Abstract
ABSTRACTNear-surface air temperature is an important determinant of the surface energy balance of glaciers and is often represented by a constant linear temperature gradients (TGs) in models. Spatio-temporal variability in 2 m air temperature was measured across the debris-covered Miage Glacier, Italy, over an 89 d period during the 2014 ablation season using a network of 19 stations. Air temperature was found to be strongly dependent upon elevation for most stations, even under varying meteorological conditions and at different times of day, and its spatial variability was well explained by a locally derived mean linear TG (MG–TG) of −0.0088°C m−1. However, local temperature depressions occurred over areas of very thin or patchy debris cover. The MG–TG, together with other air TGs, extrapolated from both on- and off-glacier sites, were applied in a distributed energy-balance model. Compared with piecewise air temperature extrapolation from all on-glacier stations, modelled ablation, using the MG–TG, increased by <1%, increasing to >4% using the environmental ‘lapse rate’. Ice melt under thick debris was relatively insensitive to air temperature, while the effects of different temperature extrapolation methods were strongest at high elevation sites of thin and patchy debris cover.
Highlights
Glaciers in high altitude, mountainous regions are a highly important source of fresh water for major river systems (Li and Williams, 2008; Immerzeel and others, 2014); modelling the future availability of such resources is often hampered by insufficient data or poor understanding of localised meteorological conditions
The 2014 Miage glacier ablation season was characterised by wet conditions and an average Ta of 8.1°C across all station observations, with extremes of −3.0 and 24.1°C (Table 1)
The main exceptions to this are the low outliers at TT7 and TT12 which reflect localised cooling associated with thin/patchy debris cover
Summary
Mountainous regions are a highly important source of fresh water for major river systems (Li and Williams, 2008; Immerzeel and others, 2014); modelling the future availability of such resources is often hampered by insufficient data or poor understanding of localised meteorological conditions. The spatial distribution of this variable over glaciers is unknown and often estimated from a single off-glacier location using simple, uniform lapse rates. Lapse rates (referred to here as temperature gradients (TGs)), which define a variable dependency on elevation, are the most common method of distributing Ta in model studies (Marshall and others, 2007; Petersen and Pellicciotti, 2011; Wheler and others, 2014). The application of a temporally constant and spatially homogenous free-air TG has, been questioned for glacierised basins, at high altitude where the effect of terrain cannot be neglected (Marshall and others, 2007; Gardner and others, 2009; Minder and others, 2010; Petersen and others, 2013)
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