Dealing with steep slopes when modeling stable boundary‐layer flow in Alpine terrain

Steep slopes in mountainous terrain are challenging for commonly used numerical weather prediction models using terrain-following coordinates since they can generate numerical errors when evaluating horizontal gradients, and numerical instabilities. The model orography is therefore generally smoothed locally so as to remove slopes exceeding a defined threshold, typically in the range 30°–40°. However, this process can lead to undesired changes to orographic features at the scale of the mountain range. So as to preserve properties of the orography, e.g., mean terrain height across the domain, a global smoothing algorithm is developed. The algorithm is then applied to a domain centred on the Alpine Grenoble valley. Exploration of the parameters of the algorithm allows to minimize the changes in the orography in the domain, especially that of the two main valleys (of width 2 and 5 km and depth about 2000 m). Results of numerical model simulations are next analyzed to evaluate the effects of the slope threshold on local- and valley-scale atmospheric dynamics for a cold-air pool episode. Model results from simulations with slope thresholds of 28° and 42° are compared, the lower threshold allowing for a reduction (albeit small in practice) of the near-surface vertical grid spacing. Remarkably, not only are the near-surface boundary-layer temperature and wind fields similar for both simulations (and in very good agreement with observations), but this similarity holds in the whole inversion layer throughout the episode. The reasons for this similarity are that, for this cold-air pool episode, the valley atmospheric dynamics is confined within the inversion layer and the orography is essentially unchanged by the smoothing algorithm in that layer for the slope threshold of 28° and 42°.