Assessment of Using GNSS for the Monitoring of the Atmospheric Water Vapour Content over Long Time Scales
Elgered, G.
Chalmers University of Technology

Radio based space geodetic methods are affected by the water vapour in the atmosphere. The velocity of the propagating signal is reduced, depending on the value of the refractive index. The atmospheric water vapour content, sometimes also called Integrated Precipitable Water Vapour (IPWV), can be inferred from the estimated propagation delay, or the excess propagation path often expressed in units of length. The observations are relative measurements of time, which makes the methods interesting from a calibration point of view — since time is the physical parameter that we can measure with the highest accuracy.

Water vapour is difficult, and costly, to measure with a high temporal and spatial resolution. Given its characteristics of variability, researchers in the atmospheric sciences have shown interest in using data from ground-based GPS receivers. Time series of the IPWV from specific sites are now longer than ten years. For example, 20 sites in the Swedish GPS network have produced continuous data since 1993/1994. In addition to GPS also additional global navigational satellite systems (GNSS), such as the European Galileo and the finalization of the Russian GLONASS, will in the future significantly improve the spatial sampling of the atmosphere, and also reduce the relative influence of orbit errors for individual satellites.

Using ground-based GPS data acquired in Sweden and Finland over a ten year period results in estimated linear trends in the IPWV ranging from –0.05 mm/yr to +0.10 mm/yr. The uncertainties in the trends are estimated to be approximately 0.04 mm/year. It is certainly worth noting that estimated trends for summer and winter data are significantly different and varies between different regions. The same is true for trends estimated from different time periods, obviously due to the large variability in weather from one year to another.

IPWV results from GPS data analysis have also been compared to Numerical Weather Models (NWM) using data from 1997 to 2002 (inclusive). The NWMs are from the European Centre for Medium Range Weather Forecasting (ECMWF) re-analysis and two different climate models from the Rossby Centre at the Swedish Meteorological and Hydrological Institute (SMHI). Typical IPWV bias values between GPS and the NWMs are less than 1 mm. There is a tendency that the models are wetter than are the GPS results. It is also interesting to note that the standard deviation for the time series of the differences of the monthly means are typically less than 1 mm. I will review results obtained form comparisons between ground-based GPS data and those from other independent techniques, such as radiosondes, microwave radiometry, and geodetic Very-Long-Baseline Interferometry (VLBI). While doing that I will focus on systematic effects that could alias with true trends in the atmospheric water vapour content. Potential causes of such effects are: changes of antennas — with different phase patterns — in both the ground-based network as well as when new satellites are taken into operation, a slowly varying electromagnetic environment at a receiver site (which can induce varying multi-path effects), slowly varying unmodelled third-order effects in the ionospheric propagation delay, and variations in the satellite constellation changing the distribution of observations as a function of elevation angle.

The overall goal for the possible use of GNSS data in climate research is to determine to which extent these independent data can be used to discriminate between different climate models — both in terms of absolute values as well as long term trends — thereby improving the quality of the models and increasing the probability to produce realistic scenarios for the future climate.