Aufeis

Sheet like mass of layered ice

Aufeis ('/aʊ//f//aɪ//s/ OWfysse) (German for "ice on top") is a sheet like mass of layered ice that forms from successive flows of ground or river water during freezing temperatures. This form of ice is also called overflow, icings,

Aufeis accumulates during winter along stream and river valleys in arctic and subarctic environments. It forms by upwelling of river water behind ice dams, or by ground water discharge. The latter mechanism prevails in high gradient alpine streams as they freeze solid. Ground water discharge is blocked by ice, perturbing the steady state condition and causing a small incremental rise in the local water table until discharge occurs along the bank and over the top of the previously formed ice. Successive ice layers can grow and expand, attaining areas of ~25+ km2 and thicknesses of 6+ m.

River aufeis form when the cross sectional area of a stream channel becomes locally restricted as ice accumulates during winter. Such restrictions or “choke points” result in bulk overflow and local increases in hydrostatic pressure, causing water to move upward through fissures onto the original ice layer. This overflowing water subsequently freezes to produce additional ice layers, thus explaining the origin of the term aufeis. and localized thicknesses of 6+ m. The cumulative area of late winter aufeis in the Sagavanirktok River drainage alone, for example, ranges from 102 to 103 km2.

Sheets of aufeis may block stream channels and cause their flood plains to widen as spring floodwaters are forced to flow around the ice. Research on aufeis has to a large extent been motivated by the variety of engineering problems the ice sheets can cause (e. g. blocking drainages and causing flooding of roads). Aufeis can present an extreme danger to recreational boaters even during summer months, who can find themselves trapped between walls of ice or pulled under aufeis by the current of the river. Breaking dams of aufeis can cause flash floods downriver. When thawed, aufeis leave footprints in the form of “aufeis glades”".

In late 2011, Mongolia planned to test the use and storage of artificial naleds as a way of cooling Ulan Bator in the hot Mongolian summer, and reducing the use of energy intensive air conditioning.

River aufeis are common and widespread features of the arctic cryosphere, particularly in northern Alaska and Siberia. In eastern Siberia, where aufeis are known as naleds,  10,000 aufeis with a cumulative area of ~ 50,000 km2 containing 30 km3 of water have been documented in 2015. Sheets of aufeis have been observed in Alaska, Russia, and Mongolia. Analysis of satellite imagery from 2000 to 2015 has shown that the extent and duration of many Alaskan river icings has decreased due to climate change.