Flow during the evening transition over steep Alpine slopes

A field campaign, the Slope Experiment near La Fouly (SELF-2010), was conducted to monitor the evening transition of slope flows on clear-sky days from July to September 2010 in a narrow valley of the Swiss Alps. A steep west-facing slope with inclinations ranging from 25◦ to 45◦ was instrumented from 1900mto 2200mabove sea-level. Detailed measurements were made along a linear transect of the slope with two turbulence towers, two weather stations, five surface temperature measurement stations and a tethered balloon system. The present study focuses on nine exemplary ‘convective’ days, characterized by weak synoptic flow and clear-sky conditions, during which thermal circulations prevail. The analysis of the observational data shows that topographic shading triggers the evening transition. The topographic configuration around the experimental site results in a sharply defined ‘shading front’ propagating up slope, causing a sudden decrease in incoming short-wave radiation on the order of several hundreds of Wm−2 within a few minutes. The slope surface rapidly responds to the advancing shading front; in some cases, reductions in surface temperatures of some 10◦C in less than 10 min are observed. This is rapidly followed by an early-evening calm period with very small turbulent kinetic energy (TKE< 0.05m2 s−2) and extremely light wind speeds (< 0.5ms−1). When the inertia-driven upslope flow is fully stopped by the katabatic acceleration, a shallow local drainage flow forms and reaches a quasi-equilibrium 1.5 h after the local sunset. An analysis of the TKE budget close to the surface shows that the buoyancy flux is much greater than the shear production in the last hours before the local sunset, possibly due to valley curvature effects. Copyright c 2012 Royal Meteorological Society