Monitoring IWV from GPS and limited - area forecast model

The GPS Networks of Germany and Switzerland, operated by the GeoForschungsZentrum and Swiss Federal Office of Topography, provide Zenith Total Delay (ZTD) measurement from more than 100 permanent GPS sites. Integrated Water Vapour (IWV) is extracted and compared to IWV derived from a non-hydrostatic mesoscale model Local Model (LM), used for operational numerical weather forecast in Germany and Switzerland. For the period April to December 2001 GPS IWV is compared against IWV fields from LM. Special attention is paid to the difference between GPS and model data in the day-to-night behaviour.