Gruber 2012 a Global View on Permafrost in Steep Bedrock

8/12/2019 Gruber 2012 a Global View on Permafrost in Steep Bedrock

1/6

131

A Global View on Permafrost in Steep Bedrock

Stephan Gruber

Department of Geography, University of Zurich, Switzerland

Abstract

Mountainous topography covers a considerable proportion of the global permafrost region. There, temperature changes

and the degradation of permafrost in steep bedrock can evoke rapid geomorphic change, some of which may result innatural hazards. Due to the strong relief of most rock walls, corresponding events can have a long runout and transform

into cascading events, such as impact to lakes or the damming of rivers that propagate far below the periglacial zone. The

identication of areas that potentially have permafrost conditions in steep rock is the important rst step in the evaluation

of corresponding hazards. During the past decade, researchers have given considerable attention to permafrost in steep

bedrock, although most investigations have been conducted in the European Alps. This contribution provides a summary

of ndings related to the delineation of permafrost in rock walls. Based on this, simple considerations and data sources are

outlined that may help the application of current knowledge in remote mountain regions.

Keywords:bedrock slopes; mountains; steep terrain; global; hazard zonation; mapping.

Introduction

A signicant proportion of the global permafrost region

has mountainous topography (Gruber 2011a). Additionally,

coastal cliffs may be subject to permafrost conditions, and

changes to permafrost there can alter the stability of rock

faces perched high above valleys and thus pose a hazard to

human life and infrastructure (cf. Carey 2005). This may

occur indirectly through, for example, impacts to lakes that

may cause long-distance secondary events (cf. Geertsema &

Cruden 2008, Geertsema & Clague 2011). However, non-

stationarity, incomplete understanding of processes, and lack

of information on local surface and subsurface conditions

challenge corresponding hazard zonation. Here, the delineation

of permafrost rock slopes can serve as a proxy for areas thatmay require special attention or surveillance, as they are

prone to rapid changes in response to climate forcing (Gruber

2011b). Absent or minimal vegetation, a reduced snow cover,

and less important active-layer processes make permafrost in

steep rock a system that is simple enough for attempting an

extrapolation of ndings to environments not measured. In this

contribution, I summarize ndings on the spatial and temporal

variability of mean annual ground temperature (MAGT) and

permafrost in steep bedrock. These ndings, often based on a

narrow range of environmental conditions, are extended with

hypotheses for generalization to other environments. This is

intended to support the estimation of permafrost conditions in

steep rock globally, and to provide a framework to test andimprove them by means of measurement, experiment, and

observation. Unless stated otherwise, the investigations cited

have been performed in the European Alps.

Rock Temperature: Factors and Processes

Basic considerations

The energy uxes at and near the ground surface are shown

for the two conceptual end members of steep and homogenous

bedrock and a horizontal debris slope in Figure 1. Longwave

radiation is mostly affected by air temperature, moisture

content, and clouds. Turbulent uxes mostly depend on airtemperature and wind.

Solar irradiation, however, is strongly dependent on

cloudiness, sun-terrain geometry, and topography, and results

in a high degree of spatial differentiation. Figure 2 shows three

conceptual scales that determine ground temperatures. When

considering global patterns, differing climate conditions affect

the strength of the topographic inuence.

For example, in areas with more abundant clouds and diffuse

radiation, topographic differentiation is less pronounced than

in areas with few clouds and strong direct solar radiation. The

Figure 1. Schematic of energy balance at and near the ground

surface in steep bedrock (A) and gentle debris slopes (B).

Figure 2. Conceptual scales of inuence on ground temperatures.


2/6

132 TENTHINTERNATIONALCONFERENCEONPERMAFROST

spatial differentiation of MAGT can be conveniently described

relative to mean annual air temperature (MAAT) using

(1)

Because MAAT is a commonly available climate variable,

dT helps to estimate local patterns relative to it. MAAT is

often subject to strong intra-annual variation, but dT for near-

vertical rock remains rather constant (cf. Noetzli & VonderMuehll 2010).

Near-vertical and compact rock

Near-vertical and compact rock faces exhibit a strong

topographic control on subsurface temperatures and a weak

thermal inuence of snow and active-layer processes typical

for debris-covered slopes (Fig. 1). Spatial patterns of dT are

dominated by direct solar radiation (cf. Hoelzle 1994) and

modied by rock albedo (cf. Hall 1997). For this reason,

they are a simple system to investigate and to then gradually

expand by considering additional processes. Gruber et al.

(2003) present a review of previous investigations and a

strategy for measuring in steep rock faces. Corresponding data

are available through PERMOS (cf. Noetzli & Vonder Muehll

2010) and a pan-Alpine inventory of permafrost evidence

(Cremonese et al. 2011). In near-vertical and compact rock,

MAGT is often measured 530 cm below the surface and

presumed to be equivalent to that just below the active layer.

Based on the work of Allen et al. (2009) summarizing 14

one-year time series from the Southern Alps of New Zealand

(44S), dT in sun-exposed slopes can be estimated to 4.5

6.5C. In the European Alps (46N), Boeckli et al. (2012)

estimated this to be 5.56C based on 57 locations and a

statistical model. A process-based modeling study (Gruber

et al. 2004) in the European Alps resulted in dT rangingfrom 6.5 to 9C. The model is driven by daily data that do

not resolve temperature differences between east and west

exposure. This may result from convective clouding in the

afternoon lowering the temperature on west-exposed faces by

reducing direct solar radiation. Generally, dT increases with

elevation due to more shortwave and less longwave radiation,

and with decreasing precipitation due to a higher amount of

direct radiation. Reection from surrounding snow-covered

slopes can locally increase shortwave irradiation and thus dT.

This effect is assumed to have affected dT at two measured

locations by 23C (cf. Allen et al. 2009). Without any solar

radiation, dT would become slightly negative due to longwave

loss.Summarizing this paragraph, dT likely ranges from 0 to 9C

in steep and compact rock at latitudes near 45, and varies

mostly as a function of solar radiation.

Surface heterogeneity and clefts

Most rock slopes differ from the near-vertical case because

micro-topography and fracturing promote patchy covers of

debris, snow, and ice (Gruber & Haeberli 2007). The thermal

inuence of snow, fractures, and active-layer processes can

thus contribute to the heterogeneity of dT. Safety issues

and the dominance of lateral heterogeneity in near-surface

measurements of MAGT, however, complicate corresponding

research. Gubler et al. (2011) found that MAGT on more

gentle slopes can already vary by up to 2.5C within an area of

10 m x 10 m. For heterogeneous bedrock slopes, this value is

likely often exceeded.

Using measurements at four depths down to 85 cm in

borings and deeper on thermistor strings lowered into clefts,

Hasler et al. (2011) estimated a local cooling effect of fractureson the order of 1.5C. This cooling resulting from ventilation

likely depends on cleft aperture and abundance, and its

functioning may resemble ventilation effects in coarse blocks

(cf. Harris & Pedersen 1998) or, in extreme cases, those in

ice caves (Obleitner & Sptl 2011, Schner et al. 2011). A

local lowering of dT by several C, even in shaded slopes,

thus appears possible in rare cases. For practical use, however,

an assumption of 12C cooling, increasing from shaded to

sun-exposed slopes, appears plausible for heterogeneous

conditions. Snow deposited in clefts may have an additional

cooling effect by consuming latent heat during melting. At

the same time, however, snow cover or inll can inhibit air

circulation and thus reduce the associated cooling (Hasler et

al. 2011). A thermal diode effect caused by seasonally varying

proportions of ice/water/air in pores and fractures can cause a

lowering of temperatures at depth (Gruber & Haeberli 2007)

similar to the thermal offset described for lowland permafrost

(cf. Smith & Riseborough 2002), but this has not been detected

in measurements (Hasler et al. 2011).

Snow and perennial ice

Snow on rock faces alters the coupling between the

atmosphere and subsurface and thus affects dT. It is usually

thinner and spatially and temporally more variable than in

gentle terrain. Due to difcult access, corresponding studiesare rare (cf. Wirz et al. 2011). Rock faces often extend far

above the equilibrium line of glaciers and receive much of their

precipitation in solid form. In those conditions, temperature

moderates the retention of snow because cohesive snow near

melting conditions is more likely to accumulate than cold

snow that is prone to be removed by wind and sliding. For

cold and shaded faces, this can cause a pronounced snow

cover during summer, whereas precipitation in winter barely

affects the bare and dark surfaces of ice and rock (cf. Gruber &

Haeberli 2007). Depending on the seasonality of precipitation,

snow during summer can be common in high-elevation

rock faces. For thin snow, the net warming through thermal

insulation during winter is reduced and the relative importanceof cooling through albedo and latent heat uptake during melt

becomes greater. Higher albedo decreases dT more strongly

on sun-exposed slopes, whereas the effect of latent heat can

cause a lowering of dT in all slope aspects.

Based on numerical experiments under the assumption of

laterally homogeneous snow cover, Pogliotti (2011) estimated

this cooling to have magnitudes of up to 2C with respect to

snow-free rock. In reality, this is further modied in laterally

heterogeneous and discontinuous snow cover because a lateral

interaction between snow-covered and snow-free rock facets

dT= MAGT- MAAT


3/6

GRUBER 133

exists by heat conduction in rock as well as by radiative and

sensible exchange at the surface (cf. Neumann & Marsh 1998).

Based on distributed measurements of rock temperature,

Hasler et al. (2011) estimated the cooling effect of snow in

radiation-exposed faces to be up to 23C.

Ice faces, hanging glaciers, or surface lowering of glaciers

can inuence the temperature regime in steep rock and can

exhibit strong spatio-temporal variations (Fischer et al.

2011). While ice faces and hanging glaciers are diagnostic ofpermafrost conditions at their rock-ice interface, their absence

does not indicate the absence of permafrost. The occurrence of

cold rn (Suter et al. 2001, Suter & Hoelzle 2002, Hoelzle et

al. 2011), and thus hanging glaciers, follows differing patterns

than rock temperature because they are subject to higher albedo

and are affected by wind through snow transport. In gently

inclined parts of hanging glaciers, the percolation of water into

the rn and heat release at depth during refreezing can cause

zones of temperate rn and ice (Haeberli & Alean 1985). As

a consequence, an inference of temperature below ice cover

is difcult. For ice faces that are subject to minimal ow and

balance change, a cooling with respect to rock is likely for

sun-exposed slopes due to the inuence of albedo. For a north-

exposed ice face, Hasler et al. (2011) report approximately the

same mean annual temperature as in nearby rock.

Three-dimensional and transient effects

Spatial variation of surface conditions and their temporal

changes inuence permafrost bodies at depth (cf. Gold &

Lachenbruch 1973), and in mountain areas this is further

complicated by geometric effects (Kohl 1999, Gruber et al.

2004). The combination of a distributed MAGT model and

three-dimensional heat-transfer modeling of steep topography

(Noetzli et al. 2007) showed complicated permafrost bodies

with steeply inclined isotherms near peaks and ridges. Even insteady-state, permafrost can be induced below non-permafrost

terrain facets due to nearby cold slopes. Transient simulations

were performed with a similar setup (Noetzli & Gruber 2009)

and accounted for temperature changes during the past. This

revealed the importance of past climate and latent heat that

can lead to permafrost bodies persisting at depth below slopes

that today would be interpreted as permafrost-free. Because

of multi-lateral warming, the reaction of convex topography,

such as ridges or rock spurs, can be much faster than that of

at terrain (Noetzli & Gruber 2009). Several new boreholes

in steep rock and in ridges (Noetzli et al. 2010, Noetzli et al.

2008, Noetzli & Vonder Muehll 2010) support many of the

general ndings from the modeling studies.

The Global View: Estimating Spatial Patterns

of Rock Temperatures for Diverse Mountain

Ranges

In this section, simple approaches for estimating permafrost

occurrence in rock faces worldwide are proposed. It is

important to keep in mind that this is based on limited point

data from a restricted range of environmental conditions.

Furthermore, the derivation of such heuristics has an element

of subjectivity, especially where it involves the delineation of

permafrost zones.

Global distribution

For strongly shaded rock faces, the 0C long-term isotherm

of the MAAT is a good pointer to determine the elevation

above which permafrost must be expected, even though

permafrost may sometimes be found at lower elevation. Due

to the absence of better data, the zonation shown in Gruber

(2011a) can provide an approximation globally at about 1 km

resolution. While few rock faces are dominated by inversion

effects, inversions are important to consider in the choice

of local meteorological stations as possible alternatives.

As the extrapolation of MAAT in rugged terrain involvesconsiderable vertical distances, the choice of appropriate lapse

rates is important. If suitable ground measurements do not

exist, the lapse rate can be approximated based on model data

(Gruber 2011a) or regional radiosonde measurements. MAAT

data covering a period of several decades helps to minimize

the inuence of short-term uctuations. It is sometimes

necessary to account for longer-term transient effects that have

occurred during the past decades or centuries with respect to

the measurement period chosen (cf. Noetzli & Gruber 2009).

Topography and solar radiation

Topography inuences dT for individual slopes, mainly

through its geometric effect on incoming solar radiation.Generally, steeper and more rugged terrain causes a wider

range of dT than more gentle terrain. This effect, however,

is modied by latitude and the proportion of direct radiation.

As a rst approximation, a ruggedness index (e.g., Gruber

2011a) can be used to identify terrain tending to have a higher

proportion of steep and strongly shaded rock slopes. Locally,

the differentiation of slopes with respect to solar radiation can

be estimated based on the calculation of mean annual potential

solar irradiation and a decameter-resolution DEM (e.g., Farr et

al. 2007). This operation is routinely available in GIS software.

Figure 3. Latitude-dependency of maximum (solid) and minimum

(dashed) potential direct solar radiation received in rugged terrain.

The experiment shown spans slope angles 090, slope expositions

0180 (because east and west are identical in this respect), and has

no horizon shading.


4/6

134 TENTHINTERNATIONALCONFERENCEONPERMAFROST

Latitude

Besides its effect on MAAT, latitude inuences the

maximum amount of total annual solar irradiation received by

the most suitably exposed slope. For this reason, the range of

dT is also likely to be affected by latitude (Fig. 3). Further,

the partitioning of radiation with topography changes with

latitude. Toward the poles, increasingly steep slopes receive

the highest amounts of radiation; near the tropics, rather gentleslopes receive the highest radiation input. Furthermore, the

sensitivity to cast shadowing by obstructed horizons increases

toward the poles with the prevalence of low solar elevation

angles.

Continentality

Continental (as opposed to maritime) climate is

characterized by comparably large seasonal thermal variations

and dryness, both generally caused by large distances from

major water bodies and altered by atmospheric circulation

patterns and rain-shadow effects. The dryness in continental

areas is usually accompanied by very high amounts of direct

solar radiation afforded by often cloud-free skies. BesidesMAAT, continentality is often used to describe spatial patterns

of cryosphere phenomena (e.g. Holmlund & Schneider 1997,

King 1986, Haeberli & Burn 2002, Shumskii 1964) and is

frequently characterized by a continentality index dened as

the average difference between the coldest and warmest month

at a location.

The continentality index shown in Figure 4 provides an

appreciation of its global patterns that may help to inform the

extrapolation of ndings from one mountain range to another.

The patterns of temperature range, the proportion of direct

sunlight, and precipitation are jointly described by the concept

of continentality. However, these affect rock temperature in

differing ways and often correlate much less than expected

when plotted based on gridded climate data. Until further

analyses help to disentangle these effects, the assumption of

subdued spatial differentiation of dT toward maritime areas

likely provides a good assumption. This effect of continentally

or precipitation on the degree of spatial differentiation due tosolar radiation is, however, not clearly detectable in current

measurements (cf. Allen et al. 2009, Boeckli et al. 2012) that

span only small gradients.

Surface characteristics

As a general rule, rough surfaces can be assumed to be colder

than smooth ones. Cooling is caused by a thin, intermittent,

and frequent snow cover and by air circulation in fractures or

debris.

At least qualitatively, the roughness of rock walls can be

assessed based on eld visits, photographs, aerial photographs,

or high-resolution satellite imagery. Increasingly, the imagery

freely available on GoogleEarth, for example, has sufcientresolution to provide valuable rst impressions.

Conclusion

Based on research from a narrow range of environmental

conditions and the consideration of what appear to be the

processes that govern permafrost distribution in steep bedrock,

I have proposed simple considerations to estimate permafrost

in rock walls worldwide. Besides the potential for practical

application, this contribution points to the wide range of

Figure 4. Continentality index derived from 20002010 mean monthly surface air temperatures based on ERA-Interim re-analysis data. Dark

areas are maritime; light areas are continental climate.


5/6

GRUBER 135

environmental conditions that drive permafrost in steep terrain.

More systematic research in integrating local and regional

research to achieve a coherent understanding would be a good

avenue to progress beyond the heuristics presented here. Such

research would consolidate understanding by basing it on a

wider range of environmental conditions.

Acknowledgments

Over the past ten years, many colleagues and friends

have contributed toward the synthesis presented here.

These contributions have come not only through the works

referenced, but also though inspiring discussions and eld

campaigns. Among these are: Jeannette Ntzli, Andreas

Hasler, Marco Peter, Adrian Zgraggen, Lorenz Bckli, Jan

Beutel, Simon Allen, Martin Hoelzle, Ludovic Ravanel, Paolo

Pogliotti, Umberto Morra di Cella, Edoardo Cremonese, and

Reynald Delaloye. ECMWF ERA-Interim data used in this

study have been obtained from the ECMWF data server.

Michael Krautblatter is acknowledged for valuable and

insightful reviewer comments.

References

Allen, S., Gruber, S., & Owens, I. 2009. Exploring steep

bedrock permafrost and its relationship with recent slope

failures in the Southern Alps of New Zealand.Permafrost

and Periglacial Processes20(4): 345356.

Boeckli, L., Brenning, A., Gruber, S., & Noetzli, J. 2012. A

statistical permafrost distribution model for the European

Alps. The Cryosphere6(1): 125140.

Carey, M. 2005. Living and dying with glaciers: peoples

historical vulnerability to avalanches and outburst oods

in Peru. Global and Planetary Change47(2-4): 122134.Cremonese, E., Gruber, S., Phillips, M., Pogliotti, P., Boeckli,

L., Noetzli, J., Suter, C., Bodin, X., Crepaz, A., Kellerer-

Pirklbauer, A., Lang, K., Letey, S., Mair, V., Morra

di Cella, U., Ravanel, L., Scapozza, C., Seppi, R., &

Zischg, A. 2011. Brief Communication: An inventory

of permafrost evidence for the European Alps. The

Cryosphere5(3): 651657.

Farr, T.G., Rosen, P.A., Caro, E., Crippen, R., Duren, R.,

Hensley, S., Kobrick, M., Paller, M., Rodriguez, E., Roth,

L., Seal, D., Shaffer, S., Shimada, J., Umland, J., Werner,

M., Oskin, M., Burbank, D., & Alsdorf, D. 2007. The

Shuttle Radar Topography Mission.Review of Geophysics

45: RG2004.Fischer, L., Eisenbeiss, H., Kb, A., Huggel, C., & Haeberli,

W. 2011. Monitoring topographic changes in a periglacial

high-mountain face using high-resolution DTMs, Monte

Rosa East Face, Italian Alps.Permafrost and Periglacial

Processes22(2): 140152.

Geertsema, M. & Clague, J.J. 2011. Pipeline routing in

landslide-prone terrain.Innovation15(4): 1721.

Geertsema, M. & Cruden, D.M. 2008. Travels in the Canadian

Cordillera. InProceedings of 4th Canadian conference on

geohazards, Quebec City: 383390.

Gold, L.W. & Lachenbruch, A.H. 1973. Thermal conditions

in permafrost - a review of North American literature. In

F. J. Sanger and P. J. Hyde, eds. Yakutsk, USSR: National

Academy of Sciences, Washington D.C.: 325.

Gruber, S. 2011a. Derivation and analysis of a high-resolution

estimate of global permafrost zonation. The Cryosphere

Discussions5(3): 15471582.

Gruber, S. 2011b. Landsides in cold regions: making a science

that can be put into practice. InProceedings of the SecondWorld Landslide Forum, 39 October 2011, Rome, Italy.

Gruber, S. & Haeberli, W. 2007. Permafrost in steep bedrock

slopes and its temperature-related destabilization

following climate change. Journal of Geophysical

Research112(F2): F02S18.

Gruber, S., Hoelzle, M. & Haeberli, W. 2004. Rock wall

temperatures in the Alps: modelling their topographic

distribution and regional differences. Permafrost and

Periglacial Processes15(3): 299307.

Gruber, S., King, L., Kohl, T., Herz, T., Haeberli, W., &

Hoelzle, M. 2004. Interpretation of geothermal proles

perturbed by topography: the Alpine permafrost boreholes

at Stockhorn Plateau, Switzerland. Permafrost and

Periglacial Processes15(4): 349357.

Gruber, S., Peter, M., Hoelzle, M., Woodhatch, I., & Haeberli,

W. 2003. Surface temperatures in steep alpine rock faces -

A strategy for regional-scale measurement and modelling.

In Proceedings of the 8th International Conference on

Permafrost 2003, Zurich, Switzerland: 325330.

Gubler, S., Fiddes, J., Gruber, S., & Keller, M. 2011. Scale-

dependent measurement and analysis of ground surface

temperature variability in alpine terrain. The Cryosphere

5: 431443.

Haeberli, W. & Alean, J. 1985. Temperature and accumulation

of high altitude rn in the Alps. Annals of Glaciology6:161163.

Haeberli, W. & Burn, C.R. 2002. Natural hazards in forests:

glacier and permafrost effects as related to climate

change. In R.C. Sidle, ed. Environmental Change and

Geomorphic Hazards in Forests. Wallingford/New York,

CABI Publishing: 167202.

Hall, K. 1997. Rock temperatures and implications for cold

region weathering. I: New data from Viking Valley,

Alexander Island, Antarctica.Permafrost and Periglacial

Processes8(1): 6990.

Harris, S.A. & Pedersen, D.E. 1998. Thermal regimes beneath

coarse blocky material. Permafrost and Periglacial

Processes9: 107120.Hasler, A., Gruber, S., & Haeberli, W. 2011. Temperature

variability and thermal offset in steep alpine rock and ice

faces. The Cryosphere5: 977988.

Hoelzle, M. 1994.Permafrost und Gletscher im Oberenga-

din. Grundlagen und Anwendungsbeispiele fr autom-

atisierte Schtzverfahren. Ph.D. thesis. Switzerland:

ETH Zrich.

Hoelzle, M., Darms, G., Lthi, M., & Suter, S. 2011. Evidence

of accelerated englacial warming in the Monte Rosa area,

Switzerland/Italy. The Cryosphere5: 231243.


6/6

Gruber 2012 a Global View on Permafrost in Steep Bedrock

Documents

Transcript of Gruber 2012 a Global View on Permafrost in Steep Bedrock