Pôle Alpin Risques Naturels (PARN) Alpes–Climat–Risques Avec le soutien de la Région Rhône-Alpes (2007-2014)
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Réf. Gubler & al. 2011 - A

Référence bibliographique
GUBLER S., FIDDES J., KELLE M., and GRUBER S.: Scale-dependent measurement and analysis of ground surface temperature variability in alpine terrain, The Cryosphere, 5, 431-443, doi:10.5194/tc-5-431-2011, 2011. PDF

Abstract : Measurements of environmental variables are often used to validate and calibrate physically-based models. Depending on their application, the models are used at different scales, ranging from fewmeters to tens of kilometers. Environmental variables can vary strongly within the grid cells of these models. Validating a model with a single measurement is therefore delicate and susceptible to induce bias in further model applications. To address the question of uncertainty associated with scale in permafrost models, we present data of 390 spatially-distributed ground surface temperature measurements recorded in terrain of high topographic variability in the Swiss Alps. We illustrate a way to program, deploy and refind a large number of measurement devices efficiently, and present a strategy to reduce data loss reported in earlier studies. Data after the first year of deployment is presented. The measurements represent the variability of ground surface temperatures at two different scales ranging from few meters to some kilometers. On the coarser scale, the dependence of mean annual ground surface temperature on elevation, slope, aspect and ground cover type is modelled with a multiple linear regression model. Sampled mean annual ground surface temperatures vary from −4 °C to 5 °C within an area of approximately 16 km2 subject to elevational differences of approximately 1000 m. The measurements also indicate that mean annual ground surface temperatures vary up to 6 °C (i.e., from −2 °C to 4 °C) even within an elevational band of 300 m. Furthermore, fine-scale variations can be high (up to 2.5 °C) at distances of less than 14m in homogeneous terrain. The effect of this high variability of an environmental variable on model validation and applications in alpine regions is discussed.

Mots-clés
 

Organismes / Contact

Authors / Auteurs :

  • Gubler S., Department of Geography, University of Zurich, Switzerland
  • Fiddes J., Department of Geography, University of Zurich, Switzerland
  • Kelle M., Computer Engineering and Networks Laboratory, ETH Zurich, Switzerland
  • Gruber S., Department of Geography, University of 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
Swiss Swiss alps Corvatsch   mean 2700m a.s.l.  

(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

Over the whole study area, measured MAGST variations go up to 9 °C. MAGST vary also at very fine scales: even in homogeneous areas, variations amount to more than 2.5 °C at distances of less than about 14m at steep slopes or in terrain of large blocks.

Inter-footprints variability :

1) Slope and solar radiations

Exposition to the sun has a large influence, resulting in a difference of 1 °C between north and south facing slopes, if the slopes are rather gentle. Steep slopes however show a much larger variations between north and south, resulting in differences of 2 °C for 10 ° steep slopes, and up to 5 °C for 40 ° steep slopes. This clearly shows the influence of the incoming solar radiation on GSTs, since northern exposed, steep slopes receive almost no direct solar radiation (especially in winter time), in contrast to more gentle slopes. South-exposed slopes show an opposite behaviour: depending on the angle of the incoming solar radiation (and thus the season), steeper slopes receive more radiation and accumulate less snow than gentle slopes, resulting in faster snow melting and thus warming of the ground in spring. East exposed slopes are approximately 0.8 °C warmer than west exposed slopes, which can possibly be attributed to the formation of convective clouds during afternoons.

2) Slope and snow cover

A thick snow cover in early winter insulates the ground from cold air temperatures. On the other hand, a thin snow cover can cool the ground during winter due to the high albedo of snow. The timing and the duration of the snow cover have a large, non-linear influence on MAGST, and that snow cover can produce both a cooling and a warming of the GST in respect to the air temperature.

3) Ground

The use of multiple linear regression has shown that MAGST variability can be explained with the topographic variables elevation, slope, aspect and ground cover type. The model shows that MAGST are 1.6 °C to 2.2 °C higher in soil than within coarse blocks. This effect can be attributed to various processes (Gruber and Hoelzle, 2008), such as the ventilation of cold air below the snow cover on block fields or contrasts in thermal conductivity. Furthermore, moisture and water content encountered in the upper layer of the ground play a crucial role for GSTs.

Intra-footprints variability :

We found that MAGST can vary from 0.2 °C up to 2.5 °C within 100m2. The variability is larger at footprints with large boulders and in steep terrain. These fine-scale variations can be attributed to differing ground properties, water availability, heterogeneous snow cover, solar radiation and local shading of small to medium boulders, etc. However, the variability is small in homogeneous grass sites. Within large blocks, logger placement probably also has an effect on intra-footprint variability due to the difficulty of defining the surface.

 

Sur l’ensemble de la zone d’étude, les variations de la température moyenne annuelle de la surface du sol (MAGST) ont une amplitude de 9°C. On observe également des variations de la MAGST a une échelle plus fine. Ainsi, même sur des zones homogènes, on peut observer des amplitudes de températures supérieures à 2.5°C sur des distances d’environ 14m et sur des terrain en pentes ou composés de larges blocs.

Variabilité entre les secteurs d’études :

1) pente et exposition aux radiations solaires :

La différence d’exposition aux radiations solaires à un rôle important dans les variations de températures, et est à l’origine d’une amplitude de 1°C entre les pentes, si ces dernières ne sont pas trop raides, d’exposition nord et sud.

Les pentes raides, au contraire, connaissent une amplitude de température plus importante entre celles exposées nord et celles exposées Sud. Ainsi, on observe des différences de 2°C pour des pentes à 10° et des différences qui vont jusqu’à 5°C pour des pentes à 40°. Ces observations démontrent l’influence des radiations solaires sur la température du sol ; les pentes raides d’exposition nord ne recevant presque aucune radiation solaire (et ce, particulièrement en hiver), à l’inverse des pentes plus douces.

Les pentes d’exposition sud suivent une logique opposée. En raison de l’angle d’incidence des radiations solaires, les pentes les plus raides reçoivent plus de radiations et connaissent une accumulation neigeuse moins importante que les pentes plus douces. Ce phénomène entraîne une fonte de la neige plus rapide sur les pentes plus raides et, par conséquent, un réchauffement plus rapide au printemps.

Les pentes d’exposition est sont approximativement 0.8°C plus chaudes que celles d’exposition ouest, ce qui peut s’expliquer par la formation de nuages convectifs au cours de l’après-midi.

2) Pente et couverture neigeuse

Une épaisse couverture neigeuse en début d’hiver joue un rôle isolant et protège le sol des températures froides extérieures. D’un autre côté, une couverture neigeuse fine aura un rôle refroidissant durant l’hiver, en raison du fort albédo de la neige.

La période d’apparition et la durée de la couverture neigeuse ont une forte influence (qui ne suit pas un modèle linéaire) sur la MAGST. La couverture neigeuse peut donc à la fois réchauffer ou refroidir la température de surface du sol en fonction des variations de la température de l’air.

3) sol

L’utilisation d’une régression linéaire multiple a permis de déterminer que les variations de la MAGST trouvent une explication à travers les différentes variables topographiques : altitude, pente, type de sol.

Le modèle montre que la MAGST est entre 1.6°C et 2.2°C plus haute dans un sol dense que dans un sol composé de plus large éléments. Ces effets peuvent avoir des causes multiples comme le passage de l’air sous la couverture neigeuse dans le cas d’un sol composé d’éléments grossiers ou encore des différences de conductivité thermale. De plus, les moisissures et les poches d’eau que l’on peut retrouver dans les couches supérieures du sol jouent également un rôle important pour les GST.

Variabilité intra-secteurs :

Nous avons mis en évidence que la MAGST peut varier, sur une surface de 100m², de 0.2°C à 2.5°C. Cette variabilité connaît une amplitude plus importante sur les secteurs d’étude à pente forte & composés de gros blocs. Ces variations, que l’on retrouve à une échelle fine, peuvent être attribuées aux différentes propriétés et caractéristiques du sol, à la présence d’eau, à l’hétérogénéité de la couverture neigeuse, à l’incidence des radiations solaires ou encore aux ombres portées des blocs plus ou moins gros … On remarque que les variations de températures sont plus faibles sur les terrains composés de pelouse.

Modélisations
 
Hypothèses
 

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

In order to resolve the spatial patterns and the variability of GST around Corvatsch, 39 locations, so-called footprints, were selected such that most of the topographic variability within this area of approximately 16 km² is represented (Fig. 1). On the one hand, the focus in footprint selection lay on the influence of the topographic variables elevation, slope and aspect, and additionally ground cover types and terrain curvature. On the other hand, the replication of GST measurements within each 10m×10m footprint reflects the variability in GST at a fine scale.

To represent GST variability due to slope, aspect and ground material, one main elevational band was selected for intense instrumentation. It ranges from 2600m to 2900ma.s.l. Some footprints lie outside this band and reflect the dependence of GST on elevation. The footprints cover all aspects, steep and gentle slopes and different ground cover types such as meadow, fine material and large blocks (Table 1).

In order to record near surface temperatures and avoid heating by direct solar radiation, the iButtons were buried approximately 5cm into the ground or placed between and underneath boulders. GST is measured every 3h at 0.0625°C resolution, enabling operation for 512 days.

Afin d’expliquer l’organisation spatiale et les variations de GST autours de Corvatsch, 39 secteurs d’études ont été selectionnés, afin que la majorité des variations topographiques, présentent dans la zone d’étude d’environ 16km², soient représentés. La démarche est double D’une part, le choix des différents secteurs d’étude se fondent sur l’influence des différentes variables topographiques comme l’altitude, la pente, le type de sol ou encore la courbure du sol. D’autre part, la démultiplication des mesures de température, sur chaque secteur d’étude de 10x10m, permet de mettre en évidence les variations de températures à une échelle plus fine.

Afin d’étudier les variations de GST engendrées par la pente, la forme et les différentes caractéristiques du sol, une bande altitudinale fut choisi afin d’y concentrer les mesures. Cette bande test se situe entre 2600 et 2900m. Certains secteurs d’études ont pourtant été placés en dehors de cette bande, comme témoins, afin de montre les effets de l’altitude sur la GST. Tous les paramètres de variabilité tels que les pentes raides et douces, les différents types de sols, les différentes tailles de matériaux etc … sont représentés dans les secteurs d’études.

Afin d’enregistrer les températures de surface du sol et d’éviter le réchauffement direct des capteurs par les rayons du soleil, ces derniers ont été enterrés à environ 5 cm dans le sol ou positionné entre des blocs de taille importante. La température est mesurée avec une périodicité de 3h et une précision de 0.0625°C.


(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
 

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