Classification of glacier zones in western Greenland using albedo and surface roughness from the Multi-angle Imaging SpectroRadiometer (MISR)

Abstract

The Greenland ice sheet has been the subject of mass balance and melt measurements over the past two decades and its margins have shown significant thinning in recent years. Surface characteristics of the ice sheet margins are strongly modified by the annual process of snow accumulation and melt. In this work, we explore spatial and temporal relationships between near-infrared albedo and surface roughness from the Multi-angle Imaging SpectroRadiometer (MISR) aboard NASA's Terra satellite. Our study area is a region in western Greenland in the vicinity of Jakobshavn Glacier and its upland drainage basin. We compute near-infrared albedo from MISR data using imagery over the April to September period for 2000 through 2005. Near-infrared albedo is inversely related to both snow age and melt intensity. We map surface roughness using the Normalized Difference Angular Index (NDAI) applied to MISR atmospherically corrected surface hemispherical-directional reflectance. In previous work, the NDAI has been correlated with surface roughness on the scale of about 70 m. We have further substantiated that relationship here. The NDAI and albedo images for each of the six years are used in an ISODATA unsupervised classification. Classification results for the individual years show year-to-year differences, which appear to depend on the number and temporal distribution of images with minimal cloud cover as well as interannual differences in ice sheet surface properties. The year 2003, which had the greatest number of images with minimal cloud cover, shows good correspondence with previously mapped glacier zones. However, we do not have an ISODATA class that corresponds to the dry-snow zone and we see greater differentiation between zones at lower elevations. Within each ISODATA class, distinct relationships between near-infrared albedo and surface roughness emerge and in all cases but one there is an inverse temporal relationship between albedo and roughness. The one class that shows a direct relationship between albedo and surface roughness is hypothesized to be the superimposed-ice zone. While still somewhat preliminary, these results suggest that concurrent measurements of near-infrared albedo and surface roughness have significant potential for ice sheet surface characterization as well as for ongoing monitoring.