Field observations of the morning transition over a steep slope in a narrow alpine valley

This paper examines the morning transition period of the atmospheric boundary layer for convective summer days over steep topography. An experimental transect composed of two turbulence towers was installed in summer 2010 on a steep, west-facing slope of Val Ferret, in the Swiss Alps. The analysis focuses on valley-scale processes as well as near-surface turbulence for nine clear-sky days with thermally driven slope flows. At the valley scale, we show that shading from surrounding topography resulted in an inhomogeneous heating of the slope surface. Atmospheric profiles collected over the slope showed that subsidence warming was the main driver of the temperature inversion breakup in the central part of the valley. As a result, it took on average about 5 h after astronomical sunrise for downslope flows to be replaced by upslope/upvalley winds, with typical 2-m wind speeds of 1 and 2 m s−1, respectively. Near the surface, the slope flow layer went through a calm period with 2-m wind speeds below 1 m s−1 starting right after local sunrise. The duration of that quiescent period was highly variable, lasting between 1 and 40 min, possibly as a result of the presence or absence of large-scale, long-lived eddies feeding their energy downward to the surface. An analysis of the turbulence kinetic energy (TKE) budget for all days of interest showed that there was buoyant production of TKE throughout the full morning transition period as a result of the positive contribution from the along-slope buoyancy flux.