Characterization of Snow and Ice Reflectance Zones On Glaciers Using Landsat Thematic Mapper Data

Landsat Thematic Mapper (TM) data have been analyzed to study the reflectivity characteristics of three glaciers: the Grossglockner mountain group of glaciers in Austria and the McCall and Meares Glaciers in Alaska, USA. The ratio of TM band 4 (0.76-{).90 ILm) to TM band 5 (1.55-1.75 ILm) was found to be useful for enhancing reflectivity differences on the glaciers. Using this ratio, distinct zones of similar reflectivity were noted on the Grossglockner mountain group of glaciers and on the Meares Glacier; no distinct zones were observed on the McCall Glacier. On the TM subscene containing the Grossglockner mountain group of glaciers , 28 .2% of the glacierized area was determined to be in the zone corresponding most closely to the ablation area, and 71.8% with the location of the accumulation area. Using these measurements, the glacier system has an accumulation area ratio (AAR) of approximately 0.72. Within the accumulation area, two zones of different reflectivity were delineated. Radiometric surface temperatures were measured using TM band 6 (10.4-12 .5 ILm) on the Grossglockner mountain group of glaciers and on the Meares Glacier. The average radiometric surface temperature of the Grossglockner mountain group of glaciers decreased from 0.9 ± 0.34°C in the ablation area, to -0.9 ± 0.83°C in the accumulation area. INTRODUCTION Glaciers and ice sheets are comprised of an ablation and an accumulation area. Within these areas, several facies are present. Facies display a distinctive group of characteristics that reflect the environment under which the snow or ice was formed. The ablation area consists of exposed ice during the summer and contains the ice facies. The accumulation area can be sub-divided into the wet-snow facies, the percolation facies and the dry-snow facies (C.S. Benson, personal communication) . Development of the discrete facies is directly related to the temperature regime and mass balance of a glacier or ice sheet. Net loss by melting occurs in the ice facies. In the wet-snow facies all snow derosited since the end of the previous summer is raised to 0 C and wetted by the end of the melt season. The superimposed ice zone consists of a mass of ice which can overlap both the ice facies and the wet-snow facies. The annual increment of new snow is not completely wetted nor does its temperature reach the melting point in the percolation facies . Negligible melting occurs in the dry-snow facies (Benson 1962; Benson and Motyka 1978). At least 104 some of these facies can be detected using Landsat Multispectral Scanner (MSS) and Thematic Mapper (TM) data. In this paper, the use of Landsat TM data for detecting glacier surface conditions and for relating these conditions to the glacier facies is studied through analysis of a glacier group in Austria and two glaciers in Alaska, USA. BACKGROUND The Landsat TM sensor acquires data in seven spectral bands. TM bands I through 5 and 7 are in the visible, near-infrared and middle infrared wavelength regions and have a spatial resolution of each picture element (pixel) of approximately 30 m. TM band 6, a thermal infrared band , is sensitive to infrared surface temperature and has a resolution of 120 m. Spectral reflectivity of snow as determined from the visible, nearand middle-infrared bands is dependent on snow parameters such as grain size and impurity content of the surface layers of the snow (Dozier 1984). Williams (1983[a], [b], 1987) found that computerenhanced Landsat MSS images were useful for analysis of ice and snow reflectivity differences present on Vatnaj6kull, an ice cap in Iceland, especially when Landsat MSS data acquired at the end of the summer melt-season were custom processed . In addition, Crabtree (1976) found that Landsat MSS imagery of an outlet glacier, Merkurjokull, of the Myrdalsj6kull ice cap, Iceland, showed a reflectivity boundary that was attributed to differences in glacier surface conditions. There has also been considerable evidence that Landsat MSS data can show the location of the equilibrium line (Hall and Ormsby 1983). The MSS band 7 (0 .8 l.l ILm) is located in a wavelength region which is close to the TM band 4 (0.76 0.90 ILm) region and has been used for detection of surface water on snow and ice (Holmgren and others 1975; Rango and others 1975). TM band 4 has been found by Dozier (1984) to be sensitive to snow grain size and TM band 2 (0.53 0.61 /Lm) to be sensitive to contamination. STUDY AREAS TM digital data of the Grossglockner mountain group of glaciers in the eastern Austrian Alps, the Meares Glacier in the Chugach Mountains in southern Alaska, and the McCall Glacier in the Brooks Range of Alaska, have been analyzed. The Grossglockner mountain group of glaciers is located at approximately 47°10'N, 12°45'E in the Noric Alps of Austria. The TM scene (50155-09272) of the Grossglockner group was acquired on 3 August 1984. The TM scene (50518-20372) of the Meares Glacier in the eastern part of the Chugach Mountains was acquired on I August 1985 and is centered at approximately 61°30'N. 148°30'W. McCall Glacier is located in the Romanzoff Mountains of the Brooks Range at 68 ° 19' N, 143 ° 48' W in northern Alaska. The McCaU Glacier TM scene (50196-20474) was acquired on 13 September 1984. RESULTS TM band 4 (O.76-{).90/lm) was found to show the greatest variability of the 6 reflective bands, in spectral response in the glacierized areas. Much of the TM band 4 variability in spectral response is caused by snow grain size difference in the accumulation area of the glaciers, and melting or refrozen, previously melted snow. The spectral response pattern of TM band 2 (O.52-{).60 /lm) generally follows that of TM band 4 but detector saturation is more common in band 2 over snow-covered areas. TM band 5 (1.55-1.75 /lm) is quite useful for distinguishing between clouds and snow, and also shows subtle surface reflectivity differences on the glaciers. TM band 6 (10.4-12 .5 /lm), the thermal band, is useful for measuring radiometric surface temperature and detecting high cirrus clouds over snow and ice. Hall alld others: Reflectance zones all glaciers Using the computer compatible tapes (CCTs), the contrast between imaged features can often be enhanced by band ratioing (Moik 1980). This technique is particularly useful in eliminating the intensity variatIOns caused by shadows. The ratio of TM band 4 to TM band 5 produces an image product that enhances snow and ice features because of the large difference in spectral response in snow and ice features between band 4 where high digital numbers (DNs, a measure of spectral reflectance) are common and band 5 where low DNs characterize snow and ice. Contrast enhancement is especially evident in the accumulation area of the glaciers, where the difference between the TM band 4 and 5 spectral response is the greatest. Grossglockner mountain group of glaciers Observations of TM imagery and transects across the Grossglockner mountain group of glaciers using TM bands 2, 4 and 5 digital data from CCTs reveal that there are three separate zones in which spectral reflectivity is distinctive. These zones relate to differences in snow and ice surface conditions, e.g. presence of surface water and differences in snow grain size. Fig.1 is an image processed by employing the TM band 4/ 5 ratio and assigning colors according to ranges of DNs as seen in Table 1. Zone I is within the ablation area or ice facies and may be underestimated due to the similarity in DN between the debris-covered margin of the ice facies and the background. The snow line delineates the ablation area from Zone 11 Fig .1. Image obtained from ratlOlfig TM bands 4 and 5 (4 / 5) and assigning colors to reflectance zones of the Grossglockner mountain group of glaciers; Landsat TM image (50155-09272) was acquired on 3 August 1984. Zone I is believed to correspond to the ablation area and Zones IT and III are within the accumulation area.