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Physical Characteristics of the Mccall Glacier, Brooks

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Physical Characteristics of the Mccall Glacier, Brooks Snow and Ice-Symposium-Neiges et Glaces (Proceedings of the Moscow Symposium, August 1971; Actes du Colloque de Moscou, août 1971): IAHS-AISH Publ. No. 104, 1975. Physical characteristics of the McCall Glacier, Brooks. Range, Alaska G. Wei 1er, D. Trabant and C. Benson Abstract. The McCall Glacier spans an altitude range from 1300 to 2600 m at 69° N, 144° W in the Brooks Range of Northern Alaska. Preliminary results of energy and mass balance studies of the glacier indicate a strongly negative annual radiation balance. Screening from the sun of a large portion of the glacier surface results in the reduction by as much as half of the possible hours of sunshine. The resulting radiation anomaly is probably typical of the protected, north-facing glaciers in the Brooks Range and may be an important factor in the survival of these small glaciers in a region of relatively low precipitation and high summer temperatures. Stratigraphie studies show a hard wind-packed snow layer underlain by a thick layer of depth-hoar. The depth-hoar is destroyed during the summer at all points on the McCall Glacier and serves as a site for localization of ice lenses which form in the firn by refreezing of percolating meltwater. Refreezing of the meltwater at various depths in the firn adds enthalpy to the subsurface and raises the firn temperature substantially. Runoff from the glacier occurs all year long; from September to early June, this runoff freezes to form large perennial 'aufeis' deposits which have been repeatedly surveyed. Résumé. L'altitude du glacier McCall varie entre 1300 à 2600 m à 69° Nord, 144° Ouest dans la 'Brooks Range' au nord d'Alaska. Des résultats préliminaires d'études d'énergie et d'équilibre de masse concernant le glacier indiquent une balance de radiation annuelle fort négative. Quand une grande partie de la surface du glacier est protégée contre le soleil, cela a pour résultat que les heures de soleil possible sont réduites à tant que la moitié. L'anomalie de radiation qui en résulte est probablement typique pour les glaciers protégés et qui donnent sur le nord de la 'Brooks Range' et peut être un facteur important pour la survivance de ces petits glaciers dans une région de relativement peu de précipitation et de hautes températures estivales. Des études stratigraphiques montrent une assise dure, tassée par le vent, de neige sous laquelle se trouve une assise épaisse de gelée blanche. La gelée blanche est détruite pendant l'été à chaque point du glacier McCall et sert à localiser des lentilles de glace qui se forment dans le névé en regelant de l'eau fondante qui filtre à travers. La regelée de l'eau fondante à des profondeurs différentes du névé augmente la chaleur de la sous-surface et lève la température du névé considérablement. Il y a écoulement du glacier pendant toute l'année; de septembre au début de juin cet écoulement gèle pour former de grands dépôts pérennants de 'aufeis' qui ont été examinés à plusieurs reprises. The McCall Glacier lies at latitude 69.3N, longitude 143.2W on the north-facing slope of the Romanzoff Mountains of the Brooks Range, Alaska. This range provides a zonal barrier between the extreme continental climate of interior Alaska and the polar climate of the Arctic Basin, with both regimes influencing the local glacier climate. The glacier lies above the prevailing summer stratus cloud decks which cover the Arctic Ocean and extend right into the foothills of the Brooks Range. Its weather is influenced mainly by storms moving north of the glacier over the ocean. Storm wind directions are westerlies but these are considerably modified by the local topography in the lower sections of the glacier. Glacier and katabatic winds characterize the meso-scale circulation between storm periods. Precipitation is relatively light, with approximately 50 cm of water, falling largely as snow in spring and fall. The glacier chronology includes five distinct advances (Keeler, 1959). The furthest advance gave the glacier a length of approximately 20 km, compared with the present 6 km, and a maximum height of 300 m above the present valley floor. However, despite these advances, glaciation in the Brooks Range was never very extensive. With the relatively low precipitation patterns and high summer temperatures characterizing Physical characteristics of the McCall Glacier, Brooks Range, Alaska 89 the climate it is perhaps surprising that glaciers survive at all. Through the present studies, relationships between climate and glacier responses typical of this Arctic region are sought. This paper treats the physical-thermal characteristics of the McCall Glacier system, another paper in this session deals with the glacier's mass balance aspects. The broad features of the heat balance of the McCall Glacier are presented in Table 1 and are based largely on data taken during the IGY (Orvig, 1961). These data are essentially confirmed by our own observations, so far only partially reduced; particularly the temperature and radiation data taken during summer. All available radiation data cover only the time span from the beginning of March to the beginning of September, and extrapolation was necessary to give a complete annual cycle. This extrapolation assumed the net radiation to change little during the winter months, as reflected by the 'coreless' patterns of temperature and radiation observed at other Arctic and Antarctic stations (Dalrymple et al, 1966). The heat balance presented here is to be considered as a first estimate only, which is subject to revision. The most interesting result of this budget is the strong negative annual radiation balance of—15 cal cm-2 day-1 or —5.5 Kcal cm-2 year-1. This is emphasized when comparing the McCall Glacier data with other data at similar latitudes as shown in Table 2. In winter, all stations listed are located on surfaces which are snow-covered. Skies are generally clear at all sites and the radiation balances match fairly well. Summer, however, shows appreciable contrasts which are also reflected in the annual values. Low cloud decks (at ARLIS II) reduce the net incoming radiation during summer, low albedos of the surface (Barrow, tundra vegetation) increase it despite the presence of low clouds. Higher elevations (McCall, 2000 m) should increase the net incoming radiation, but screening by surrounding mountains reduces it below sea-level values of areas with unobstructed horizons (Mawson). This latter effect is particularly interesting and measurements show a reduction by 50% of the total possible hours of sunshine during summer on McCall Glacier. The resulting radiation anomaly is probably typical of the protected, north-facing glaciers in the Brooks Range. Computations of sensible and latent heat fluxes by eddy diffusion are in progress but the results cannot be presented yet. Initial short-period results, however, seem to indicate ablation rates in summer (approximately 1 cm day-1) and hoar frost deposition rates in spring (approximately 0.3 mm day-1) which seem physically realistic. The ice and firn temperatures raise some interesting questions which will be discussed below in connection with the physical characteristics of the glacier. Stratigraphie studies, which include detailed temperature and density profiles in the snow and ice, demonstrate that the McCall Glacier spans an interesting and (so far) little-studied part of the glacier facies spectrum. The annual snow accumulation has the same physical characteristics as the seasonal snow on Alaska's Arctic Slope, namely, a hard, wind-packed layer underlain by a thick layer of depth-hoar. The depth-hoar layer is destroyed during the summer at all points on the McCall Glacier and serves as a site for localization of ice lenses which form in the firn by refreezing of percolating meltwater. The ice layers within the firn apparently increase in thickness each year for at least 5 years because superimposed ice forms on them. The complex network of nearly horizontal ice lenses and layers together with nearly vertical ice glands forms a structure which locally protects snow and firn from vertical stress of the overburden. Layers of anomalously low density (0.30 to 0.35 g cm-3) containing depth-hoar crystals have been observed 2.5 m below the surface under large ice layers. These low density layers which occur in firn several years old may be due to a combination of (1) protection from vertical stress, (2) drainage of mass through and from them, and (3) growth of depth-hoar crystals. 90 G. Weller, D. Trabant and C. Benson o 7 o Z 3 < I O ON + o + + ê <* 3 tu n p. S o U t3 o Z 2 Physical characteristics of the McCall Glacier, Brooks Range, Alaska 91 TABLE 2. Net radiation values at several Arctic and Antarctic stations Winter Summer Annual (cal cm-2 (cal cm-2 (Kcal cm"' Station Latitude day-') day"') y-1) Author McCall 69.3N -100 +100 - 5.5 Present Study Barrow 71.4N - 65 +260 + 5.3 Weaver (1969) ARLIS II Drifting - 70 + 30 -12.8 Roulet (1969) 88-67N Mawson 67.5S -100 +130 - 5.6 Weller (1968) Refreezing of percolating meltwater at various depths in the firn adds enthalpy to the subsurface and raises the temperature. The 10 m temperature was about —3.0°C in 1969 and 1970; this is close to measurements made to depths of 91 m during 1958 (Orvig and Mason, 1963). In the lower parts of the glacier where bare ice is exposed during the summer, no vertical percolation of meltwater occurs. Consequently, lower temperatures are measured in the ice at lower altitudes on the glacier than in firn at higher altitudes. The 10 m temperature in the ice at 1700 m altitude is —8°C, which is close to the mean annual temperature.
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