Réf. Gruber & al. 2004b - A

Référence bibliographique complète
GRUBER S., HOELZLE M., HAEBERLI W. Rock wall temperatures in the Alps - modelling their topographic distribution and regional differences. Permafrost and Periglacial Processes, 2004, Vol. 15, p. 299-307.

Abstract: Rising temperatures or the complete thaw of permafrost in rock walls can affect their stability. Present as well as projected future atmospheric warming results in permafrost degradation and, as a consequence, makes knowledge of the spatial distribution and the temporal evolution of rock temperatures important. Rock-face near-surface temperatures have been measured over one year at 14 locations between 2500 and 4500 m a.s.l. in the Alps. Different slope aspects have been included in order to capture the maximum spatial differentiation of rock temperatures. These data were used to further develop and verify an energy-balance model that simulates daily surface temperatures over complex topography. Based on a 21-year (1982–2002) run of this model, spatial patterns of rock-face temperatures in the Swiss Alps are presented and discussed. This model provides a basis for the reanalysis of past rock-fall events with respect to permafrost degradation as well as for the simulation of future trends of rock temperatures.

Mots-clés
Rock temperatures, rock faces, Alps, mountain permafrost, energy balance, slope instability, rock fall.

Organismes / Contact
Department of Geography, Glaciology and Geomorphodynamics Group, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland. stgruber@geo.unizh.ch

(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
Temperature Permafrost - Rock wall temperature    

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Switzerland Alps     2500-4500 m  

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations
The inter-annual variability in mean annual air temperature (MAAT) is of the order of 23°C for Swiss mountain stations and global radiation has fluctuated by about +/- 5% in recent decades (based on Aschwanden et al., 1996).
Modélisations
 
Hypothèses

Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 

(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 
Observations
 
Modélisations
Generally, mean annual temperatures and the temperature fluctuations are represented well. The south-facing site has a much greater variability because it receives a strong signal of short-wave radiation in addition to the long-wave signal related to air temperature. The amplitude of the day-to-day noise on the north-facing rock wall is in the order of 2.5°C and about twice this amount on the south-facing slope. Temperature variability as well as model uncertainty are greater in locations with a high input in solar radiation. The overall simulation of rock-wall temperatures with a mean coefficient of determination of 0.88 and a mean absolute difference in the mean annual ground surface temperature of 1.2°C is considered very encouraging.

The elevation of the modelled mean 0°C isotherm for both areas (Corvatsch and Jungfraujoch) shows that between slope angles of 90° and 50°, temperatures generally increase (and so the isotherm rises) with decreasing slope due to increased solar irradiation. The disturbance due to snow cover (Goodrich, 1982; Keller and Gubler, 1993; Zhang et al. 2001) that is not taken into account in this model increases at the same time, so the temperatures calculated for shallower slopes should be interpreted with care.

To illustrate the differentiation between the two locations, the difference between Corvatsch and Jungfraujoch mean temperatures modelled for a slope angle of 70° has been studied. Generally, the differences increase from north to south because they are related to short-wave radiation. The increase from lower towards higher elevations is caused by the growing importance of shortwave over long-wave radiation. Short-wave incoming radiation increases with elevation whereas long-wave incoming radiation decreases due to lower air temperature, air pressure, vapour pressure and other factors.

A large inter-annual fluctuation of the mean annual ground surface temperature has been observed and thus supports the combined approach of measurement and modelling used in this study.
Hypothèses
The combination of MAAT and global radiation fluctuations can lead to an expected compound inter-annual signal having a range of up to 5°C. These signals are progressively dampened by heat diffusion with increasing depth in the rock.

It is obvious that in rugged topography air temperature alone is a poor surrogate for rock surface temperatures. Net short-wave radiation is likely to be the major controlling factor causing the lateral variation of several degrees Celsius that can be deduced from previous research (Gruber et al., 2003).

In comparison with debris-covered slopes, rock faces react quickly to climate change. This is due to the absence of a block layer (Harris, 1996; Harris and Pedersen, 1998; Mittaz et al., 2000; Hoelzle et al., 2001) and corresponding direct coupling of surface and sub-surface conditions, combined with a low water content and a small transfer of latent heat during melt. This rapid reaction together with the effect of destabilization make rock fall due to permafrost degradation a likely and well perceivable impact of climate change in the near future (Gruber et al., 2004).

Due to 20th century warming (Haeberli and Beniston, 1998; Beniston et al., 1997; Diaz and Bradley, 1997) the actual lower limit of permafrost in rock walls is likely to be about 1.0°C or 150 m lower. This is due to possible remnants of inactive permafrost at greater depths within a rock face.

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)

Air temperature and net short-wave radiation are the major controlling factors causing rock wall temperature variations.

14 loggers, installed at elevations between 2000 and 4500 m a.s.l., yielded complete one-year time series from 15 October 2001 to 14 October 2002. Loggers were placed in the following areas: Gornergrat/Stockhorn, Monte Rosa and Kleinmatterhorn/Gandegg close to Zermatt, Birg/Schilthorn and Jungfraujoch in the Bernese Alps and Corvatsch/Furtschellas close to St. Moritz. For the recording of realistic daily surface temperatures, thermistors were placed at a depth of 10 cm. Temperatures were logged every 2 hours. All measurement sites were roughly vertical and several metres above flat ground to ensure snow-free conditions.

The model PERMEBAL (Stocker-Mittaz et al., 2002) has been extended and adapted for the simulation of rock temperatures in rugged topography. The model uses daily meteorological time series of air temperature, vapour pressure, air pressure, precipitation, wind speed, wind direction and global radiation. Based on these data, atmospheric variables are extrapolated over complex topography and the surface energy balance is simulated. All 14 locations with measured time series were simulated using PERMEBAL.

One year of near-surface rock-temperature data is not sufficient to deduce permafrost distribution because of the large inter-annual fluctuations of air temperatures and global radiation. Therefore, rock-wall surface temperatures are modelled for the period 19822002 based on existing meteorological data from the stations Corvatsch and Jungfraujoch (data source: MeteoSwiss). Corvatsch represents central-Alpine conditions with high radiation budgets and little clouding, whereas the data from Jungfraujoch is typical for the northern Alps, having comparably little direct solar irradiation and frequent clouding. For both areas, slopes of 50°, 70° and 90° steepness are modelled. Calculations are performed every 500 m at elevations between 2000 and 5000 m a.s.l. and for eight aspects. The same conditions as in the model verification were used. The depth discretization was 10 cm, having 200 nodes.

(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

Paramètre de l'aléa
Sensibilité du paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 
 

(4) - Remarques générales
 

(5) - Syntèses et préconisations
The retreat of glaciers and the melt of ice faces change the temperature (and possibly also the hydrological) regime of many steep mountain-sides at an increasing speed. The tools developed here can assist the combined, quantitative assessment of ice/rock faces under transient conditions. Northern slopes have the most abundant permafrost, and, at the same time the least spatial differentiation in rock temperature. This provides the opportunity to model a large proportion of existing rock-wall permafrost in Switzerland with relatively low computational effort and to estimate its total area. The successful simulation of temperatures over a wide range of air temperature and radiation regimes establishes the validity of the model in the Alps. It is expected to yield results of equal reliability for other years using measured past time series of the Swiss meteorological network.

Références citées :

Beniston M, Diaz HF, Bradley RS. 1997. Climatic change at high elevation sites: an overview. Climatic Change 36(3): 233251.

Diaz HF, Bradley RS. 1997. Temperature variations during the last century at high elevation sites. Climatic Change 36(3): 253279.

Goodrich LE. 1982. The influence of snow cover on the ground thermal regime. Canadian Geotechnical Journal 19: 421432.

Gruber S, Peter M, Hoelzle M, Woddhatch I, Haeberli W. 2003. Surface temperatures in steep Alpine rock faces - a strategy for regional-scale measurement and modelling. In Proceedings of the Eighth International Conference on Permafrost 2003. Swets & Zeitlinger: Zurich; 325330.

Gruber S, Hoelzle M, Haeberli W. 2004. Permafrost thaw and destabilization of Alpine rock walls in the hot summer of 2003. Geophysical Research Letters 31: doi:10.1029/2004GL020051. [Fiche Biblio]

Haeberli W, Beniston M. 1998. Climate change and its impacts on glaciers and permafrost in the Alps. Ambio 27(4): 258265. [Fiche Biblio]

Harris SA. 1996. Lower mean annual ground temperature beneath a block stream in the Kunlun Pass, Qinghai Province, China. In Proceedings Fifth Chinese Permafrost Conference, Lanzhou; 227237.

Harris SA, Pedersen DE. 1998. Thermal regimes beneath coarse blocky material. Permafrost and Periglacial Processes 9: 107120.

Hoelzle M, Mittaz C, Etzelmüller B, Haeberli W. 2001. Surface energy fluxes and distribution models of permafrost in high mountain areas: an overview of current developments. Permaf rost and Periglacial Processes 12(1): 5368.

Keller F, Gubler HU. 1993. Interaction between snow cover and high-mountain permafrost, Murtèl/Corvatsch, Swiss Alps. In Proceedings of the Sixth International Conference on Permafrost, Beijing, Vol. 1. South China University of Technology Press: Beijing; 332337.

Mittaz C, Hoelzle M, Haeberli W. 2000. First results and interpretation of energy-flux measurements over Alpine permafrost. Annals of Glaciology 31: 275280.

Stocker-Mittaz C, Hoelzle M, Haeberli W. 2002. Modelling alpine permafrost distribution based on energybalance data: a first step. Permafrost and Periglacial Processes 13(4): 271282.

Zhang T, Barry RG, Haeberli W. 2001. Numerical simulations of the influence of the seasonal snow cover on the occurence of permafrost at high latitudes. Norsk Geografisk Tidskrift 55: 261266.