Attributing floods to anthropogenic climate change using a hydrological model forced with climate simulations under nudged atmospheric circulation

Anthropogenic climate change can affect wide-spread river floods through changes in atmospheric circulation patterns and thermodynamic effects. Changes of atmospheric circulation patterns due to climate change are rather uncertain, while the thermodynamic effects can be simulated more accurately. Here, we employ a storyline approach to attribute the magnitude of recent extreme European floods to the thermodynamic effect of climate change. We use Newtonian nudging to constrain the zonal and meridional winds in simulations of the Community Earth System Model (CESM2) to reanalysis data, to generate factual and counterfactual weather conditions based on historical and pre-industrial greenhouse gases and aerosol concentrations respectively. Downscaled precipitation and temperature, along with observed climatology for leaf area index and fixed land cover from 2009, are used to force the mesoscale Hydrological Model (mHM), thereby simulating counterfactual and historical discharge at 0.125 degree resolution. However, spatial shifts in precipitation extremes observed in nudged circulation simulations present challenges for event attribution, particularly for smaller-scale phenomena. Thus, our focus is on large-scale floods, where we examine changes in flood patterns and explore the intensification of historical events in the context of climate change. By focusing directly on discharge, our approach is closer to actual flood impacts and thus goes beyond traditional flood attribution approaches that usually rely on precipitation extremes only. The presented approach also provides a blueprint for other types of impact attribution using impact models forced with nudged circulation climate model simulations.