Synthetic aperture radar detection of the snowline on Commonwealth and Howard Glaciers,TaylorValley, Antarctica P atrick Bardel , 1 Andrew G. Fountain , 1 Dorothy K. Hall, 2 Ron K wok 3 1 Departments of Geography and Geology, Portland State University, Portland, OR 97207, U.S.A. 2 Hydrological Sciences Branch, NASA Goddard Space Flight Center, Code 974, Greenbelt, MD 20771, U.S.A. 3 Jet Propulsion Laboratory, California Institute ofTechnology, 4800 Oak Grove Drive, Pasadena, CA 91109-8099, U.S.A. ABSTRACT . Synthetic aperture radar (SAR) images of TaylorValley, Antarctica, were acquired inJanuary 1999 in coordination with ground-based measurements to assess SAR detection of the snowline on dry polar glaciers. We expected significant penetration of the radar wave resulting in an offset of the SAR-detected snowline relative to the true snowline. Results indicated no detectable displacement of the SAR snowline. Snow depths of 15 cm over ice can be detected on the imagery.We hypothesize that the optical depth of thin snowpacks is enhanced by reflection and refraction of the radar beam by internal snow layers. The enhanced optical depth increases the volume scattering, and thereby enhances backscatter sufficiently to be detected by the SAR. Consequently, SAR imagery may be used directly to image the position of transient snowlines in dry polar regions. INTRODUCTION Assessing glacier change in the polar regions, particularly in the Antarctic, has been difficult because of the distance and expense involved in conducting the fieldwork. Fieldwork also limits the spatial scope of the measurements due to the logistics of surface movement. Monitoring glaciers in the polar regions is an important component of studying the effects of climatic change because the rate and magnitude of change are expected to be greatest in those regions (Serreze and others, 2000). To monitor glacial change efficiently over broad regions requires the use of remote-observation methods. Currently, remote monitoring includes airborne laser altimetry of alpine glacier altitude (Echelmeyer and others, 1997), and satellite remote sensing (Advanced Very High Resolution Radiometer and Landsat) of ice-shelf extent (Scambos and others, 1998). An important glacial feature that can be tracked on dry polar glaciers is the posi- tion of the equilibrium line. The equilibrium line separates the zone of net mass accumulation from the zone of net mass loss (ablation) at the end of the melt season. The equilibrium-line altitude (ELA) relative to the distribution of glacier area with altitude is an important characteristic for determining the state of the glacier’s mass balance (Paterson, 1994). This presumes that the gradient of mass balance with altitude is relatively constant from year to year, but the intercept is shifted. Unlike glacier-length (terminus-position) changes, the ELA is entirely controlled by climatic processes and is not influenced by the behavior of glacier flow. In contrast to temperate or subpolar glaciers, the ELA on polar glaciers is the same as the snowline because there is no melting. One of the most attractive techniques for tracking the ELA on polar glaciers is satellite synthetic aperture radar (SAR) since it covers broad areas, it images through clouds, and it is self- illuminating.These attributes are particularly useful for the cloudy coastal conditions and long polar nights common to polar marine environments. For dry-snow conditions, typ- ical of polar glaciers, however, the SAR beam penetrates snow (Ulaby and others, 1981, 1982, 1986; Jezek and others, 1993; Lucchitta and others,1995) and would displace the location of the ELA up-glacier. Maximum penetration in homogeneous snow is estimated to be about 20m (Ulaby and others, 1981, 1982, 1986; Rott and MÌtzler, 1987). This effect should be quite pronounced for thin snow accumula- tions on low-sloping surfaces, characteristics common to polar glaciers. For example, with a small mass-balance gra- dient with altitude, typical of polar glaciers, we would expect roughly a 2 km shift in the apparent snowline for a glacier with a 5 slope. The purpose of our study was to examine the extent of ELA displacement on a polar glacier and deter- mine a correction for the SAR-estimated ELA. STUDY SITE We examined two glaciers in Taylor Valley, one of the McMurdo DryValleys in Antarctica (Fig.1). This area was chosen because of ongoing glaciological investigations in the valleys on four glaciers that include measurements of mass balance, energy balances, ice velocity and ice depth. Mean annual temperature in TaylorValley is ^17C (Clow and others, 1988), and summer temperatures have not often exceeded freezing since 1993 (unpublished information from P.T. Doran). The glaciers show little or no evidence of melt except in the lowermost fringe near the glacier terminus and along the cliff face, which forms the terminus (Lewis and others,1995; Fountain and others,1998). Most of the mass loss is from sublimation, which during the summer represents 40^90% of the ablation (Lewis and others,1995). The two glaciers we studied were Commonwealth and Howard Glaciers. In this paper we examine Commonwealth Glacier in detail, and briefly mention the results from Howard Glacier in the discussion section. Commonwealth Glacier (Fig. 2) faces south-southeast with a gentle (5) slope. Annals of Glaciology 34 2002 # International Glaciological Society 177