Surface layer climate and turbulent exchange in the ablation zone of the west Greenland ice sheet

摘要

A comprehensive description is presented of the Surface layer (SL) wind, temperature and humidity climate and the resulting sensible and latent heat exchange in the ablation zone of the west Greenland ice sheet. Over a four-year period (August 2003-August 2007), data were collected using three automatic weather stations (AWS) located along the 67 degrees N latitude circle at 6, 38 and 88 km from the ice sheet margin at elevations of 490, 1020 and 1520 m asl. In the lower ablation zone, surface momentum roughness peaks in summer, which enhances the mechanical generation of turbulence in the stable SL. The SL is stably stratified throughout the year: in summer, the surface temperature is maximised at the melting point and therefore remains colder than the overlying air, in winter the surface is cooled by a radiation deficit. The resulting downward directed sensible heat flux cools the SL air. Humidity gradients between surface and air Lire small in winter, in response to low temperatures, but peak in spring, when the surface is not yet inching and can freely increase its temperature. This is especially true for the lower ablation zone, where winter accumulation is small so that the dark ice surface is already exposed at the onset of spring, allowing significant convection and sublimation. During summer, when the surface is melting, the sensible heat flux becomes directed towards the surface and sublimation changes into deposition in the lower ablation zone. The SL wind climate is dominated by katabatic forcing, with high directional constancy in summer and winter. The katabatic forcing is important to maintain turbulent exchange in the stable Greenland SL.