Physical characteristics of the McCall Glacier, Brooks Range, Alaska

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.
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Physical characteristics of the McCall Glacier, Brooks Range, Alaska
91
TABLE 2. Net radiation values at several Arctic and Antarctic stations
Station
Latitude
Winter
(cal cm - 2
day-')
Summer
(cal cm - 2
day"')
Annual
(Kcal cm"'
y-1)
Author
McCall
69.3N
-100
+100
- 5.5
Present Study
Barrow
71.4N
- 65
+260
+ 5.3
Weaver (1969)
ARLIS II
Drifting
88-67N
- 70
+ 30
-12.8
Roulet (1969)
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.
There is some runoff from the McCall Glacier all year long. From September to
early June, this runoff freezes to form a perennial aufeis deposit which extends
2 km down the McCall Creek at the end of winter and 1 km at the end of summer. The
aufeis has been surveyed during fall of 1969 and 1970 and spring 1970 in an attempt
to measure its mass balance. A cross-section 100 m from the glacier terminus has ice
up to 10 m thick and 80 m wide at the time of minimum mass.
Acknowledgements. This study was supported by NSF Grant GA-10090. Logistic support was
provided by ONR through the Naval Arctic Research Laboratory at Barrow, Alaska.
REFERENCES
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Orvig, S. (Ed.) (1961) McCall Glacier, Alaska. Meteorological Observations 1957-58. Arctic
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Orvig, S. and Mason, R. W. (1963) Ice temperatures and heat flux, McCall Glacier, Alaska. IASH,
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