AbstractGround‐based receiver networks of the Global Positioning System (GPS) provide observations of atmospheric water vapour with a high temporal and horizontal resolution. Variational data assimilation allows researchers to make use of zenith total delay (ZTD) observations, which comprise the atmospheric effects on microwave signal propagation. An observing‐system experiment (OSE) is performed to demonstrate the impact of GPS ZTD observations on the output of the High Resolution Limited Area Model (HIRLAM). The GPS ZTD observations for the OSE are provided by the EUMETNET GPS Water Vapour Programme, and they are assimilated using three‐dimensional variational data assimilation (3D‐Var).The OSE covers a five‐week period during the late summer of 2008. In parallel with GPS ZTD data assimilation in the regular mode, the impact of a static bias‐correction algorithm for the GPS ZTD data is also assessed.Assimilation of GPS ZTD data, without bias correction of any kind, results in a systematic increase in the forecast water‐vapour content, temperature and tropospheric relative topography. A slightly positive impact is shown in terms of decreased forecast‐error standard deviation of lower and middle tropospheric humidity and lower tropospheric geopotential height. Moreover, verification of categorical forecasts of 12 h accumulated precipitation shows a positive impact. The application of the static bias‐correction scheme is positively verified in the case of the mean forecast error of lower tropospheric humidity and when relatively high precipitation accumulations are considered. Copyright © 2010 Royal Meteorological Society