STARTWAVE

STudies in Atmospheric Radiative Transfer
and Water VApour Effects

Comparison of GPS and ERA40 IWV data

A network of fixed GPS receivers, such as the AGNES (Automated GPS Network of Switzerland) network operated by Swiss Topography, can be used to determine the Integrated Water Vapour. Measurements from the 31 AGNES stations have only been available since November 2000. This is not a long time for determining a water vapour climatology for the Alpine region and is certainly too short to observe any long term changes in water vapour due to climatic forcing. On the other hand, data from the ERA40 reanalysis project are available for the period 1957-2002. This dataset was produced by running a global meteorological model for this period with all available measurements. Integrated Water Vapour is one of the many parameters output by the model. Before using reanalysis data, it is important to validate it against actual measurements, particularly in topographically challenging areas such as the Swiss Alps. We carried out a comparison between GPS and ERA40 IWV for nine Swiss stations for the November 2000 to August 2002 period. The stations lay at altitudes between 330 and 3584 m.

In the plot below, we show an example of the comparison between ERA40 and GPS data at Davos for the months of January and July 2002. The GPS station lies at 1597 m but the height of the ERA40 meteorological model is 1296 m as the model has a relatively low spatial resolution (125 km) and artificially flattens the Alps. Because of the model-station altitude difference, we expect the IWV estimated by the model to be higher than that actually measured at Davos. This is indeed what we see in June 2002. However, in January 2002, due to persistent inversions which were not well modelled, the the ERA40 IWV is somewhat lower than that measured at the GPS station. What is noticeable, is that the ERA40 IWV captures the water vapour variation very well in comparison to the observed IWV.

For the two year comparison, we generally found a very good agreement between ERA40 and GPS IWV. The bias in the ERA40 IWV compared to the GPS IWV was what we would expect from the altitude differences between the model and the GPS stations.

This study is described in: J. Morland, M. A. Liniger, H. Kunz, I. Balin, S. Nyeki, C. Mätzler, N. Käpfer, Comparison of GPS and ERA40 IWV in the Alpine region, including correction of GPS observations at Jungfraujoch (3584 m), in press at Journal of Geophysical Research: Atmospheres.