On the determination of atmospheric water vapour from GPS measurements

Surface based GPS-based measurements of zenith path delay (ZPD) can be used to derive vertically integrated water vapour (IWV) of the atmosphere. ZPD data are collected in a global network presently consisting of 260 stations as part of the International GPS Service. In the present study ZPD data from this network are converted into IWV using observed surface pressure and mean atmospheric column temperature obtained from the ECMWF operational analyses. For the four months of January/July 2000/2001, the GPS derived IWV values are compared to the IWV from the ECMWF operational analyses with a special focus on the monthly averaged difference (bias) and the standard deviation of daily differences. This comparison shows that the GPS derived IWV values are well suited for the validation of operational analyses of IWV. For most GPS stations, the IWV data agree quite well with the analysed data indicating that they are both correct at these locations. Larger differences for individual days are interpreted as errors in the analyses. A dry bias in the winter is found over central USA, Canada and central Siberia suggesting a systematic analysis error. Larger differences were mainly found in mountain areas. These were related to representation problems and interpolation difficulties between model height and station height. In addition, the IWV comparison can be used to identify errors or problems in the observations of ZPD. This includes errors in the data itself, e.g. erroneous outlier in the measured time series, as well as systematic errors that affect all IWV values at a specific station. Such stations were excluded from the intercomparison. Finally long term requirements for a GPS-based water vapour monitoring system are discussed