High-resolution observations of separated flow in the lee of steep orography: Gaudex 2003

A dense observational network was deployed over a 3 month period on a steep-sided ridge to provide a uniquely detailed description of flow separation and turbulence at sharp edges. The ridge, called Gaudergrat, is located on the alpine plateau in eastern Switzerland and has been the focus of snow drift and accumulation studies for a number of years. These data have been used to develop models for applications such as avalanche warning. Gaudergrat is triangular in cross-section, 1 km long and rises about 200 m above the surrounding topography. The particularly steep orography with maximum slopes of about 40 degrees facilitates turbulent flow separation on the lee slope. The field measurements are aimed specifically at generating a comprehensive dataset for airflow over steep orography for use in the development and validation of a new microscale numerical model for the UK weather research community.

Observations were made using 20 automatic weather stations (AWS), synchronised to GPS, recording wind and pressure data at a 1 s time resolution and temperature and humidity at a 4 s time resolution. These supplemented a permanent array of 6 AWS measuring 3D winds at two heights. Three 15 m turbulence towers were located at sites across the ridge crest with 3-axis sonic anemometers at two heights recording at 5 Hz. In addition, Doppler sodars upwind and downwind of the ridge provided 10 m vertical resolution and 15 s time resolution wind data. Upwind temperature and wind profiles were measured by radiosondes launched daily from a site 1 km upwind of the ridge.

This paper presents initial analysis of the wind and pressure fields over Gaudergrat ridge. Three distinct flow features are discussed and quantified. (i) A vertically orientated recirculation region in the lee associated with flow separation at the ridge crest. The extent of the reversed surface layer wind which results is unsteady in time and its strength varies with both time and position along the lee slope. (ii) Horizontally-oriented trailing vortex flow about the ridge end indicative of bluff-body separation. The interaction between these vortices and the crest-induced separation is assessed. (iii) A cross-ridge flow at the ridge crest, independent of the flow at lower levels. Approximately westerly winds persist at the crest even when northerly or southerly ridge-parallel winds are observed a few metres below the crest on the ridge slopes. A physical interpretation for this feature will be presented.

The turbulence data is used to calculate heat and momentum fluxes in mean physical streamline coordinates. The interpretation of turbulent flux quantities in a steep-terrain environment is discussed.