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. Western Norway suffered major flooding after 4 d ofintense rainfall during the last week of October 2014. While events likethis are expected to become more frequent and severe under a warmingclimate, convection-permitting scale models are showing their skill with respect to capturing their dynamics. Nevertheless, several sources of uncertainty needto be taken into account, including the impact of initial conditions onthe precipitation pattern and discharge, especially over complex, mountainousterrain. In this paper, the Weather Research and Forecasting ModelHydrological modelling system (WRF-Hydro) is applied at aconvection-permitting scale, and its performance is assessed in westernNorway for the aforementioned flood event. The model is calibrated andevaluated using observations and benchmarks obtained from the HydrologiskaByrans Vattenbalansavdelning (HBV) model. The calibrated WRF-Hydro modelperforms better than the simpler conceptual HBV model, especially in areaswith complex terrain and poor observational coverage. The sensitivity of theprecipitation pattern and discharge to poorly constrained elements such asspin-up time and snow conditions is then examined. The results show the following: (1) the convection-permitting WRF-Hydro simulation generally captures theprecipitation pattern/amount, the peak flow volume and the timing of theflood event; (2) precipitation is not overly sensitive to spin-up time, whereas discharge is slightly more sensitive due to the influence of soil moisture,especially during the pre-peak phase; and (3) the idealized snow depthexperiments show that a maximum of 0.5 m of snow is converted to runoffirrespective of the initial snow depth and that this snowmelt contributes todischarge mostly during the rainy and the peak flow periods. Although furthertargeted experiments are needed, this study suggests that snow coverintensifies the extreme discharge instead of acting as a sponge, whichimplies that future rain-on-snow events may contribute to a higher flood risk.

The impact of initial conditions on convection-permitting simulations of a flood event over complex mountainous terrain