A problem with high uaX warming rates in the Column model

As the warming rate required by the uaX term in the Column model increases, the curvature of the temperature profile increases, and the minimum and basal temperatures decrease (see Figure 6.11 and discussion on p. 134). Near the equilibrium line, the downglacier warming rate is high because meltwater percolating into the firn raises near-surface temperatures. (In the modeling for Figure 6.12 this effect was included by using an effective value of X that is higher than the atmospheric lapse rate.) If the ice at depth is assumed to be warming at the same rate, calculated minimum temperatures in profiles near the equilibrium line are often lower than the minima in profiles just upglacier. In plots such as Figure 6.12, this appears as a pocket of cold ice beneath the equilibrium line that is surrounded by warmer ice (Hooke, 1977, Figure 4c). Such a temperature distribution is physically impossible; to have cooled off, this ice would have had to have lost heat to colder ice, yet it is surrounded by warmer ice.

To circumvent this problem, as noted, a warming rate of 1 uaX was used.

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