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Air temperature and glacier melting in the outer-tropics

Congress: 2008
Author(s):
Jean Emmanuel Sicart Great Ice IRD, Case MSE, UMII 300, avenue du Professeur Emile Jeanbrau 34 095 Montpellier cedex 5, France Tel.: 0033 (0)4 67149035 / Fax: 0033 (0)4 67149071 sicart@msem.univ- montp2.fr

Keyword(s): tropical glaciers, temperature, melting discharge, water resources
AbstractThe air temperature is a measure of the internal energy of the air under fixed conditions of pressure and humidity. It is no-linearly linked to most of the melting energy fluxes at a glacier surface. Numerous studies revealed a high correlation between ice or snow melt and air temperature. This relation explain the efficiency of degree-day models to forecast ice or snow melting in various climates: The degree-day model relates the melting to the sum of positive temperatures, generally at a daily time scale, the factor of proportionality being the degree-day factor. This statistical model does not make the distinction between causes and effects; the temperature being more a consequence than a cause of the energy fluxes. The degree-day model has been applied to investigate the sensitivity to climate change of glaciers throughout the world. However, rather few studies investigated the physical causes of the good correlation between air temperature and ice melting. The paradox is that radiation generally is the greater incoming energy flux, but is poorly correlated to the air temperature. In the Arctic Canada, Braithwaite [1981] showed that the high correlation between temperature and glacier melting is due to the sensible heat flux: although net radiation is the largest source of energy, the sensible heat flux is more variable than the radiation and better correlated with the temperature. Ohmura [2001] stressed the role of the atmospheric long-wave radiation. The sky long-wave irradiance usually is the largest incoming energy flux and comes from the near-surface layer of the atmosphere, whose temperature is well correlated to the temperature measured at screen level. These results may not be valid in all climates. Here, we analyze the relationship between temperature and the melting energy fluxes on a tropical glacier to test these hypotheses in a low latitude climate. The cased study is the tropical Zongo Glacier, Bolivia. We show that the sensible heat flux is less important on tropical glaciers than on mid-latitude glaciers because of smaller temperature gradients (low latitude) and smaller air density (very high altitude). Melting is controlled by the radiation fluxes. Solar radiation is positively correlated to the temperature, whereas long-wave radiation is negatively correlated to it. As a result, the melting of Zongo Glacier is poorly correlated to the air temperature at a daily time scale. This study suggests that the degree-day model may no be appropriate to forecast the melting of tropical glaciers at short time steps.
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