Anabatic Winds over a Steep Alpine Slope: Observations of the Turbulence Structure

Anabatic flows are buoyantly-driven up slopes and valleys by daytime surface heating and gradients in the near-surface virtual potential temperature field. These winds occur under weak synoptic and clear-sky conditions over sloped terrain, most prominently in mountainous regions. The turbulence structure of anabatic flows has received much less attention than their katabatic (downslope) counterparts; yet these winds are drivers of important phenomena like convergence at peaks and ridges, cloud formation, and convective precipitation. As such, a better understanding of the physical mechanisms driving heat and momentum transport are important to improving meteorological forecasting, pollutant transport, and hydrologic modeling in mountainous regions. We present observations of the mean flow and turbulence structure over a steep (35.5 deg) slope in a narrow Alpine valley in Val Ferret, Switzerland. Here, the anabatic winds are characterized by a multi-scale, superposition of upslope and up-valley flows with wind component directions. With height above the surface, wind directions tend toward the up-valley direction and exhibit oscillatory behavior. Wind speeds strengthen throughout the afternoon with increasing temperatures until a topographic shadow front triggers the evening transition period. The near-surface virtual potential temperature profiles generally indicate a very shallow convective layer. Although surface-normal buoyancy fluxes also build consistently throughout the day, the slope-parallel buoyancy fluxes tend to oscillate between positive and negative throughout the earlier part of the day, which weakens and enhances the vertical buoyancy fluxes and buoyant production of turbulence kinetic energy, respectively (for a coordinate system with positive x directed down the slope). In the latter part of the day, a strong flux divergence occurs, causing the vertical buoyancy flux to weaken despite a continued increase in temperatures and the surface-normal flux. Many of these phenomena indicate non-local forcing, which pose a challenge for numerically modeling these flows.