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 optionsSpatial and temporal characteristics of the signalSummary of conclusionsRecommendation

The GGOS Vision

Geometry, kinematicsGPS, altimetry, INSAR, mobile SLRRemote sensingLevelingTide gauges

Reference frame

VLBI, SLR, LLR, DORIS, PRARE, GPS

Earth RotationSee: reference frame, VLBI, LLR, SLR,GPS, DORISClassical: astronomyFuture: terrestrial gyroscopes

Gravitational fieldOrbit analysisHi-lo & lo-lo SSTSatellite GradiometryShip/air-borne gravimetryAbsolute gravimetryGravity-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.

Access to Reference Frame

Problem: Common Mode Variations (CMV) degrade the accuracy of the access

CMVs are due to: orbit, clocks and ERP errorsunmodeled 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

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)

Surface Loading

Model predictions

Based on: - theory (continuum mechanics) - Earth model - surface loads

Comparison of predictions

Range:-12 to 12 mm

Time:2000.0 to 2004.o

Steps to compute atmospheric loading signal

SLP

T

SUP

REP

PAN

UP

Range of Pressure anomaly

Mean

Std

Maximum

Daily Weekly

mbar

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

Difference between air pressure data sets

Reference surfaces for air pressure

ECMWF: Pressure at sea surfaceNCEP: Pressure at topographic

height

Comparison: at topographic heightResolution: 2.5 x 2.5 degrees

NCEP ref. surf. ECMWF ref. surf.

ECMWF-NCEP

Air pressure anomaly

ECMWF - NCEP

Year 1990

lower left: DOY 021upper right: DOY 121lower right: DOY 171

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

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 => ...