Moisture sources of extreme precipitation events in Northern Europe

In August 2023, an extreme precipitation event named Storm Hans occurred in Northern Europe, which produced flooding and landslides in southeastern Norway and large parts of Sweden, resulting in casualties and considerable infrastructural damage. We used a Lagrangian moisture tracking algorithm and a global Lagrangian reanalysis dataset to identify the moisture source regions that contributed to precipitation during Storm Hans. Additionally, we applied the moisture tracking algorithm over an 83-year period to compare climatological patterns with key source regions for extreme precipitation events in southeastern Norway and Sweden. For Storm Hans, a temporal evolution of moisture uptake regions indicates a shift of origins for the different phases of the event. Eastern Europe contributed the most moisture during the first phase of the event, which occurred between August 6 and 8. During the second phase between 9 and 10 August, moisture sources were mostly located in the Atlantic, the precipitation region, the North Sea, the Baltic Sea, and Eastern Europe. Overall, the majority of the moisture came from Eastern Europe, which is rare for extreme precipitation events that occurred in southeastern Norway and Sweden. This case study of the extreme event in August 2023, along with the climatological analysis, helps in determining which processes are most important for these kinds of events. Identifying recurring pathways, key source regions, and their trends can further help climate and forecast model evaluation and development by pointing out areas where land-atmosphere coupling or transport requires better parameterizations.