Radar glacier zones in southeast Alaska, USA: field and satellite observations

The ability of synthetic-aperture radar (SAR) to record change in glaciers and icefields on seasonal to interannual time-scales is useful in maritime mountain regions where visible data are often obscured by clouds. A time series of RADARSAT and second European Remote-sensing Satellite (ERS-2) SAR images shows dramatic changes related to the onset and progression of glacier melting on the Juneau Icefield, southeast Alaska, U.S.A. Four "radar glacier zones" are interpreted from SAR images as snow that is dry during winter, an early snowmelt (M), a second phase of snowmelt (M2) and bare ice. These zones develop every year on the Juneau Icefield and other mid-latitude glaciers. Summer field observations on the Juneau Icefield during 1997 and 1998 constrain interpretations of the C-band radar glacier zones. Of the two zones that occur in melting snow (M, M2), M has low radar backscatter coefficients (sigma degrees < -12), in contrast to the higher backscatter coefficients (<sigma>degrees > -12) of the subsequent M2 zone. Snow moisture and surface roughness at the scale of the radar wavelength (5.6 cm) were measured to characterize the melt zones. Correlation length, wetness and grain-size in the two zones are not distinguishable in the late ablation season. Mean surface roughness, due to the presence of suncups, is higher in the M2 zone than in the M zone and probably causes the higher backscatter due to greater scattering.