The objective of this thesis was to evaluate the representation of thermally driven winds in the Inn Valley, Austria, in the Weather Research and Forecasting (WRF) model and its sensitivity to the horizontal grid size (Δx) and planetary boundary layer (PBL) parameterization scheme. Furthermore, the interaction between the along-valley flow, the mountain-plain wind circulation (MPWC), and the synoptic flow was investigated. For this purpose, IOP8 of the CROSSINN campaign was simulated using three different WRF setups. M1 used Δx = 1 km and a traditional 1D PBL scheme (MYNN 2.5), M02 used Δx = 200 m and the same 1D PBL scheme, and S02 used Δx = 200 m but a new scale-adaptive PBL scheme (SMS-3DTKE).
It was found that the along-valley flow in the Inn Valley strongly interacts with the synoptic flow. When WRF overestimated the north(-easterly) synoptic flow in the close proximity of the Inn Valley entrance, forced channeling of this synoptic flow supported an up-valley flow during the second night which was not observed during IOP8. This highlights that small differences in the synoptic flow field can impact the along-valley wind simulations in the Inn Valley. Furthermore, an interaction of the MPWC with the valley atmosphere was found. During the afternoon of IOP8, the MPWC interacted with the synoptic flow above the mountains north of the Inn Valley and penetrated into the Inn Valley at several locations. The resulting downslope wind supported the formation of a cross-valley temperature gradient in the lower valley atmosphere and enhanced turbulent mixing in the upper valley atmosphere. These results demonstrate that the different scales strongly interact with each other even during a day of weak synoptic forcing.
The reduction of Δx from 1 km to 200 m did neither strongly affect the mean structure and timing of the thermally driven winds nor the scale interaction. Nevertheless, M1 did not resolve the quasi-stationary gravity waves formed in M02 due to their too short wave length and produced weaker mixing at the top of the valley wind as well as near the penetration of the MPWC.
The new scale adaptive PBL scheme provided reasonable results but no noticeable improvements compared to the 1D scheme. In fact, the flow fields of S02 are smoother than of M02 (e.g., weaker gravity wave amplitudes and smaller wind speed fluctuations) which may result from stronger horizontal and/or vertical diffusion.