Abstract. Western Norway suffered major flooding after 4 d of
intense rainfall during the last week of October 2014. While events like
this are expected to become more frequent and severe under a warming
climate, convection-permitting scale models are showing their skill with respect to capturing their dynamics. Nevertheless, several sources of uncertainty need
to be taken into account, including the impact of initial conditions on
the precipitation pattern and discharge, especially over complex, mountainous
terrain. In this paper, the Weather Research and Forecasting Model
Hydrological modelling system (WRF-Hydro) is applied at a
convection-permitting scale, and its performance is assessed in western
Norway for the aforementioned flood event. The model is calibrated and
evaluated using observations and benchmarks obtained from the Hydrologiska
Byråns Vattenbalansavdelning (HBV) model. The calibrated WRF-Hydro model
performs better than the simpler conceptual HBV model, especially in areas
with complex terrain and poor observational coverage. The sensitivity of the
precipitation pattern and discharge to poorly constrained elements such as
spin-up time and snow conditions is then examined. The results show the following: (1) the convection-permitting WRF-Hydro simulation generally captures the
precipitation pattern/amount, the peak flow volume and the timing of the
flood 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 depth
experiments show that a maximum of 0.5 m of snow is converted to runoff
irrespective of the initial snow depth and that this snowmelt contributes to
discharge mostly during the rainy and the peak flow periods. Although further
targeted experiments are needed, this study suggests that snow cover
intensifies the extreme discharge instead of acting as a sponge, which
implies that future rain-on-snow events may contribute to a higher flood risk.