Reconciling the moisture sources of the extreme flood event in July 2021 over western Europe

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 south of The Netherlands. Locally, almost thrice as much as the monthly precipitation amount was observed, culminating in 175 mm of rain in just two days. Dynamically, a stationary upper-level cut-off low was the driver of moisture transport to the region resulting in the extreme precipitation over a large area. A follow-up step to unravel the hydrometeorology of the event, is to understand the evaporative regions (moisture sources) that contributed to the event. In literature, these different source region contributions were presented, either indicating the importance of transpiration from vegetation over North America and Europe, or highlighting the role of the Baltic sea, which experienced a heatwave and high evaporation rates at the same time.

Here, we reconcile the moisture sources of the flood event in July 2021 and its uncertainties by comparing the results from three different moisture tracking models (WaterSip, HAMSTER & WAM-2layers) forced with ERA5. By further addressing model-internal sensitivities, we (can) provide a thorough estimate of the uncertainty of contributions from different regions to precipitation during the extreme event, and we ascertain the mechanisms that played a role. Our first results confirm that central Europe is the largest contributor of moisture for precipitation during the event (45 – 90%), whereas the Baltic contributed very little (0 – 5%), thereby contrasting results from recent single-model studies. However, substantial differences were found between the moisture tracking models indicating the need to better understand where those difference arise from and employ multi-model moisture tracking intercomparison studies in the future.