Impact of soil moisture on the heavy precipitation during the July 2021 flood

Backward trajectories have been used to assess the relative importance of the moisture source regions contributing to the heavy precipitation during the July 2021 flooding event. Mohr et al. (2023) suggest that the moisture fueling the extreme event was mostly advected from the North Sea and Baltic Sea, which displayed unusually high sea surface temperatures. In turn, Weijenborg et al. (2022) suggest that the water mostly originated from land surfaces in the proximity of the event. The water, which then would have had to be evaporated over adjacent regions, presumably was available due to high amounts of rainfall leading up to the event. They find smaller contributions from the North and Baltic Seas.

To quantify the input from these source regions towards the heavy precipitation, we perform a sensitivity study. Here, we carry out free ensemble forecasts of the event using the ICON model starting from DWD analysis fields. We then vary the initial conditions with respect to soil moisture. Our sensitivity study can also be seen as a storyline approach, which allows us to assess the role of high soil moisture as a forcing of the heavy precipitation. This is directly related to our work in the collaborative research center DETECT, where we hypothesize that regional changes in land and water use alter the onset and evolution of extremes. This study allows us to test how well we are able to detect changes in the characteristics of the July 2021 flood event based on variations in the terrestrial water budgets.