The observed katabatic flow at the edge of the Greenland ice sheet during GIMEX-91

Observations performed in the melting zone of the Greenland ice sheet and over the adjacent tundra in the summer of 1991 are described. The experimental area is the region near Søndre Strømfjord (67°N, 54°W), which is relatively dry and sunny, resulting in the highest mean temperature in Greenland in July. The katabatic wind is dominantly present over the ice; it influences the energy balance near the surface through the sensible and latent heat flux. With the aid of a tethered balloon it was observed that the thickness of the katabatic layer is typically 100 to 200 m. An interesting aspect of the katabatic wind appears to be the acceleration of the flow in the late afternoon due to the large temperature gradient at the border between tundra and ice. Further on the ice, this effect is no longer important for the dynamics of the katabatic flow. The net radiation is the main driving force there. An attempt is made to estimate the importance of these thermal wind effects compared to the buoyancy forcing. It is concluded that near the edge of the ice surface winds are driven by the horizontal pressure gradient, imposed by the thermal contrast between tundra and ice. A comparison is made between the observed katabatic wind and those in the Antarctic.