Résumé des résultats (Abstract)
(Anglais)
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Little knowledge exists to date on the complex glacier-atmosphere interactions leading to the formation of cold firn on high-elevation Alpine glaciers. Although cold firn and ice is not a wide-spread phenomenon on Alpine glaciers, it is of importance for many glaciological problems and has a major impact on glaciers as an environmental and climatic archive.The present study is a contribution to a better and more quantitative understanding of the atmospheric and glaciological processes in cold firn areas. The study is divided into four main parts dealing with the atmosphere-ground interactions (surface energy-balance), the distribution of cold firn in space (spatial occurrence of cold firn), the thermometric evidence of observed firn- and ice-temperature profiles in terms of a climate signal (borehole thermometry) and the (future) energy balance and englacial thermal conditions in space (coupled spatial energy-balance/firn-temperature model).The atmospheric impact was investigated with the help of an energy-balance study at the cold 4300 m high Seserjoch firn saddle, Monte Rosa area (Italy and Switzerland). Measurements of short- and longwave radiation, wind speed and wind direction, air temperature, humidity, snow height and snow- and firn temperatures were effected between September, 1998 and August, 2000 under difficult meteorological conditions. A one-year time series of energy-balance measurements covering the period from May, 1999 to April, 2000 shows that the net radiation and turbulent heat fluxes form the major contribution to the energy balance. The heat fluxes due to surface melt in summer and re-freezing events (re-freezing of meltwater at the surface or rime accretion) cannot be neglected. Their precise magnitude is difficult to interpret as these fluxes also comprise the instrumental and methodological errors of the energy-balance calculation. Single surface melt events and the prevailing meteorological conditions favouring or preventing surface melt could be identified by precise high-resolution surface-temperature measurements.Near-surface firn temperatures were measured in 22 steam-drilled boreholes in the summit region of Mont Blanc (France and Italy) between 3800 and 4800 m a.s.l. in June, 1998, and in 31 boreholes in the Monte Rosa area (Italy and Switzerland) between 3900 and 4500 m a.s.l. in May/July 1999. Borehole temperatures were sampled with removable thermistor chains to a depth of 22 m. The temperatures at 18 m depth ranged between temperate conditions and approximately -15°C. The thermal distribution pattern of cold firn suggests a strong influence of solar radiation and turbulent heat exchange. During the melt season in summer, these two energy fluxes mainly determine the melt-energy input into the snow and firn and, thereby, the observed near-surface firn temperatures. Mean annual air temperature is of secondary importance, although the observed mean annual firn temperatures generally increase with decreasing elevation. A statistical analysis of the measured firn temperatures revealed that the parameters elevation, potential direct solar radiation, slope and accumulation are able to explain more than 80 % of the variation of the mean annual firn temperatures. The aspect-dependent lower boundaries for the cold firn occurrence in the Mont Blanc and Monte Rosa areas range between 3500 and 3700 m a.s.l. in north and between 3800 and 4100 m a.s.l. in south aspects.Theoretical calculations, using a one-dimensional time-dependent thermo-mechanical firn-temperature model including the effect of latent heat originating from surface melt, show that the englacial thermal regime is extremely sensitive to the magnitude and duration of surface melt and that melt events disturb the pure surface-temperature signal, considerably. A typical surface-temperature perturbation penetrates a 100 m thick glacier within 18 to 30 years, only. Therefore, the possible time horizon for surface-temperature reconstructions using englacial temperature profiles is limited to a few centuries at best. Englacial temperature profiles were measured with an absolute accuracy of ±0.01-0.03°C in a 29 m deep borehole at Seserjoch (4300 m a.s.l., Monte Rosa area), in a 25 m deep borehole at the saddle point of Colle Gnifetti (4450 m a.s.l., Monte Rosa area) and in a 40 m deep borehole on top of Dôme du Goûter (4300 m a.s.l., Mont Blanc area). These records suggest a surface-temperature increase on the order of 0.5-1°C for the last decade.A spatial energy-balance model was coupled with a one-dimensional thermal firn-temperature model and applied to the Monte Rosa study area. Although the energy-balance model yielded some encouraging results, the errors in the calculated surface temperature turned out to be too large for a direct application in a coupled energy-balance/firn-temperature model. A simplified formulation of the upper boundary condition in terms of surface temperature and melt-energy input is proposed and coupled with the firn-temperature model. The model is considered robust enough to give a statement on the future thermal evolution of the cold firn saddles of Seserjoch and Colle Gnifetti.
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