Réf. Noetzli & al. 2007 - A

Référence bibliographique complète
NOETZLI J., GRUBER S., KOHL T., SALZMANN N., HAEBERLI W. Three-dimensional distribution and evolution of permafrost temperatures in idealized high-mountain topography. Journal of Geophysical Research, 2007, Vol. 112, 14 p.

Abstract: Permafrost degradation is regarded as a crucial factor influencing the stability of steep rockwalls in alpine areas. Discernment of zones of fast temperature changes requires knowledge about the temperature distribution and evolution at and below the surface of steep rock. In complex high-mountain topography, strong lateral heat fluxes result from topography and variable surface temperatures and profoundly influence the subsurface thermal field. To investigate such three-dimensional effects, numerical experimentation was conducted using typical idealized geometries of high-mountain topography, such as ridges, peaks, or spurs. The approach combines a surface energy balance model with a three-dimensional ground heat conduction scheme to investigate belowground temperature distribution and permafrost occurrence in high-mountain topography. Time-dependent simulations are based on scenario data gained from regional climate models. Results indicate complex three-dimensional patterns of temperature distribution and heat flow density below mountainous topography for equilibrium conditions, which are additionally perturbed by transient effects. Permafrost occurs at many locations where temperatures at the surface do not indicate it, e.g., on the south face of ridges or below the edges of a peak. The modeling tools applied have potential for a number of studies in high mountains addressing questions related to permafrost distribution and evolution at depth in real topographies, for instance, the reanalysis of temperature-related instabilities.

Mots-clés
Permafrost, heat fluxes, modeling, distribution, evolution, climate change.

Organismes / Contact
Glaciology and Geomorphodynamics Group, Department of Geography, University of Zurich, Zurich, Switzerland.
Laboratoire Environnements, Dynamiques et Territoires de la Montagne, UMR 5204, CNRS, Université de Savoie, Chambéry, France.
GEOWATT AG, Zurich, Switzerland.

(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
  Permafrost    

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Switzerland Alps     2000-4500 m asl  

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

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

(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 
Observations
 
Modélisations
Steady State Conditions
The temperature distribution is governed by the difference in surface temperatures between the two flanks of the ridges, which leads to near-vertical isotherms in the top part of the ridge. Isotherms are curved as a result of the geometry. The main heat flux is directed horizontally from the warmer to the colder side in the top part and diagonally upward in the middle part of the ridge and vertical heat fluxes only exist at the base of the geometry. The heat flux density is largest at the top due to the largest temperature difference on the shortest distance. In addition, a zone of increased temperature gradients and heat flux can be found at the foot of the colder side. A corresponding zone of lower heat flux density is located in the middle of the warmer side.

Changes in elevation have no major effect on the relative temperature distribution pattern. At elevations up to 4000 m asl thick permafrost exists on both sides. Permafrost thickness decreases with increasing height mainly on the warmer side. At elevations lower than ~3500 m asl surface temperatures are above 0°C on the southern side of our model. The influence of the geothermal heat flux decreases exponentially toward the top and becomes negligible in the upper half of the geometry. A lower thermal conductivity slightly increases the influence of the geothermal heat flux, whereas steeper and more extreme topography decreases it.

In a pyramid geometry similar effects can be observed but with four sides having different surface temperatures. Additionally, irregularities on rockwall surfaces such as spurs with sides exposed to different aspects may modify the subsurface temperature field. This leads, for instance, to local permafrost occurrence in a south facing rockwall that generally does not contain permafrost.

Transient Ground Temperatures
The main heat flux is directed horizontally from the warmer to the colder side, but strongest heat fluxes exist in the upper part on the warmer side. Temperature gradients and heat fluxes near the surface increase strongly due to the larger temperature difference between the warming surface and the temperatures at depth that still remain unchanged. On the colder side a reversal of the direction of heat fluxes takes place, and hence at the depth reached by the temperature signal a zone exists where heat flows toward it from both sides.

Where the temperature raises toward 0°C, energy is needed to melt ice contained in pore spaces. This effect leads to a delay in the propagation of the temperature signal into the subsurface and increases the time lag between changes in surface conditions and temperatures at depth.

Possible changes in MAGST between the time periods 1982-2002 and 2071-2091 were simulated for a 60° slope at an elevation of 3500 m asl. For north oriented slopes the authors assumed a linear temperature change over the next 100 years of +3.5°C, for south and east/west oriented slopes the change was set to 2.5°C and 3°C, respectively. It can mainly be attributed to differing amounts of direct solar radiation received.

For the next 100 years, the lower limit of permafrost distribution at the surface rises to nearly 4000 m asl on south facing rockwalls, which is higher than most peaks in the European Alps. However, a substantial permafrost body remains in the underground but surface conditions no longer indicate it. Within only one or two centuries after a temperature increase permafrost occurrence several hundred meters below the surface is not substantially affected. A permafrost body can remain unaffected by changes at the surface over centuries to millennia where temperatures little below the surface have changed significantly and many parts of the surface have become free of permafrost. The 20th century warming [Beniston et al., 1997; Diaz and Bradley, 1997; Haeberli and Beniston, 1998] has not yet penetrated to greater depth but affects temperatures in the upper decameters.

Main results
The steady state temperature field below highmountain topography is mainly controlled by spatially varying surface temperatures between different mountain sides and is little influenced by the geothermal heat flux in the higher parts. Isotherms are nearly vertical and a strong heat flux is directed from the warmer to the colder side of the mountain.

Permafrost may occur underground at locations where surface temperatures do not indicate it, even in steady state conditions.

Irregularities on the surface, such as spurs, may modify ground temperatures and induce local permafrost occurrence.

Permafrost degradation in steep topography takes place from different sides, affecting both the permafrost table and the permafrost base. This leads to an increase in the pace of deeper permafrost degradation as compared to permafrost in flat terrain, where warming typically penetrates vertically into the ground.

Owing to the long time needed for a temperature signal to penetrate to greater depth, permafrost can remain inside mountains over centuries. At some locations where surface temperatures rise above 0°C substantial permafrost occurrence can be found. Timescales involved in deep permafrost degradation are on the order of millennia, even without the retarding effect of latent heat. The influence from past cold periods such as the last ice age is likely to still be found in the interior of mountain peaks.

With rising surface temperatures, heat fluxes strongly increase near the surface.
Hypothèses
 

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 
This study explores the influence of three-dimensional high-mountain topography on the subsurface thermal field in steady state, and its evolution over time given changing surface temperatures. A modeling chain that considers the processes in the atmosphere (climate), at the surface (energy balance) and in the deeper subsurface (heat conduction) was developed. A surface energy balance model was combined with a three-dimensional heat conduction scheme, and for time-dependent calculations the authors obtained scenario climate time series from Regional Climate Models (RCMs).

This study is based on numerical experimentation with idealized test cases of typical topographic features in high mountains. Ridges and mountain peaks were identified as the main forms and simplified to triangular prisms and pyramid geometries. The authors generated corresponding artificial digital elevation models (DEMs) and varied their topographic attributes: elevations were set between 2000 and 4500 m asl, slope values were set between 50° and 70°, and the four main slope orientations were considered.

Ground temperatures for geometries were modeled by coupling an energy balance model and a heat conduction scheme. The energy balance model TEBAL [Gruber, 2005; Stocker-Mittaz et al., 2002] is driven by climate time series (Corvatsch, 3315 m asl and Jungfraujoch, 3580 m asl) and calculates mean annual ground surface temperatures (MAGST). The overall simulation of daily mean rock temperatures resulted in a mean absolute difference in MAGST of 1.7°C. MAGST was calculated for a 10-year period (1990-1999) using hourly time series from the high-elevation site at Corvatsch (3315 m asl), Upper Engadine (Data source: MeteoSwiss) for the DEMs. These are imposed as upper boundary condition in the numerical heat conduction scheme FRACTURE [Kohl and Hopkirk, 1995], which computes a three-dimensional subsurface temperature field. The authors ignore annual temperature variations that may penetrate up to about 12 m in bedrock [Gruber et al., 2004b] and only consider long-term variations as related to climate change (timescales of decades to centuries). Therefore the climatologic perturbations are treated as transient effects defined for the upper boundaries. The transient thermal field is then superimposed on a steady state temperature regime defined from the lower basal heat flow boundary condition.

The scenario climate time series were generated from output of RCM simulations in the scope of a study by Salzmann et al. [2007b]. A set of 12 RCM-based daily climate time series was created from the results of five RCM simulations that were performed within the European project PRUDENCE [see Christensen et al., 2002]. The results of the RCM control (1961-1990) and scenario runs (2071-2100) were adapted for high-mountain situations using the so-called delta and bias approaches that are discussed by Salzmann et al. [2007a]. The constructed scenario climate time series were applied to TEBAL and the average change in surface temperature was calculated for 36 specific topographical situations.

(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
 

Références citées :

Beniston, M., H. F. Diaz, and R. S. Bradley (1997), Climatic change at high elevation sites: An overview, Clim. Change, 36, 233 251.

Christensen, J. H., T. R. Carter, and F. Giorgi (2002), PRUDENCE employs new methods to assess European climate change, Eos Trans. AGU, 83, 147.

Diaz, H. F., and R. S. Bradley (1997), Temperature variations during the last century at high elevation sites, Clim. Change, 36, 253279.

Gruber, S., L. King, T. Kohl, T. Herz, W. Haeberli, and M. Hoelzle (2004b), Interpretation of geothermal profiles perturbed by topography: The Alpine permafrost boreholes at Stockhorn Plateau, Switzerland, Permafrost Periglacial Processes, 15, 349 357.

Gruber, S. (2005), Mountain permafrost: Transient spatial modelling, model verification and the use of remote sensing, Ph.D. thesis, 121 pp. Univ. of Zurich, Zurich, Switzerland.

Haeberli, W., and M. Beniston (1998), Climate change and its impacts on glaciers and permafrost in the Alps, Ambio, 27, 258 265. - [Fiche biblio]

Kohl, T., and R. J. Hopkirk (1995), FRACTURE: A simulation code for forced fluid flow and transport in fractured porous rock, Geothermics, 24, 345359.

Salzmann, N., C. Frei, P. L. Vidale, and M. Hoelzle (2007a), The application of regional climate model output for the simulation of high-mountain permafrost scenarios, Global Planet. Change, 56, 188202.

Salzmann, N., J. Noetzli, C. Hauck, S. Gruber, and W. Haeberli (2007b), Ground surface temperature scenarios for complex high-mountain topographies based on regional climate model results, J. Geophys. Res., doi:10.1029/2006JF000527, in press.

Stocker-Mittaz, C., M. Hoelzle, and W. Haeberli (2002), Permafrost distribution modeling based on energy-balance data: a first step, Permafrost Periglacial Processes, 13, 271282.