Determination of the equilibrium line altitude ELA

The equilibrium line altitude marks the area or zone on the glacier where accumulation is balanced by ablation. The ELA is sensitive to variations in winter precipitation, summer temperature, and wind transport of dry snow. When the annual net mass balance is negative, the ELA rises, and when the annual net balance is positive, the ELA drops. The steady-state ELA is defined as the ELA when the annual net balance is zero, and can be calculated by linear regression analysis of annual net balance data and corresponding ELAs over some years (Fig. 4.4).

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Nigardsbreen, western Norway

y = 1560.2 - 133.02* R2 = 0.909

Net mass balance (m water equivalents)

Figure 4.4 The steady-state ELA (= 1560 m) at Nigardsbreen, an eastern outlet glacier from Jostedals-breen, western Norway. Based on data (observation period 1962-1997) in Kjollmoen (1998)

PREVAILING WIND DIRECTION

PREVAILING WIND DIRECTION

Glacier Diagram Decreasing Altitude
Figure 4.5 Schematic examples showing the difference between the TP-ELA (temperature-precipitation equilibrium line altitude) at plateau glaciers and the TPW-ELA (temperature-precipitation-wind equilibrium line altitude) at cirque glaciers. (Modified from Dahl and Nesje, 1992)

The climatic ELA is the average ELA over a 30-year period (corresponding to a 'normal' climatic period).

Climatic processes influencing the ELA on glaciers commonly involve ablation (mainly determined by the summer temperature) and the winter accumulation (reflecting the winter precipitation) giving the TP-ELA (see below). In addition, however, wind transport of dry snow is an important factor for the glacier mass balance. On plateau glaciers, snow deflation and drifting dominate on the windward side, while snow accumulates on the leeward side. By calculating the mean ELA in all glacier quadrants, the influence of wind on plateau glaciers can be neglected. The resulting ELA is therefore defined as the TP-ELA (temperature/precipitation ELA). The TP-ELA reflects the combined influence of the regional ablation-season temperature and accumulation-season precipitation (Dahl and Nesje, 1992; Dahl et al, 1997).

In deeply incised cirques and valleys surrounded by wide, wind-exposed mountain plateaux, the snow may deflate from the plateaux and accumulate in the cirques and valleys, either by direct accumulation on the cirque/ valley glaciers, or by avalanching from the mountain slopes. This may thereby increase significantly the accumulation on the cirque/valley glaciers (Dahl and Nesje, 1992; Tvede and Laumann, 1997). Consequently, the mean ELA on a plateau glacier (average for all quadrants) defines the TP-ELA, while the ELA on a cirque glacier, commonly influenced by wind-transported snow, gives the TPW-ELA. Therefore, the TPW-ELA is commonly lower than the TP-ELA (Fig. 4.5).

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  • Tesfalem
    How to calculate equilibrium line altitude?
    1 year ago

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