Abstract
ABSTRACTThe spatio-temporal distribution of air temperature over mountain glaciers can demonstrate complex patterns, yet it is often represented simplistically using linear vertical temperature gradients (VTGs) extrapolated from off-glacier locations. We analyse a network of centreline and lateral air temperature observations at Tsanteleina Glacier, Italy, during summer 2015. On average, VTGs are steep (<−0.0065 °C m−1), but they are shallow under warm ambient conditions when the correlation between air temperature and elevation becomes weaker. Published along-flowline temperature distribution methods explain centreline observations well, including warming on the lower glacier tongue, but cannot estimate lateral temperature variability. Application of temperature distribution methods improves simulation of melt rates (RMSE) in an energy-balance model by up to 36% compared to the environmental lapse rate extrapolated from an off-glacier station. However, results suggest that model parameters are not easily transferable to glaciers with a small fetch without recalibration. Such methods have potential to improve estimates of temperature across a glacier, but their parameter transferability should be further linked to the glacier and atmospheric characteristics. Furthermore, ‘cold spots’, which can be >2°C cooler than expected for their elevation, whose occurrence is not predicted by the temperature distribution models, are identified at one-quarter of the measurement sites.
Highlights
Near-surface air temperature (Ta) is a crucial component of the glacier surface energy balance (Ohata, 1992; Hock, 1999; Ohmura, 2001) and a fundamental control on the melt rate at a snow or ice surface (Petersen and Pellicciotti, 2011)
For the warmest ambient conditions, this overestimation can be as high as 4°C for GCELR (Fig. 6b) leading to an RMSE of 3.6°C compared to the measured data
The weak dependence of centreline temperature on elevation under warm ambient conditions recorded at Tsanteleina Glacier concurs with the majority of previous findings regarding katabatic effects on Ta distribution (e.g. Greuell and Böhm, 1998; Shea and Moore, 2010; Petersen and others, 2013; Ayala and others, 2015)
Summary
Near-surface air temperature (Ta) is a crucial component of the glacier surface energy balance (Ohata, 1992; Hock, 1999; Ohmura, 2001) and a fundamental control on the melt rate at a snow or ice surface (Petersen and Pellicciotti, 2011). In the absence of local data, or due to modelling constraints, a constant and uniform linear ‘lapse rate’ such as the environmental lapse rate (ELR = −0.0065 °C m−1) is commonly applied when distributing air temperature across entire glaciers (e.g. Arnold and others, 2006; Nolin and others, 2010). Several studies have demonstrated that, over melting ice surfaces, VTGs tend to be shallow or absent due to the development of a katabatic boundary layer (KBL) under warm ambient conditions (Greuell and Böhm, 1998; Shea and Moore, 2010; Petersen and others, 2013; Ayala and others, 2015) which often result in thermal inversions (Strasser and others, 2004; Carenzo, 2012). Evidence of erosion into the KBL by warm up-valley winds and/or synoptically forced flow has been shown to lead to more negative VTGs and a stronger dependency on elevation (Pellicciotti and others, 2008; Petersen and Pellicciotti, 2011)
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