Precipitable Water Vapour Estimation from GNSS Observations: Methodology and Evaluation during a High-Precipitation Month in Santa Maria (RS)

Abstract. Precipitable Water Vapour (PWV) represents the total amount of water vapour contained in a vertical column of the atmosphere that is available for precipitation. It is a key variable in meteorological studies, as well as in the monitoring of extreme weather events. This study proposes and evaluates a method for estimating PWV using Global Navigation Satellite System (GNSS) data (PWV-GNSS), obtained from the Zenith Total Delay (ZTD), which comprises the Zenith Hydrostatic Delay (ZHD) and the Zenith Wet Delay (ZWD). The approach combines data from GNSS stations with external meteorological sources to improve atmospheric monitoring systems. The evaluation was conducted in the region of Santa Maria, Brazil (RS), selected due to the proximity (~2km) between the radiosonde (SBSM), GNSS (SMAR), and meteorological (A803) stations and its variable climate, characterised by frequent precipitation events. As a case study, the month of September 2023 was analysed, during which accumulated rainfall exceeded 450 mm. Results indicated strong agreement between PWV estimates derived from GNSS (PWV-GNSS) and radiosonde observations (PWV-RDS) used as a reference, with a root mean square error (RMSE) of 2.37 mm, a bias of –1.90 mm, and a coefficient of determination (R²) of 0.97 between PWV-RDS and PWV-GNSS. Furthermore, PWV-GNSS demonstrated a characteristic response to heavy rainfall events, with reductions of up to 40 mm following intense precipitation.