A Global Assessment of Diurnal Discontinuities in ERA5 Tropospheric Zenith Total Delays Using 10 Years of GNSS Data

The diurnal cycle of tropospheric Zenith Total Delays (ZTD) is critical for refining tropospheric models and understanding key geophysical and atmospheric processes. The fifth European Centre for Medium-Range Weather Forecasts reanalysis ERA5, with its unique 1-hr temporal resolution, provides valuable data for these purposes. However, we identified obvious discontinuities in ERA5-derived ZTD at 09:00 and 21:00 UTC. A comparison with 10-year GNSS data from 6,437 stations showed that 32.8% of the stations experienced discontinuities in mean diurnal anomalies of ERA5 ZTD, with an average magnitude of 2.2 mm, far exceeding the 0.3 mm hour-to-hour variations observed by GNSS. The discontinuities are more pronounced in summer and are primarily due to errors in ERA5's humidity modeling, with a few pressure-related issues limited to Antarctica. The discontinuity is attributed to the transition between ERA5's 12-hr assimilation windows and suggest that other ERA5 variables may also be impacted, warranting caution in ERA5-based diurnal cycle studies.