Homogenization of integrated water vapour time series using twin GNSS stations and vertical step offsets

Abstract. Accurate long-term GNSS-derived atmospheric water vapour time series are essential when used in climate research. However, these records are frequently affected by interventions, e.g., change of hardware at the station. We investigate the homogenization of integrated water vapour (IWV) time series using two GNSS stations on site. The ERA5 reanalysis product from European Centre for Medium-Range Weather Forecasts (ECMWF) was used as a reference for validation. The results demonstrate that estimating step offsets at the time epoch of the interventions based on the IWV differences between the co-located (twin) GNSS stations significantly improve the agreement with IWV trends from the ERA5 reanalysis. In addition, we find a strong negative correlation coefficient, up to −0.92, between step offsets in a single station's vertical component and the corresponding IWV step offsets derived from the twin stations. A 1 mm step offset in the vertical component corresponds to an IWV step offset of approximately 0.03 kg m⁻². Such a relationship can be used as a tool for the homogenization of IWV time series for single stations.