Global Characterization of IWV Diurnal Variability from GNSS and Its Relevance to ERA5 Reanalysis Products

The diurnal variability of Integrated Water Vapor (IWV) plays an important role in land–atmosphere coupling, convection initiation, and the diurnal water cycle, yet its global observational characterization remains limited. Global Navigation Satellite Systems (GNSS) observations provide a unique capability for resolving IWV diurnal variability through continuous, all-weather, high–temporal-resolution measurements with long-term stability. In this study, we analyze a decade of GNSS-derived IWV observations from a global network of thousands of stations to characterize the climatological features of the IWV diurnal cycle. The analysis focuses on the spatial structure and harmonic characteristics of sub-daily IWV variability across different latitude bands and climate regimes. The results reveal a coherent global diurnal signal, with systematic variations in amplitude and phase that exhibit strong geographic dependence. In addition, we examine the representation of IWV diurnal variability in the ERA5 reanalysis by analyzing temporal features in ERA5 IWV time series and their potential influence on estimated diurnal harmonics. The comparison highlights the importance of accounting for reanalysis-related temporal artifacts when interpreting sub-daily variability. Based on the unique strengths of long-term, globally distributed GNSS observations, this work provides a robust observational framework for studying IWV diurnal variability and offers methodological insight for evaluating reanalysis and satellite-based water vapor products. The results are relevant for studies of atmospheric processes operating at sub-daily timescales and for the interpretation of water vapor observations from observing systems with limited temporal sampling.