Evaluation of a sub-kilometre NWP system in an Arctic fjord-valley system in winter

Terrain challenges the prediction of near-surface atmospheric conditions, even in kilometre-scale numerical
weather prediction (NWP) models. In this study, the ALADIN-HIRLAM NWP system with 0.5 km
horizontal grid spacing and an increased number of vertical levels is compared to the 2.5-km model system
similar to the currently operational NWP system at the Norwegian Meteorological Institute. The impact of
the increased resolution on the forecasts’ ability to represent boundary-layer processes is investigated for the
period from 12 to 16 February 2018 in an Arctic fjord-valley system in the Svalbard archipelago. Model
simulations are compared to a wide range of observations conducted during a field campaign. The model
configuration with sub-kilometre grid spacing improves both the spatial structure and overall verification
scores for the near-surface temperature and wind forecasts compared to the 2.5-km experiment. The subkilometre experiment successfully captures the wind channelling through the valley and the temperature field
associated with it. In a situation of a cold-air pool development, the sub-kilometre experiment has a
particularly high near-surface temperature bias at low elevations. The use of measurement campaign data,
however, reveals some encouraging results, e.g. the sub-kilometre system has a more realistic vertical profile
of temperature and wind speed, and the surface temperature sensitivity to the net surface energy is closer to
the observations. This work demonstrates the potential of sub-kilometre NWP systems for forecasting
weather in complex Arctic terrain, and also suggests that the increase in resolution needs to be accompanied
with further development of other parts of the model system.