alignment of solution to ITRF (correction of the reference time series after the analysis)

5. Surface Loading Model predictions Based on: - theory (continuum mechanics) - Earth model - surface loads

6. Comparison of predictions Range: -12 to 12 mm Time: 2000.0 to 2004.o

7. Steps to compute atmospheric loading signal SLP T SUP REP PAN UP

8. Range of Pressure anomaly Mean Std Maximum Daily Weekly mbar

9. Decadal variability of Surface Pressure 1960-1969 1970-1979 1980-1989 1990-1999 Differences between Decadal Mean and Long-term Mean Range: -4 to 4 mbar Left: Mean 1958 - 2002

10. Difference between air pressure data sets Reference surfaces for air pressure ECMWF:Pressure at sea surface NCEP: Pressure at topographic height Comparison: at topographic height Resolution:2.5 x 2.5 degrees NCEP ref. surf. ECMWF ref. surf. ECMWF-NCEP

11. Air pressure anomaly ECMWF - NCEP Year 1990 lower left:DOY 021 upper right:DOY 121 lower right: DOY 171

12. Summary * Largest uncertainties: - pressure field itself - ocean response - estimated total uncertainty in vertical: +/- 3 mm (+/- 5 mm) * Decadal variability: order of 3 to 4 mbar (1 to 2 mm) * Weekly variability: - mean range: 2... 35 mbar - maximum range: 5... 90 mbar * Daily variability: - mean range: 2... 10 mbar - maximum range: 3... 50 mbar

13. Recommendations * Surface load: one pressure field should be recommended. * Daily analyses: Post-correction may be sufficient in most cases (some extremes will cause significant errors). * Weekly analyses: Loading signal should be included in station motion model/combination. * Reference frame alignment/combination: loading needs to be accounted for. To take into account the uneven station distribution and associated uncertainties in frame origin, it is suggested to use =>...