The impact of valley geometry on daytime thermally driven flows and vertical transport processes

The influence of the valley geometry on thermally driven flows is studied by means of large-eddy simulations (LES). An idealised valley-plain topography and a spatially constant but time dependent surface sensible heat flux is used to generate upslope, upvalley and plain-to-mountain winds. A systematic variation of valley depth, width and length induces differences in the cross- and along-valley flow field and thermal structure of the boundary layer. The deeper the valley the stronger are the upvalley winds and the more favoured is the formation of vertically stacked circulation cells and an elevated valley inversion layer. Upvalley winds become weaker for short and wide valleys. The development of plain-to-mountain circulations considerably increases vertical exchange processes between the boundary layer and the free atmosphere compared to vertical transport processes over a plain. The analysis of mass flux budgets and forward trajectories indicates that mass is transported three to four times more effectively from the surface to the free atmosphere over valleys than over flat terrain. Vertical transport processes are strongest for deep and narrow valleys.