High-resolution simulations over a sub-kilometre scale valley during stable night conditions

Complex terrain creates many challenges for modelling aerologic phenomenon. Local variations can have drastic effects on airflows; a good representation of local orography is, therefore, crucial in models. While finer-resolution improves the orographic description in models, it creates a “grey zone” problem for boundary layer schemes. The grey zone, for PBL schemes, is where model-resolution comes close to the size of turbulent structures, leading to the structures being partly resolved and partly sub-grid scale. Research on the grey zone has focused mostly on daytime, neutral-to-unstable conditions. In thinner nocturnal stable layers, local processes become predominant, such as channelling, the simulation of which demands higher resolution in models.

In this research, we run a nested grid version of WRF with a resolution of the innermost domain at 111 m and the coarsest domain at 9 km. We focus on the 1 km wide pre-Alpine valley of Cadarache, a tributary of the Durance River. Previous simulations did not succeed in representing the valley with resolutions of 1 km. We test the indicative value of turbulent length-scales for decisions on the grey zone. Based on the KASCADE 2017 data of several sonic anemometers, placed along the valley thalweg, we determine the turbulent length scales to verify whether the turbulence remains sub-grid or not.

Our first results indicate that turbulent structures in this sub-kilometre scale valley remain well below the model resolution for night time conditions. First runs with 111 m resolution show realistic night time winds and produced the correct overall-structure in the valley. A general under prediction of 2 m-temperatures was found, especially during daytime. Further runs aim to improve forecast precision.