Spatial Distribution

The processes that govern the existence and evolution of mountain permafrost can be categorized into the scales and process domains of climate, topography and ground conditions (Fig. 3.2). The climate scale governs the global distribution of cold climates in mountains. It refers to the influence that latitude and global circulation have on the general climatic characteristics of an area. These climatic conditions are then further modified by topography, which affects ground temperatures because of its strong influence on surface micro-climatology. This influence is due to differences in ambient air temperature caused by elevation, differences in solar radiation caused by terrain shape, or snow transport by wind and avalanches. Locally, the influence of topographically altered climate conditions on ground temperatures are modified further by ground properties and their influence on heat transfer. Here, coarse block layers result in relative ground cooling when compared to bedrock or fine-grained substrate, and a high ice content can significantly retard warming and permafrost degradation at depth.

The distinction between these three scales and process domains is not sharply defined. The effect that topography has on regional precipitation patterns, for instance, spans the scales of climate and topography, and the effect of snow redistribution on ground temperatures spans the scales of topography and ground conditions.

Climate Topography Ground conditions

Fig. 3.2 Conceptual hierarchy of scales and process domains that influence ground temperature and permafrost conditions in mountain areas. The white disk in the two leftmost images refers to a location that is then depicted in the image to the right — and has its conditions further overprinted by the respective conditions of that scale

Climate Topography Ground conditions

Fig. 3.2 Conceptual hierarchy of scales and process domains that influence ground temperature and permafrost conditions in mountain areas. The white disk in the two leftmost images refers to a location that is then depicted in the image to the right — and has its conditions further overprinted by the respective conditions of that scale

maritime continental

Fig. 3.3 Schematic of mountain permafrost distribution (changed from King et al. 1992). Lines indicate a general trend, but the shape (straight line) is a strong generalization (glaciation limits, for instance, rather rise exponentially with increasing continentality)

maritime continental

Fig. 3.3 Schematic of mountain permafrost distribution (changed from King et al. 1992). Lines indicate a general trend, but the shape (straight line) is a strong generalization (glaciation limits, for instance, rather rise exponentially with increasing continentality)

Nevertheless, this concept of scales is useful for understanding the diverse influences on mountain permafrost characteristics. The overall magnitude of the effect of topography and ground conditions can be as high as 15°C within a horizontal distance of 1 km — a similar difference in ground temperature in polar lowland areas would normally occur over a latitudinal distance of roughly 1,000 km.

In the European Alps, a mean annual air temperature below -3°C can be used for first-order classification of altitudinal belts that have significant amounts of permafrost. However, this rule is subject to many exceptions, and may not hold for other mountain areas. Figure 3.3 illustrates the influence that continentality has on mountain permafrost distribution. We speak of continental climates where total precipitation and cloudiness are low and total solar radiation as well as annual and diurnal temperature amplitudes are high. Maritime areas have high precipitation, often overcast skies, and rather small temperature amplitudes and solar radiation sums. The upper limit of closed forests rises along with summer air temperatures, which are higher in continental climates. The glaciation limit rises with decreasing precipitation towards continental areas, whereas the permafrost limit rises towards maritime areas because thick snow cover provides insulation during winter and results in warmer ground temperatures. However, this only holds true for gently inclined slopes that accumulate a thick snow cover. The regional boundary for permafrost in steep bedrock is probably much less affected by continentality. The relative difference between sun exposed and shaded slopes is usually greater in steep than in moderately inclined terrain, because of the dampening effect of snow cover, and it is higher in continental areas because of the increased solar radiation. As a consequence of these patterns, permafrost can exist in forested mountain areas in continental climate, whereas in the European Alps even alpine meadows usually are a reliable sign of the absence of permafrost. In maritime climates, the glaciation limit is lower than the regional limit of permafrost. As a consequence, perennially frozen talus and rock glaciers are often absent, because their potential locations are covered by glaciers, and permafrost only exists in steep bedrock.

Permafrost in mountain areas occurs in a wide range of materials and surface cover types, which decisively influence ground temperatures. One of the most prominent surface covers are coarse block layers. They exert a cooling influence on ground temperatures and thus affect permafrost distribution patterns. For this reason, coarse rock has also received considerable attention from the engineering community as a construction material (Goering and Kumar 1996). The cooling influence of blocky layers is mainly based on three processes:

(a) Temperature-driven convection of air

(b) A reduced warming effect of the winter snow

(c) The advection of latent heat by snow that enters deep into the voids of the active layer.

During winter, ground temperature is higher than near-surface air temperature and, in deposits with sufficient permeability, free convection of air can thermally couple the atmosphere and the sub-surface effectively. Because a closed snow cover reduces or inhibits convection, the effectiveness of this cooling mechanism is greatest in areas or during times with little snow. The warming effect of the winter snow cover is based on a contrast in thermal resistance between cold and warm periods. This contrast reduces the influence that cold winter temperatures have on ground temperatures at depth. Because block layers have a very low thermal conductivity, they reduce the contrast between summer and winter by increasing the overall thermal resistance. In this way, block covers can result in significant ground cooling by reducing the warming effect of the winter snow (Gruber and Hoelzle 2008). The magnitude of this relative cooling is greatest in areas with thick snow cover. In very coarse deposits, snow can penetrate deeply into the voids of the active layer. Especially in areas with high wind speed, this process can advect significant latent heat into the ground, which is only slowly removed by heat conduction from the warming surface during summer.

Permafrost and ground temperatures in steep bedrock are discussed in depth by Gruber and Haeberli (2007). Unfortunately, little quantitative understanding exists with respect to the many intermediate conditions in the spectrum between steep bedrock and moderately inclined coarse blocks that make up a large proportion of mountain permafrost areas. For example, the influence of water flow and summerwinter contrasts of thermal conductivity in fine-grained soil, or the influence of snow on temperatures in moderately steep rock walls, are hardly known at present.

Active talus slopes as well as active volcanic areas (Kellerer-Pirklbauer et al. 2007) often accumulate permafrost deposits consisting of debris or scoria mixed and inter-layered with snow deposits. Very ice-rich talus often begins to creep and ultimately forms rock glaciers. Figure 3.4 shows a buried perennial snow patch in aggrading permafrost, and illustrates the influence of topography and strong winds on the spatial pattern of such mixed deposits.

Unusual forms of permafrost can sometimes be found in areas that have a mean annual air temperature several degrees above freezing. Ice caves, for instance, preserve ice (and thus permafrost conditions) over several years (see Luetscher et al. 2005). The main process responsible for this effect is strong density-driven exchange of air through the cave system during winter, which terminates during summer when the cold air is stratified stably in the cave. Additionally, winter snow sometimes falls through the cave opening (bringing with it significant latent heat) and does not melt during summer because almost no solar radiation arrives inside the cave, and air exchange with the warm surface is minimal. Steeply inclined slopes of coarse blocks often have permafrost conditions at the foot of the slope, which are caused by a seasonal sub-surface ventilation pattern ("chimney effect") which can reduce the mean temperature in the lower parts of steep and blocky slopes locally by several degrees (Delaloye and Lambiel 2005).

Fig. 3.4 The interplay of strong winds and topography governs the spatial distribution of permafrost characteristics and small glaciers on Deception Island, Maritime Antarctic. The contrast of Light-colored substrate on the ridge in the foreground of the left panel and the darker lower slopes is due to wind transport of fine scoria from convex to concave areas. Similarly, snow is transported and deposited. On the right panel, a cross section through aggrading permafrost is shown. The sequence from top to bottom is: active layer in fine scoria; permafrost in fine scoria (above buried snow patch); buried snow patch consisting of dense ice in the lower and compact snow in the upper part; permafrost in fine sediments; and unfrozen sediments where the permafrost has been undercut by a stream

Fig. 3.4 The interplay of strong winds and topography governs the spatial distribution of permafrost characteristics and small glaciers on Deception Island, Maritime Antarctic. The contrast of Light-colored substrate on the ridge in the foreground of the left panel and the darker lower slopes is due to wind transport of fine scoria from convex to concave areas. Similarly, snow is transported and deposited. On the right panel, a cross section through aggrading permafrost is shown. The sequence from top to bottom is: active layer in fine scoria; permafrost in fine scoria (above buried snow patch); buried snow patch consisting of dense ice in the lower and compact snow in the upper part; permafrost in fine sediments; and unfrozen sediments where the permafrost has been undercut by a stream

Fig. 3.5 Left: hanging glaciers and ice faces on the northern side of the ridge extending between the Matterhorn and the Dent d'Herens along the border between Switzerland and Italy. Right: an incipient rock glacier (arrow) at sea level as well as ice faces and hanging glaciers only a few hundred meters higher on Livingston Island, Maritime Antarctic

Fig. 3.5 Left: hanging glaciers and ice faces on the northern side of the ridge extending between the Matterhorn and the Dent d'Herens along the border between Switzerland and Italy. Right: an incipient rock glacier (arrow) at sea level as well as ice faces and hanging glaciers only a few hundred meters higher on Livingston Island, Maritime Antarctic

Two types of phenomena often visually indicate the presence of permafrost in mountain areas (Fig. 3.5). Rock glaciers and other creep phenomena form distinct landforms caused by the slow deformation of cohesive, ice-rich sediments (Haeberli et al. 2006). When thawed, relict forms can be used to infer past permafrost conditions. Ice faces and hanging glaciers, on the other hand, only indicate current permafrost conditions, because they leave no long-lived remnants after degradation. Ice faces, hanging glaciers and active rock glaciers are reliable indicators of permafrost. Their absence, however, does not indicate the absence of permafrost.

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