Extreme Weather Events and Atmospheric Water Vapor Trends from Homogenized GNSS Tropospheric Observations over Türkiye

Atmospheric water vapour plays a critical role in climate change and in the occurrence of hydro-climatic extreme weather events; however, its long-term monitoring is subject to considerable uncertainties. GNSS-derived tropospheric products represent an independent, high-temporal-resolution observational data source capable of addressing this gap. Nevertheless, the reliable use of these data in climate analyses requires the identification and removal of potential inhomogeneities related to instrumentation and processing changes.

In this study, atmospheric water vapour variability, long-term trends, and extreme moisture conditions over Türkiye are investigated using GNSS Zenith Total Delay (ZTD) time series. The analyses primarily employ GNSS tropospheric products reprocessed by the University of Nevada, Reno (UNR). Station-based homogenization is applied to all time series to eliminate artificial discontinuities and to ensure their suitability for climate analysis. Integrated Water Vapour (IWV) is derived using consistent meteorological inputs, and trend behaviour is assessed using robust non-parametric methods. Hydro-climatic extremes are defined based on percentile-based thresholds (P10 and P90).

Selected long-term GNSS stations are further examined to assess the sensitivity of the results to different processing strategies using IGS Repro3 solutions. Radiosonde observations are used to evaluate the physical consistency of GNSS-derived IWV, while ERA5 reanalysis data provide a reference for comparison and contextual interpretation. The results indicate that consistent long-term trends and changes in extreme moisture conditions can be robustly identified in homogenized GNSS IWV series, including shifts in the frequency of extreme weather conditions. Furthermore, GNSS observations are shown to capture rapid moisture variations more clearly than reanalysis products, in which such signals are often smoothed.

This study highlights the contribution of homogenized GNSS tropospheric observations to monitoring atmospheric water vapour variability and hydro-climatic extremes over Türkiye.