M1 Accumulation

Topic

The accumulation mode encompasses all the processes by which the cryosphere gains mass: solid precipitation falling as snow, hail, or ice pellets; the direct deposition of frost as water vapor turns straight into ice on a cold surface; and the redistribution of the snowpack as wind pushes snow from one location to another. Accumulation represents the positive component of a glacier's mass balance; indeed, it determines the extent of the glacier's accumulation zone—the area situated above the equilibrium line. The accumulation rate recorded at a given elevation is the primary climatic parameter controlling long-term glacial growth.

Solid precipitation forms within clouds containing both supercooled liquid water and ice crystals; this occurs when supercooled water droplets freeze upon contact with freezing nuclei—tiny mineral, biological, or combustion particles suspended in the air. Within such mixed-phase clouds, water vapor reaches saturation more easily over ice than over liquid water; consequently, vapor migrates from the liquid droplets toward the ice crystals, allowing the crystals to grow at the expense of the droplets, which lose vapor in the process. The colder the cloud and the higher the water vapor supersaturation within it, the faster each ice crystal grows.

Once snow has fallen to the ground, wind can remobilize it, transporting tens of tons of snow per linear meter of terrain in a single season; this process fills entire ravines and deposits additional mass on the leeward side of mountain ridges. As the wind transports this suspended snow, a fraction of the particles sublimates directly into the air; this process can remove between 10% and 40% of the original solid precipitation in areas where the wind blows most intensely.

An altitudinal precipitation gradient model—which distributes precipitation based on how it varies with elevation—serves as the standard tool for allocating snow accumulation within hydrological models of entire glaciated basins; it applies the same accumulation rate to each elevation within the basin, ultimately determining the growth of each glacier in the basin described.

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