How uncertain are precipitation and peak flow estimates for the July 2021 flooding event?

The disastrous July 2021 flooding event made us question the ability of current hydrometeorological tools in providing timely and reliable flood forecasts for unprecedented events. This is an urgent concern since extreme events are increasing due to global warming, and existing methods are usually limited to more frequently observed events with the usual flood generation processes. For the July 2021 event, we simulated the hourlystreamflows of seven catchments located in western Germany by combiningseven partly polarimetric, radar-based quantitative precipitation estimates(QPEs) with two hydrological models: a conceptual lumped model (GR4H) and aphysically based, 3D distributed model (ParFlowCLM). GR4H parameters werecalibrated with an emphasis on high flows using historical dischargeobservations, whereas ParFlowCLM parameters were estimated based onlandscape and soil properties. The key results are as follows. (1) With nocorrection of the vertical profiles of radar variables, radar-based QPEproducts underestimated the total precipitation depth relative to raingauges due to intense collision-coalescence processes near the surface, i.e., below the height levels monitored by the radars. (2) Correcting the vertical profiles of radar variables led to substantial improvements. (3) The probability of exceeding the highest measured peak flow before July 2021 was highly impacted by the QPE product, and this impact depended on the catchment for both models. (4) The estimation of model parameters had alarger impact than the choice of QPE product, but simulated peak flows ofParFlowCLM agreed with those of GR4H for five of the seven catchments. Thisstudy highlights the need for the correction of vertical profiles ofreflectivity and other polarimetric variables near the surface to improveradar-based QPEs for extreme flooding events. It also underlines the largeuncertainty in peak flow estimates due to model parameter estimation.