OBSERVATION AND SIMULATION OF BARRIER WINDS AT THE WESTERN MARGIN OF THE GREENLAND ICE-SHEET

Meteorological observations performed in the melting zone of the Green land ice sheet during GIMEX-90/91 show the regular occurrence of moder ately strong barrier-winds. The barrier wind is a thermally generated jet that develops when the large-scale now advects warm tundra air tow ards the cold, melting ice sheet, creating a large local horizontal gr adient of temperature. As a result, the geostrophic wind in the atmosp heric boundary-layer acquires a component perpendicular to the tempera ture gradient. In the melting zone of the ice sheet, barrier winds int eract with persistent surface-based katabatic winds. The forcing mecha nisms and detailed two-dimensional structure of the barrier wind have been investigated with a mesoscale meteorological model. The model res ults confirm the important role of the warm tundra during the developm ent of barrier winds. Barrier winds cause a pronounced rise in tempera ture and a strong increase of turbulent exchange in the lower melting zone: friction velocity and sensible-heat flux in the melting zone sho wed peak values of 1.3 m s(-1) and 300 W m(-2) respectively. In combin ation with large positive net radiation, the daily melt rate increased to 10 cm water equivalent, which is twice the rate in typical katabat ic wind conditions. This shows that barrier winds could have a signifi cant impact on the melt regime of areas where the ice sheet ends in th e tundra. Although barrier winds in this part of Greenland may occur l ess frequently than the observations during GIMEX-90 suggest, they are of great importance for the climate of this and of other polar region s, such as the Antarctic, and so merit careful study.