The aim of this Master's thesis was to investigate the impact of thermally driven winds in selected tributary valleys on the flow in the Inn Valley and vice versa by means of numerical simulations conducted with the Weather Research and Forecasting (WRF) model. This study focused on IOP7b (27 - 28 August 2019) of the CROSSINN field campaign. Six different tributaries near the village of Kolsass were chosen to cover a range of different sizes and orientations: four smaller valleys (Volder, Watten, Weer and Pill Valley) and two larger tributaries (Ziller and Achen Valley). Only the Achen Valley runs north towards the Alpine foreland, while the other tributaries run south. A mass flux analysis was performed for different control volumes at the entrance of each tributary valley to quantify the horizontal and vertical mass flux in the main valley and through the tributary gates. This analysis was mainly based on a simulation with a horizontal grid spacing of 200 m (DX200), but the sensitivity to the model resolution was evaluated with an additional run with a grid spacing of 1 km (DX1).
It was found that the integrated mass flux through the entrance of the small tributaries is about an order of magnitude smaller than the mass flux in the Inn Valley. Nevertheless, the impact of the tributaries on the circulation in the main valley cannot be neglected. Differences in the mass flux of individual small tributaries can be attributed to a combination of topographic factors, such as the individual orientation of the tributary, and characteristics of the flow in the main valley, such as direction and pathway. Among the two larger tributaries, the Ziller Valley was found to have the greatest influence on the flow in the main valley, with a mass flux reduction (increase) of about two thirds (half) of the mass flux in the Inn Valley during upvalley (downvalley) winds. Conversely, the contribution of the Achen Valley is often opposite and much smaller than the contribution of the Ziller Valley. During the investigated period the circulation in both smaller and larger tributaries is influenced by large-scale winds. These winds affect the vertical mass flux in the main valley, making it not possible to verify the simplified concept of a closed circulation at the entrance of the side valley with compensating subsidence in the main valley proposed in previous studies. Furthermore, no clear relationship was found between the topographic amplification factor (TAF) and the mass flux in the tributaries.
Although the simulation with hectometer resolution (DX200) captures better than the kilometer resolution run (DX1) details of boundary layer structure, slope wind circulation and nocturnal drainage flow, the integrated mass flux at the tributary entrances agrees surprisingly well between the two simulations, suggesting that an increase in horizontal resolution to the hectometer scale might not necessarily result in an improved representation of the tributaries' bulk effects. Moreover, the comparison with available observations shows that the simulation with hectometer resolution exhibits deficiencies too. Nevertheless, the study shows that high-resolution simulations are an indispensable tool for studying scale interaction in complex terrain.