Atmospheric drivers of extreme floods

• Applicant: Professor Dr. Bodo Ahrens
• Subject Area: Atmospheric Science
• Term: from 2017 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 278017089

Floods and flood generating mechanisms are investigated in the SPATE project from different perspectives. The sub-project 2 takes the hydroclimatic/-meteorological perspective and focuses on climate and atmospheric drivers of large-scale floods. In phase 1 of SP2, we focused on large-scale heavy rain events with regional and local amplification factors like frontal activity, convective and orographic processes. Phase 2 focuses on process understanding of clustering of rain events (esp. of Vb cyclone type), the concurrence with other flood factors (esp. snowmelt), and the event predictability of 2nd kind (i.e. with change of boundary conditions of the event processes and thus predictability of event probability). Therefore, there is the following overarching objective: What are the climatologies, predictabilities, and possible future changes of the atmospheric flood factors and their climate-scale drivers? We want to understand and quantify, how and why the large- and local-scale atmospheric flood factors have changed in the past. For this objective, we want to identify the role of the low-frequency atmospheric variability, like the wintertime northern annular mode and the North Atlantic oscillation, on the large-scale atmospheric factors and on the local precipitation factors. This very broad objective has to be limited to two, more specific and closely linked research questions:(1) What explains the observed clustering and variability of large-scale rain-on-snow flood events in Central Europe and what can we expect under climate change?Better understanding is necessary if the coincidence of heavy rainfall and snow melting yielding heavy floods and flood clustering occurs by chance or is caused by a hidden driver. This is linked to the question if there are changes in persistency of large-scale circulation patterns. For example, an earlier snowmelt over Eastern Europe, potentially reduces effective continentality, increases zonal advection variability, and, in consequence, increases the possibility for heavy rain and snowmelt coincidence. On the contrary, increasing temperatures due to climate change likely reduce snow cover extent and lead to unfrozen soils year-round in Europe. Thus, the predictive power of 2nd kind of climate drivers shall be investigated. (2) What is the impact of atmospheric blocks on extreme floods in Central Europe and what can we expect under climate change?There are indications that atmospheric blocks yield precipitation anomalies and Vb clustering. Therefore, it is necessary to understand this potentially flood producing process. Additionally, it is important to investigate the predictability of 2nd kind (i.e. the dependence on climate drivers like the arctic oscillation or jet stream) of the blocking-flood chain. Before future change of climate predictability can be investigated, the representation of climate drivers and blocking events in the new CMIP6 global climate simulations have to be evaluated.

• DFG Programme: Research Units
• Subproject of: FOR 2416: Space-Time Dynamics of Extreme Floods (SPATE)