MOUNTAIN VALLEY EVACUATION BY UPPER LEVEL FLOWS - A SCALE-MODEL STUDY

The motivation for this study was to understand the complex shear flow associated with the evacuation of inversion layers and the removal of pollution from mountain valleys. An upper level flow across an invers ion layer within a mountain valley can create strong wind shear and tu rbulence at the inversion interface. In our experiments, the inversion layer was forced over the downstream mountain followed by the establi shment of a new steady-state equilibrium position for the inversion. T he evacuation process was dependent on the onset of a standing wave in the mountain valley and the formation of a vortex on the leeside of t he upstream mountain. This phenomenon only occurred at higher flow vel ocities. Subsequently, the inversion layer was no longer forced over t he downstream mountain, but drawn up the leeside of the upstream mount ain by the vortex and transported away by the upper level flow. This s ituation has important implications for airports in mountainous region s, suggesting it may be feasible to predict the onset and time scale o f the erosion process. We include comparisons with atmospheric flows.