GPS water vapour estimates compared with a NWP model within the BALTEX region

Water vapour is a crucial parameter in short term numerical weather prediction due to its high variability. Two networks of Global Positioning System receivers within the BALTEX area are used to compare the water vapour derived from GPS data with the hydrostatic numerical weather forecast model HRM (High resolution Regional Model) of the German Weather Service (DWD): (1) the SWEPOS network covering Sweden (together with 5 stations belonging to Finland) during PIDCAP (Pilot Study for Intensive Data Collection and Analysis of Precipitation; August 1 to November 17, 1995) and (2) the network of the GeoForschungsZentrum, Potsdam, covering Germany, between May and December 2000. The mean values of the differences between the vertically integrated water vapour (IWV) of the HRM model and the GPS IWV data are below 1 kg/m² for both datasets, mainly caused by the difference of the analysis data used as initial fields for the HRM model and the GPS IWV. The standard deviations are about 2.5 kg/m² and increase with increasing forecast time due to the reduced influence of the analysis data