SURFACE-ENERGY BALANCE AND KATABATIC FLOW OVER GLACIER AND TUNDRA DURING GIMEX-91

The energy balance observed in the summer of 1991 during the Greenland Ice Margin EXperiment (GIMEX-91) is described. Several masts were erected along a transect perpendicular to the ice edge; from a point 90 km up the ice cap, through the ablation zone, down to a point 5.8 km within the tundra zone. The diurnal variation of the friction velocity, sensible heat flux and latent heat flux, both on a single day (22 July) and averaged over the complete observational period are discussed. The mean daily values of the sensible heat flux (positive upwards) over the ice were negative (about -30 W/m2), as the ice was melting during most of the observational period. On average the latent heat flux was positive, which moistened the boundary layer air flowing down the ice cap. Up the ice cap, more than about 20 km from the ice edge, the energy balance is driven by the net radiation which causes higher fluxes during the day than during the night. The smaller (more negative) sensible heat fluxes during the night accelerate the katabatic flow; this leads to a wind maximum late at night or early in the morning. This diurnal cycle in energy balance is very similar to that observed on the slopes of Antarctica. At the ice edge, the large difference between the radiative and thermal properties of the ice and the tundra exerts a strong thermal forcing on the katabatic flow, analogous to the land-sea breeze circulation. During day-time the thermal wind accelerates the katabatic flow, which leads to an enhancement of the turbulent exchange rate and turbulent fluxes.