Investigation of Medicane Daniel Evolution Based on GNSS-Derived Integrated Water Vapor

Extreme weather events have become more frequent and severe, making nowcasting increasingly important. Integrated Water Vapor (IWV) estimates from global navigation satellite systems (GNSS) are accurate and provide high spatiotemporal resolution. This study investigates GNSS–IWV evolution and regional variability in relation to extreme rainfall for the first time during Storm Daniel from 4 to 12 September 2023, one of the most destructive Mediterranean cyclones of recent decades, which caused heavy rainfall in Thessaly, Central Greece and the Peloponnese. GNSS-IWV data were compared with rainfall observations, while ERA5 reanalysis data were additionally used due to GNSS data gaps caused by a temporal unavailability of the data-processing server. Strong agreement was found between GNSS- and ERA5-derived IWV (mean correlation coefficient (r) of 0.96 and mean root mean square error (RMSE) of 3.60 mm), whereas weaker temporal agreement was observed between meteorological station rainfall observations and ERA5-derived total precipitation (mean r = 0.35; mean RMSE = 2.90 mm). GNSS-IWV systematically increased before heavy rainfall, consistent with ERA5–IWV patterns. ERA5–IWV peaks occurred 1–5 h before ERA5 precipitation maxima in Thessaly and 1–3 h before rainfall peaks in Central Greece and the Peloponnese. Spatial analysis showed high GNSS-IWV values over affected regions, reaching 46–48 mm during the storm peak. These findings support GNSS-IWV as a potential high-resolution indicator for short-term monitoring in Mediterranean storm events.