Paul & al. 2005 - On the impact of glacier albedo under conditions of extreme glacier melt: the summer of 2003 in the Alps

Réf. Paul & al. 2005 - P

Référence bibliographique complète

EARSel eProceedings 4, (février 2005). On the impact of glacier albedo under conditions of extreme glacier melt: the summer of 2003 in the Alps. PAUL F., MACHGUTH H., KÄÄB A., p. 139-149.

Organismes / Contact

(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

Massif / Secteur

Site(s) d'étude

Période(s) d'observation

(1) - Modifications des paramètres atmosphériques

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

(2) - Effets du changement climatique sur le milieu naturel

The extraordinary heat wave of the summer 2003 caused record breaking glacier melt with a corresponding mean specific mass loss of -2.5 m water equivalent (we), which is eight times the annual mean of the period 1960-2000 (Hoelzle & al. 2003).
Under extreme climatic conditions, glaciers can loose more mass in their accumulation area than in their (shaded) ablation area, inverting a normal mass balance profile.

Several field studies (Greuell & al. 1997, Strasser & al. 2004, Oerlemans 2000) have confirmed that direct radiation is the most important energy source for glacier melt in the rough topography of the Alps. This is also due to the long ablation period (sometimes exceeding 90 days) and the comparably low albedo of bare glacier ice (about 0.3). As such, glacier albedo becomes the most sensitive variable for glacier melt. However, glacier albedo exhibits a high temporal (e.g. retreat of the snow line) and spatial (e.g. debris cover) variability, constant or even decreasing albedo with altitude and much lower albedo values in the ablation area than generally applied (0.15 instead of 0.35).

The modelled mass balance reveals a distribution pattern that is governed by the potential solar radiation, increasing glacier mass loss with altitude using the 2003 albedo, and a three times higher mass loss for the meteorological conditions of 2002/03 compared to the climatic means. The potential solar radiation governs the mass balance distribution in the case of low glacier albedo and long melt periods.

Sensibilité du milieu à des paramètres climatiques

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

Glacier melt and mass balance
Direct and potential radiation

In this study, the authors compare TM-derived albedo values for several glaciers and three distinct years (1985, 1998, 2003) and assess the influence of the albedo on glacier mass balance and melt with a distributed mass balance model that is forced by the 2002/2003 balance year meteorological conditions (temperature, precipitation, clouds) as well as climatic mean values.

Glacier melt can be calculated from so-called distributed glacier mass balance models, which utilize a digital elevation model (DEM) to ‘distribute’ measured meteorological input variables (e.g. temperature, precipitation) to the topography by means of elevation-dependent lapse rates and DEM modelling for incoming solar radiation (Arnold & al., 1996, Brock & al., 2000, Klok & Oerlemans, 2002). Such models have proven to calculate mass balance or discharge at well calibrated sites from a prescribed meteorological forcing very accurately.

The spatial pattern of the albedo can be obtained from multispectral Landsat Thematic Mapper (TM) data over large regions with high accuracy (Knap & al., 1999). However, acquisition at the end of the ablation period in a year with a minimum amount of remaining snow is mandatory.

(3) - Effets du changement climatique sur l'aléa

Paramètres de l'aléa

Sensibilité du paramètre de l'aléa à des paramètres climatiques et du milieu

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

(4) - Remarques générales

(5) - Préconisations et recomandations

If glacier mass balance is computed from mass balance models over larger catchments, actual satellite data from the end of the ablation period should be used as an input. The model should also account for an ice-albedo ageing, starting with somewhat higher values at the beginning of the ablation period, as well as an initial snow depth map from the previous year. Under extreme climatic conditions, glaciers can loose more mass in their accumulation area than in their (shaded) ablation area, inverting a normal mass balance profile. This has to be considered if ablation stake measurements are interpreted, or simple degree-day models are used to calculate glacier melt. In the rough topography of the Alps an accurate calculation of global radiation from a high-resolution DEM is mandatory. If glacier melt is to be assessed by a mass balance model over large regions, the usage of a glacier-specific albedo is most valuable.

Références citées :

Arnold N S, I C Willis, M J Sharp, K S Richards & W J Lawson, 1996. A distributed surface en-ergy-balance model for a small valley glacier. I. Development and testing for Haut Glacier d’Arolla, Valais, Switzerland. Journal of Glaciology, 42 (140): 77-89.

Brock B W, I C Willis, M J Sharp & N S Arnold, 2000. Modelling seasonal and spatial varia-tions in the surface energy balance of Haut Glacier d’Arolla, Switzerland. Annals of Glaciology, 31: 53-62.

Greuell W, W H Knap & P C Smeets, 1997. Elevational changes in meteorological variables along a mid-latitude glacier during summer. Journal of Geophysical Research, 102 (D22): 25941-25954.

Hoelzle M, W Haeberli, M Dischl & W Peschke, 2003. Secular glacier mass balances derived from cumulative glacier length changes. Global and Planetary Change, 36(4): 295-306. [Fiche Biblio]

Klok E J & J Oerlemans, 2002. Model study of the spatial distribution of the energy and mass balance of Morteratschgletscher, Switzerland. Journal of Glaciology, 48 (163): 505-518.

Knap W H, C H Reijmer & J Oerlemans, 1999. Narrowband to broadband conversion of Landsat-TM glacier albedos. International Journal of Remote Sensing, 20 (10): 2091-2110.

Oerlemans J, 2000. Analysis of a 3 year meteorological record from the ablation zone of Morteratschgletscher, Switzerland: energy and mass balance. Journal of Glaciology, 46 (155): 571-579.

Strasser U, J Corripio, F Pellicciotti, P Burlando, B Brock & M Funk, 2004. Spatial and temporal variability of meteorological variables at Haut Glacier dArolla (Switzerland) during the ablation season 2001: Measurements and simulations. Journal of Geophysical Research, 109.

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

(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
Observations	The extraordinary heat wave of the summer 2003 caused record breaking glacier melt with a corresponding mean specific mass loss of -2.5 m water equivalent (we), which is eight times the annual mean of the period 1960-2000 (Hoelzle & al. 2003). Under extreme climatic conditions, glaciers can loose more mass in their accumulation area than in their (shaded) ablation area, inverting a normal mass balance profile.
Modélisations	Several field studies (Greuell & al. 1997, Strasser & al. 2004, Oerlemans 2000) have confirmed that direct radiation is the most important energy source for glacier melt in the rough topography of the Alps. This is also due to the long ablation period (sometimes exceeding 90 days) and the comparably low albedo of bare glacier ice (about 0.3). As such, glacier albedo becomes the most sensitive variable for glacier melt. However, glacier albedo exhibits a high temporal (e.g. retreat of the snow line) and spatial (e.g. debris cover) variability, constant or even decreasing albedo with altitude and much lower albedo values in the ablation area than generally applied (0.15 instead of 0.35). The modelled mass balance reveals a distribution pattern that is governed by the potential solar radiation, increasing glacier mass loss with altitude using the 2003 albedo, and a three times higher mass loss for the meteorological conditions of 2002/03 compared to the climatic means. The potential solar radiation governs the mass balance distribution in the case of low glacier albedo and long melt periods.
Hypothèses

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

Pays / Zone	Massif / Secteur	Site(s) d'étude	Exposition	Altitude	Période(s) d'observation
Alps					1985, 1998, 2003 summers