Extreme events are often the result of interactions between large-scale atmospheric patterns and local scale features, e.g. orography. Traditional approaches to study extreme events typically focus on the importance of either the large scale (e.g. atmospheric rivers) or local scale dynamics. Multi-scalar dynamics investigates the full range of scales, taking a more holistic approach to studying extreme events. For example, a study (Hughes et al., 2014) of atmospheric rivers impinging upon western America showed that while changes in the angle of incidence of the atmospheric rivers resulted in only small changes (~6%) to the precipitation on regional scales, they caused large increases (~30%) in extreme precipitation in individual catchment zones. This was due to the local-scale alignment of the moist air-flow with the slopes of the orography.

Similarly, extreme precipitation in western Norway is often associated with the onshore flow of a moist jet of air interacting with local orography. One example of this situation is the severe flooding that occurred in Voss on the 28th of October 2014. A previous study of this event by Pontoppidan et al. (2017) suggests that the extreme precipitation was a result of the dynamical interactions between the air mass and the terrain. This case study will form the basis of our analysis on multi-scalar dynamics.

Our hypothesis is that the dynamics across scales are important to the occurrence of the event. Two different approaches will be used to test this hypothesis. The first approach will analyse existing datasets (e.g., ERA5) to identify and investigate occurrences of the same large-scale dynamics as occurred during the 2014 flood event in Voss that may/may not have been accompanied by flooding due to the presence/absence of the local scale interactions. The second approach will build on existing work undertaken by Pontoppidan et al. (2107) by reproducing the event described therein and performing additional idealized simulations with the Weather Research and Forecasting (WRF) model that systematically alters the local topography to test the importance of flow interaction with the local dynamics to generating this extreme event.