The primary objectives of this research were to investigate the potential of precipitable water vapour (PWV) estimates derived from Global Navigation Satellite Systems (GNSS) measurements, firstly, for short-term weather forecasting based on numerical weather prediction (NWP) in Luxembourg and its surroundings and, secondly, for monitoring climate on regional and global scales. The suitability of real-time (RT) zenith total delay (ZTD) estimates obtained from three different precise point positioning (PPP) software packages was assessed by comparing them with the state-of-the-art product from the International GNSS Service (namely the IGS Final troposphere product) as well as collocated radiosonde (RS) observations. It was found that the RT-PPP ZTD estimates from two of the three software packages meet the threshold requirements for NWP nowcasting. The biases between the RT-PPP ZTD and the reference ZTD were found to be stable over time for all the RT-PPP ZTD solutions. A millimetre-level impact on the RT-PPP ZTD estimates was also observed when integer ambiguities were resolved. The impact of assimilating GNSS-derived near real-time (NRT) ZTD in the Applications of Research to Operations at Mesoscale (AROME) NWP model using a three-dimensional, variational (3D-VAR) assimilation scheme on the quality of weather forecasts for Luxembourg was studied. It was found that the assimilation of GNSS-derived ZTD systematically improves the atmospheric humidity short-range forecasts in comparison to other water vapour observing systems (radio soundings, satellite radiances, surface networks). Examination of several case studies revealed the ability of the ZTD observations to modify the intensity and location of predicted precipitation in accordance with previous studies. The addition of ZTD from the dense GNSS network in Wallonie (Belgium) was also found to be beneficial by improving the prediction of rainfall patterns in Luxembourg. The 2D maps of IWV obtained from the hourly NRT system were compared with cloud ...