Moisture origin of the extreme precipitation event in Western Europe in July 2021

From the 12^th to the 15^th of July 2021, Western Europe was confronted with an abnormal amount of precipitation leading to extreme floods and enormous damage in western Germany, Belgium, Luxembourg, and the Netherlands. Locally, almost thrice as much as the monthly precipitation amount was observed, up to 175 mm in two days. In this study, the atmospheric conditions resulting in this extreme precipitation are investigated, with a focus on understanding the enhanced moisture supply leading to the extreme precipitation amounts. Prior to the event, both the Baltic Sea and the Mediterranean Sea were exceptionally warm, and it was hypothesized that high evaporation rates over these regions could be transported towards western Europe to result in these enormous amount of rain.

To test this hypothesis we apply two state-of-the-art moisture tracking approaches. First, we use the Lagrangian moisture attribution diagnostic WaterSip, based on trajectories computed from ERA5 with the Lagranto trajectory model. Second, we use the Eulerian moisture tracking tool WAM-2layers, applied to ERA5 re-analysis data. Comparison of ERA5 with Eobs shows that the precipitation during the event was captured rather well. The moisture sources are mostly located over the continent, and to a lesser extent over the North and Baltic Seas. There is a height dependency of the atmospheric transport of air and moisture. For the surface layer, the moisture that led to precipitation mainly originated from the region of the Baltic Sea, North Sea, Denmark and Scandinavia. Higher up in the atmosphere, this was changing towards the European continent and parts of the Mediterranean Sea.

In addition, the impact on precipitation by adapting the sea surface temperature (SST) of both the Baltic and the Mediterranean was studied using WRF. This analysis showed that SST changes in the Mediterranean had the largest impact on precipitation in western Europe. Furthermore, our results suggest that the Mediterranean Sea, which had a positive SST anomaly of 2˚C, was the main moisture source preceding the precipitation event.