Thresholds for Ice Lens Development
in Glacier Snow and Firnpack
Mauri S. Pelto1 and Maynard M. Miller2
On the Juneau Icefield, Alaska, ice lenses and ice layers make up 515% of the entire annual accu-
mulation layer volume in midsummer. In contrast, on North Cascade, Washington glaciers in mid-
summer, ice lenses make up less than 0.1% of the entire annual accumulation layer volume.
The presence of ice lenses that can develop within glacier snowpacks have several important ramifi-
cations: 1) slowing the percolation of meltwater downward through the snowpack, allowing deeper
areas to remain below freezing for longer; 2) retarding meltwater movement through, and conse-
quent meltwater release from, the glacier system, and 3) refreezing of meltwater, which in turn is
now internal accumulation and can be an important aspect of glacier mass balance. If climate warm-
ing prevented this internal accumulation from occurring, than this meltwater would be lost and the
entire glacier would have a more significantly negative mass balance.
Ice lens formation requires the presence of cold-dry snow at the beginning of the melt seasons. Late
winter snowpack temperatures in the North Cascades range from 0 to 1C, versus 3 to 7C in
southern Alaska. The threshold snowpack temperature for ice lens formation is then below 2C.
Winter air temperatures at three stations in the North Cascades adjacent to glaciers indicate mean
winter temperatures of 3 to 6C. In southern Alaska mean winter temperatures have been noted on
several glaciers to be 8 to 11C. Another important factor reflected in the temperature record is the
frequency and intensity of melting events. From 1 December to 1 March the number of melt event
days (temperature maximum above 3C) at glaciated levels in the North Cascades averaged 12 days.
These events provide critical meltwater that is either refrozen as ice lenses if the snowpack is cold
enough, or warm up the snowpack to 0C via release of latent heat and percolation. On Lemon Creek
Glacier, Alaska, the mean number of winter melting days is 3.
Ice lens development thresholds in the maritime snowpack are minimal melt events in the winter, air
temperatures below 8C, and snowpack temperatures below 2 or 3C. The transition from cold-
dry snowpack to 0C snowpack that does not allow ice lens development is identifiable using SAR
imagery. Thus, areas of internal accumulation and retarded meltwater release can be identified
readily. This also provides an important climate marker for mean winter temperature conditions.
1
Nichols College, North Cascade Glacier Climate Project, Dudley, Massachusetts 01571, USA
2
Glaciological and Arctic Sciences Institute, University of Idaho, Moscow, Idaho 83843, USA
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