Orographic and stability effects on valley-side drainage flows

The effects of orography and stability on valley-side drainage winds were investigated with the aid of a numerical model. The model is three-dimensional, non-hydrostatic, cast in terrain-following co-ordinates, has a surface energy budget and a 1.5 order TKE closure scheme. Experiments were conducted over a schematic three-dimensional valley to assess the influences on airflow of valley-side slope magnitude, valley cross-section shape, tilt of the valley floor and stability.In drainage flow, magnitudes of horizontal and vertical velocities and heights of their maxima are directly related to slope angle. The velocities are either insensitive to, or slightly inversely related to stability. The cooling which drives the flows is strongest over steep slopes and in large stabilities. The depth of the cooled layer, whilst increasing over steeper slopes, is inversely related to the stability. TKE increases with slope angle and decreases with increasing stability. In the downslope direction, the near-surface cooled layer significantly increases whereas the inversion intensity decreases by about 20%. These two features are due to mixing between the drainage flow and the overlying air. Tha drainage flow accelerates down the slope until it reaches the accumulated pool of cold air in the valley bottom, whereupon it slows down markedly and is accompanied by uplift over the centre of the valley.The cross-valley circulation is influenced by valley-side slope angle, valley cross-section shape and tilt of the valley floor, in addition to the effects of stability. For a given shape, the circulation is a direct function of the valley-side slope and an inverse function of the ambient stability. This relationship is described mathematically.V-shaped valleys generate stronger flows than doU-shaped valleys and a tilted valley floor also leads to a significant increase in velocities.