Dynamics, predictability, impacts, and climate change considerations of the catastrophic Mediterranean Storm Daniel (2023)

Abstract. In September 2023, storm Daniel formed in the centre of the Mediterranean Sea as an intense Mediterranean cyclone. Its formation was accompanied by significant socioeconomic impacts in Greece including several fatalities and severe damages to agricultural infrastructures. Within a few days, the cyclone evolved into a tropical-like storm, i.e., medicane, that made landfall in Libya, probably marking the most catastrophic and lethal weather event that was ever documented in the region. In this study, we place storm Daniel as the centrepiece of the catastrophic events in Greece and Libya. We thus consider that there is a direct link between the atmospheric processes that turned Daniel into a catastrophic storm and the actual socioeconomic impacts that a single weather system has produced in the two countries. We perform a holistic analysis that articulates between atmospheric dynamics, precipitation extremes, and quantification of impacts, i.e., floods and sea state. This is done by taking into account the predictability of Daniel at weather scales and the attribution of impacts to climate change.

Our results show that Daniel initially formed like any other intense Mediterranean cyclone. At this stage, the cyclone produced significant socioeconomic impacts on Greece, in an area far from the cyclone centre. In later times, Daniel attained tropical-like characteristics while gradually reaching its maximum intensity. Impacts over Libya coincided with the cyclone's landfall at its maturity stage. The predictability of the cyclone formation was rather low even in  relatively short lead times -of the order of four days- while higher prediction skill was found when addressing the landfall in Libya for the same lead times. Our analysis of impacts shows the adequate capacity of numerical weather forecasting to capture the extremeness of precipitation amounts and floodings in Greece and Libya.

Therefore, state-of-the-art numerical weather prediction has provided information on the severity of the imminent flood events. We also analyse the moisture sources contributing to extreme precipitation. Results show that moisture sources were majorly driven by large-scale atmospheric circulation, while in maturity, Daniel drew substantial amounts of water vapor from local maritime areas within the Mediterranean Sea. In a climatological context, Daniel was indeed shown to produce extreme precipitation amounts, and our analysis allows us to interpret Daniel's impacts as an event whose characteristics can be ascribed to human-driven climate change.