Structure and diurnal, variation of the atmospheric boundary layer over a mid-latitude glacier in summer

During the summer of 1994, a meteorological experiment (PASTEX) was performed over the Pasterze Glacier, Austria. In this paper we describe the average horizontal and vertical structure of the atmospheric boundary layer (ABL) above the melting glacier, as well as its diurnal variation during a period of fair weather. It was found that very persistent glacier winds with a vertical extent of 100 m dominate the summertime structure of the ABL, because the gravity force acting on the near surface air parcels is many times larger than the synoptic-scale pressure gradient. During fair weather, we find a well developed mountain-valley wind circulation above the katabatic layer. During daytime, the valley wind advects warm and humid air from the ice-free valley towards the glacier, limiting the development of the glacier wind. During the night, the downslope flows that develop above the ice-free valley walls (mountain wind) merge with the glacier wind and enhance the downslope transport of air. The associated subsidence is the most probable cause for the drying of the lower part of the atmosphere during the night. During periods of weak synoptic winds, the glacier wind effectively generates turbulence in the strongly stratified surface layer. On average, the turbulent fluxes of sensible and latent heat provide 25% of the total melting energy at the surface of the glacier tongue, and the influence of the glacier winds on the surface energy budget can therefore not be neglected.