Global GNSS IWV diurnal variability over 6,000 stations and its implications for assessing the impact of artificial ERA5 discontinuities

Abstract

The diurnal cycle of Integrated Water Vapor (IWV) plays a key role in radiation, convection, and land–atmosphere interactions, yet its global characteristics and representation in atmospheric reanalysis products remain insufficiently quantified. Using a decade of GNSS observations from more than 6,000 stations, this study characterizes the global IWV diurnal variability and assesses the performance of ERA5 in reproducing these signals. GNSS IWV exhibits a coherent diurnal cycle dominated by the S₁ (24 h) harmonic, with annual-mean amplitudes decreasing from 3 kg m⁻² in the tropics to near zero at high latitudes, a global mean of 0.40 kg m⁻², and peak times clustered around 17.4 local time; the semidiurnal S₂ (12 h) component is weaker but similarly systematic. In contrast, ERA5 exhibits sharp artificial discontinuities at the transition times (09–10 and 21–22 UTC) of its 12-hour assimilation windows, affecting 54% of stations and reaching magnitudes that are 146% of the standard deviation of observed diurnal anomalies. These discontinuities distort the estimated diurnal cycle, producing a median S₁ phase lag of 1.1 h relative to GNSS and weakening S₂ amplitude agreement, with the coefficient of determination (R²) decreasing from 0.48 to 0.38. The results highlight both the robustness of GNSS IWV as a benchmark and the need for caution when using ERA5 for diurnal-cycle applications.

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