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

Référence bibliographique
GILBERT, A., VINCENT C., SIX D., WAGNON P., PIARD L., GINOT P. 2014. Modeling near-surface firn temperature in a cold accumulation zone (Col du Dôme, French Alps): from a physical to a semi-parameterized approach, The Cryosphere, 8, 689 - 703.

Abstract: Analysis of the thermal regime of glaciers is crucial for glacier hazard assessment, especially in the context of a changing climate. In particular, the transient thermal regime of cold accumulation zones needs to be modeled. A modeling approach has therefore been developed to determine this thermal regime using only near-surface boundary conditions coming from meteorological observations. In the first step, a surface energy balance (SEB) model accounting for water percolation and radiation penetration in firn was applied to identify the main processes that control the subsurface temperatures in cold firn. Results agree well with subsurface temperatures measured at Col du Dôme (4250m above sea level (a.s.l.)), France. In the second step, a simplified model using only daily mean air temperature and potential solar radiation was developed. This model properly simulates the spatial variability of surface melting and subsurface firn temperatures and was used to accurately reconstruct the deep borehole temperature profiles measured at Col du Dôme. Results show that percolation and refreezing are efficient processes for the transfer of energy from the surface to underlying layers. However, they are not responsible for any higher energy uptake at the surface, which is exclusively triggered by increasing energy flux from the atmosphere due to SEB changes when surface temperatures reach 0°C. The resulting enhanced energy uptake makes cold accumulation zones very vulnerable to air temperature rise.

Mots-clés
 glacier, model, firn temperature, surface energy balance, melting

Organismes / Contact
  • CNRS, LGGE (UMR5183), 38041 Grenoble, France
  • Univ. Grenoble Alpes, LGGE (UMR5183), 38041 Grenoble, France
  • IRD, LGGE (UMR5183), 38041 Grenoble, France
  • IRD, LTHE (UMR5564), 38041 Grenoble, France
  • ICIMOD, GPO Box 3226, Kathmandu, Nepal
  • Corresponding author: gilbert@lgge.obs.ujf-grenoble.fr

(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, snow accumulation, solar radiation firn temperature, surface energy balance, water percolation, melting glacier retreat  

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
France Alpes Col du Dôme   4250 m  

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations
 As the climate is expected to change in the future (IPCC, 2007), cold glacier temperatures will be modified. The response of subsurface firn temperature to air temperature rise will be largely amplified by an increase in the duration and frequency of melt events. This will lead to strong changes in ice temperature fields

Meteorological data and firn temperatures

An automatic weather station (AWS) located near the center of the saddle ran continuously between 3 July and 23 October 2012. The measurements were carried out within the surface boundary layer. Wind speed, air temperature, humidity, incident and reflected short-wave radiation and incoming and outgoing long-wave radiation were recorded as half-hourly means of measurements made every 10 s. Instantaneous values of surface position and wind direction were collected every half hour. The Vaisala hygrothermometer was artificially ventilated in the daytime to prevent measurement errors due to radiation.

Five meters from the AWS, 16 thermistors (PT100) were set up in the firn at depths of 0.11 m, 0.24 m, 0.34 m, 0.44 m, 0.65 m, 1 m, 2 m, 3 m, 5 m, 8 m, 12m and 16m and at heights of 0.15 m, 0.30 m, 0.45m and 0.60m above the surface from 3 July 2012 to 13 June 2013. The purpose of the sensors initially located in the air was to measure the subsurface temperature in the event of snow accumulation. Firn temperature was recorded as half-hourly means of measurements made every minute. Due to surface melting, the first three sensors were found at the same depth at the end of the melting period, making the first 40 cm deep temperature measurements not reliable after September. Air and surface temperatures were also recorded at this location from 3 July 2012 to 13 June 2013.

Deep borehole temperature profiles

Englacial temperature measurements using a thermistor chain were performed from surface to bedrock in seven boreholes drilled between 1994 and 2011 at three different sites located between 4240 and 4300m a.s.l.

Site 1 was measured in January 1999 and March 2012; site 2 in June 1994, April 2005 and March 2010; site 3 in January 1999 and March 2012.

Modélisations
 Two distinct one-dimensional and spatially distributed models are used to calculate the near-surface firn temperatures at Col du Dôme. The first is used to calculate the firn surface temperature and melting from a surface energy balance model using the meteorological data recorded by the AWS. The second is a heat flow model coupled to a water percolation model with temperature and melting rate at the surface as input data. This model is used successively with different input data sets obtained first from the energy balance model and then from a parameterized approach.
Hypothèses
 

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

(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 Application of a simplified model at multi-decennial scale to reconstruct deep borehole temperature profiles

 

Observations
 Site 2 experiences higher surface melting rates than site 3, leading to a stronger firn temperature rise over the first decade of this century. Differences between the two sites are amplified by stronger vertical advection velocities at site 2.
Modélisations
 Surface energy balance (model 1)

Each melting event is associated with a significant energy transfer to the firn pack and the duration of the event therefore has a very strong impact on the total energy balance of the firn pack during summer.

For similar atmospheric conditions, firn surfaces are able to absorb more energy when the surface temperature is at 0 °C than when it is negative, explaining why warming of cold accumulation zones of glaciers is more efficient when melting conditions are encountered than when they are not.

Spatial variability of melting and subsurface temperature have also been calculated by the model.

Hypothèses

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
  With expected higher air temperatures, melting events will become more frequent and the energy will be transferred more efficiently to the firn in the accumulation zones of glaciers.
 

(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations
  Our measurements show that the spatial variability of melting is highly dependent on the potential solar radiation
Modélisations
 
Hypothèses
 The climate warming could imply future changes in cold hanging glacier stability

Paramètre de l'aléa
Sensibilité du paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 melting events of firn surface
 air temperature, solar radiation
 

(4) - Remarques générales
 

(5) - Syntèses et préconisations
 

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