Evaluation of remote-sensing techniques to measure decadal-scale changes of Hofsjökull ice cap, Iceland

Abstract

Dynamic surficial changes and changes in the position of the firn line and the areal extent of Hofsjo« kull ice cap, Iceland, were studied through analysis of a time series (1973^98) of synthetic-aperture radar (SAR) and Landsat data. A digital elevation model of Hofsjo« kull, which was constructed using SAR interferometry, was used to plot the SAR backscatter coefficient (³) vs elevation and air temperature along transects across the ice cap. Seasonal and daily ³ patterns are caused by freezing or thawing of the ice-cap surface, and abrupt changes in ³ are noted when the air temperature ranges from $^5³ to 0³C. Late-summer 1997 ³ (SAR) and reflectance (Landsat) boundaries agree and appear to be coincident with the firn line and a SAR ³ boundary that can be seen in the January 1998 SAR image. In January 1994 through 1998, the elevation of this ³ boundary on the ice cap was quite stable, ranging from 1000 to 1300 m, while the equilibrium-line altitude, as measured on the ground, varied considerably. Thus the equilibrium line may be obscured by firn from previous years. Techniques are established to measure long-term changes in the elevation of the firn line and changes in the position of the ice margin.

Monitoring glacier zones and snow/firn line changes in the Qinghai–Tibetan Plateau using C-band SAR imagery

“…In Brandt et al (2008) and Langley et al (2007), the authors give the backscatter coefficient extent of superimposed ice in C-band co-polarization. However, other authors have suggested that superimposed ice is not distinguishable from the bare ice zone (Casey & Kelly, 2010;Hall et al, 2000;König et al, 2001a). The difficulty in detecting superimposed ice zones complicates the acquisition of the equilibrium line location from remote sensing, which will be discussed below.…”

“…Therefore, the maximum late summer snow line elevations acquired from frequent SAR observations have been used to estimate the equilibrium line, and the closeness of these values suggests a potential use for SAR in complementing traditional in situ equilibrium line monitoring for mountain glaciers (Casey & Kelly, 2010;Smith et al, 1997). However, in a negative mass balance year, the equilibrium line is difficult to detect on SAR images because it is obscured by older firn with similar backscatter coefficients (Hall et al, 2000;Jaenicke et al, 2006).…”

Using SAR satellite data time-series for regional glacier mapping

“…6a). The backscatter signal from the ice is changing seasonally to changes in surface roughness, as melt water is creating a rougher ice surface due to local melt water streams (Shi and Dozier, 1995;Hall et al, 2000) (Fig. 6a, see yellow to orange colors on ice in the melt season, compared to 5 the light blue color in the cold season).…”

“…The superimposed ice zone has lower backscatter response compared to the firn in the cold season, but higher backscatter than glacier ice, as the ice transmits much of the microwaves (e.g. Langley et al, 2009) and additionally the ice surface are controlled by roughness (Shi and Dozier, 1995;Hall et al, 2000). 20…”

“…Consequently, roughness is considered as a highly important scattering variable (e.g. Hall et al, 2000). Wet snow absorbs SAR-waves, and little energy is transmitted back to the SAR sensor in comparison to a dry snow and firn surface (Rott, 1984).…”