Synoptic analysis and numerical modelling of the storm Daniel and its extreme precipitation over Greece in September 2023

The Mediterranean basin is frequently affected by various intense and high-impact weather phenomena that pose significant threat to life, property and the environment. Cyclone Daniel developed in the Ionian Sea in early September 2023 and strongly influenced the southern Balkans and Libya with thousands of deaths and a cost of billions of euros. This study focuses on the extreme precipitation that affected Greece due to Daniel, causing unprecedented floods and claiming 17 lives in this country. Extreme rainfall amounts were recorded at various Greek stations from 4 to 7 September, locally exceeding 1000 mm in four days. Severe flooding, livestock deaths, damages and economic losses occurred primarily in the region of Thessaly, central Greece. 

The formation of Daniel over the Ionian Sea was associated with the penetration of an upper air cold trough at the eastern flank of an omega blocking. The phase space diagrams show that the cyclone exhibited a symmetric deep warm-core near Libya. Large moisture amounts were transported from the Black Sea and the Aegean Sea to the eastern mainland of Greece, due to the combination of Daniel with an anticyclone over eastern Europe. Additionally, the sea surface temperatures (SSTs) were warmer than the climatology of September 1981-2020, by more than 2°C and 3°C in large parts of the Aegean Sea and Black Sea, respectively. 

The study aims to investigate (a) the synoptic conditions that influenced the evolution of Daniel and the occurrence of extreme precipitation over Greece and (b) their sensitivity to the SSTs. The non-hydrostatic Weather Research and Forecasting numerical model has been employed at high resolution over the area of interest. Sensitivity experiments have been performed with spatially uniform SST anomalies, climatological SSTs and without surface fluxes of enthalpy, in order to understand their role in the development of Daniel and the intensity of the precipitation event. The numerical experiments suggest that the warm SSTs were important for the high intensity of the precipitation over Greece, but large precipitation amounts would occur even with colder climatological SSTs.

Acknowledgments: This research was carried out in the project “Valorization and dissemination of research results on weather analysis and forecasting” (project number 76675 of the Special Account for Research Funds of the Aristotle University of Thessaloniki, Greece). Results presented in this work have been produced using the Aristotle University of Thessaloniki (AUTh) High Performance Computing Infrastructure and Resources.