Melt-water accumulation on the surface of the Greenland ice sheet: Effect on albedo and mass balance

Satellite-derived albedo maps of the western part of the Greenland ice sheet (between 64.5 and 70.5 degreesN) reveal a north-south extending zone with relatively low albedos at some distance from the ice margin. In the literature it has been hypothesized that this "dark zone" is due to a local maximum in melt-water accumulation on the ice-covered surface. A plausible explanation for this maximum in melt-water accumulation is that relative to the situation within the "dark zone", melt-water accumulation is reduced at higher elevations by a smaller melt-water production rate whereas runoff occurs more easily at lower elevations where slopes are generally steeper. For the present paper AVHRR images from eight years (1990-1997) were analysed. The following indications confirming the "melt-water accumulation hypothesis" were found: (1) there is a significant correlation between the annual mean albedo lowering within the "dark zone" and the annual amount of melt as inferred from local mass-balance measurements: and (2) within each summer season the albedo lowering within the "dark zone" seems to respond to the melt-water production rate as inferred from local temperature measurements. The effect of melt-water accumulation on the albedo implies a positive feedback between the albedo and the amount of melt. It is estimated that approximately 40% of the interannual mass-balance variations in the ''dark zone" are due to this feedback.