Un nuovo metodo per la valutazione degli effetti delle condizioni termiche e pluviometriche sulle oscillazioni frontali recenti dei ghiacciai alpini (applicate al Ghiacciaio del Dosegù, Alta Valtellina)

A new method for the evaluation of the effects of temperature and precipitation conditions on recent front variations of Alpine glaciers (applied to the Ghiacciaio del Dosegù, Alta Valtellina)

Abstract

A new methodology is proposed for the study of the relations between glaciers and climate, and applied to a sample glacier in the Lombardy Alps, the Ghiacciaio del Dosegu in Alta Valtellina. The temperature data from the Bormio station for the 1926–1995 period were adjusted with a vertical rate (°C/m) to the glacier front altitude and then transformed into the sum of the annual degree days. The data series of degree days at the specified altitude was then correlated with the corresponding historical data series regarding front variation, which were reconstructed for the years in which data were lacking by means of a specially calculated m/degree day coefficient. The two temperature and front historical data series were then compared, with a synchronic analysis and a time-shifting analysis in order to obtain the "reaction time" of the glacier examined. As regards precipitation, the Bormio station data were used for the comparison without any adjustment. Two types of correlations were used for the quantitative definition of the front variations and the climatic parameters, the traditional Bravais-Pearson correlation and the so-called cross-correlation. The two statistical correlation methods used yielded slightly different results, although both revealed a greater influence of temperature factors on the glacier front dynamics, compared to precipitation. With the Bravais-Pearson correlations, the highest coefficients (exceeding -0.6) were reached with the net and positive degree days and they indicate that the temperature factors manifest their greatest efficacy after 20–25 years. On the other hand, the cross-correlations suggest that the impact of temperatures is not manifested through a well-defined and temporally-localized reaction time, but that frontal response to temperature stimuli is prolonged continuously and homogeneously over a long period of time ranging from 0 to 20 years.