The influence of cross ridge winds on nocturnal downvalley flow patterns: observations from the Perdigão field campaign

The current thesis investigates the evolution of downvalley flows (local flows developing near the surface that blow along the valley axis towards its exit) developing under cross-ridge winds (large-scale winds blowing perpendicularly to the valley axis above its ridge). The investigation is based on the analysis of meteorological data collected during the Perdigão field campaign (2017). Three nocturnal cases were selected based on the following criteria to investigate the interaction between cross-ridge winds and valley circulation. Those criteria included the persistence of a cross-ridge wind of a minimum duration of 1 hour and the persistence of a deep downvalley flow of at least 100m of a minimum duration of 20 minutes. Results show that in each of the three cases, a down-valley wind layer always developed near the valley bottom, with its depth and intensity influenced by the valley's stability, wind strength and direction above the ridges. Specifically, it was observed that wind above ridges with a significant along valley component can be channeled in the valley and can either intensify the downvalley flow, or weaken or disrupt it. On the other hand, observations suggest that winds above ridges with direction perpendicular to the valley axis, have a tendency to descend along the valley sidewall and to propagate across the valley, thus weaking the downvalley flow. The depth of such penetration seemed weaker under well-developed stably stratified conditions in the valley. Overall, the present thesis emphasizes the complex interplays among valley circulation and its forcings, i.e., wind speed and direction at the top of the ridge, atmospheric stability within the valley, and the entrainment of external winds in the valley. The findings emphasize the need for additional follow-up studies in order to understand better how each single component and all of them combined contribute to the modification of the valley circulation.