This thesis investigates the behavior of atmospheric flows over complex terrain using data collected in a field site in the Vale de Cobrao near Perdiagao, Portugal.The study investigates and analyzes the complex interaction between the land surface, near-surface down-valley flow, and ambient flow aloft. Flow near the land surface is primarily driven by topography and temperature, whereas ambient flows aloft are primarily pressure driven. The valley of interest is encompassed by parallel ridges, with a rough vertical distance of 160m from the valley base, running from South-East to North-West. General climatology and regional forcing in the site were conducive to the formation of parallel countercurrents in the valley. Main dataset for this study comes from a 100m instrumented tower located in the valley base and a halo lidar located close the tower in the valley base. A novel mechanism of break-up of down-valley flow, with downward momentum transport from above, was identified. Richardson number, governing the physical processes at the interface, was utilized to provide insight into a series of events leading to the break-up. The results illustrates that turbulence, low below the interface and high above the interface, is detached at the interface before the break-up. This suggests that turbulence is generated at upper levels where stratification is weak and diffused downward via a top-down mechanism to eliminate the cold pool in the valley. After the break-up more traditional boundary layer, in which turbulence decreases with height and heat flux is almost zero above the ground, was restored. Additionally, this study also presented how the ridge-top speed relates to the depth of down-valley flow.