Remote sensing of the albedo and detection of the slush line on the Greenland ice sheet

Advanced very high resolution radiometer images of a section of the Greenland ice sheet (western part between 64.5 degrees and 70.5 degrees N with a size of 667 (N-S) by 700 (W-E) km) were processed in order to retrieve the surface albedo. The images were for the summer seasons of 1990-1997 inclusive, and results were compared to simultaneous mass balance measurements carried out along the transect (67 degrees N). The main findings are as follows: First, many images show a distinct transition from low spatial variability of the albedo at higher elevations to higher spatial variability of the albedo at lower elevations. It is argued that these transitions most likely correspond to transitions at the surface ("slush line," in short) from the area that is uniformly covered by snow to a mosaic of snow patches, slush, and ice. Second, 70% of the interannual variations in the mean specific mass balance can be explained by interannual variations in the maximum elevation of the slush line (i.e., the "slush limit"). However, a higher percentage (82%) can be explained by interannual variations in the sum of positive degree-days, Third, in 3 years with a relatively large amount of melt the inland migration of the slush line stopped or slowed considerably down at a fixed distance from the ice sheet margin. We believe that this so-called "maximum slush line" coincides with an abrupt change in the density profile. The maximum slush line forms an important limitation for the use of satellite data to monitor interannual variations in the mass balance. Its position is a climate indicator and was determined for the entire section of the Greenland ice sheet investigated.