Abstract
Abstract. Near-surface air temperature (Ta) is highly important for modelling glacier ablation, though its spatio-temporal variability over melting glaciers still remains largely unknown. We present a new dataset of distributed Ta for three glaciers of different size in the south-east Tibetan Plateau during two monsoon-dominated summer seasons. We compare on-glacier Ta to ambient Ta extrapolated from several local off-glacier stations. We parameterise the along-flowline sensitivity of Ta on these glaciers to changes in off-glacier temperatures (referred to as “temperature sensitivity”) and present the results in the context of available distributed on-glacier datasets around the world. Temperature sensitivity decreases rapidly up to 2000–3000 m along the down-glacier flowline distance. Beyond this distance, both the Ta on the Tibetan glaciers and global glacier datasets show little additional cooling relative to the off-glacier temperature. In general, Ta on small glaciers (with flowline distances <1000 m) is highly sensitive to temperature changes outside the glacier boundary layer. The climatology of a given region can influence the general magnitude of this temperature sensitivity, though no strong relationships are found between along-flowline temperature sensitivity and mean summer temperatures or precipitation. The terminus of some glaciers is affected by other warm-air processes that increase temperature sensitivity (such as divergent boundary layer flow, warm up-valley winds or debris/valley heating effects) which are evident only beyond ∼70 % of the total glacier flowline distance. Our results therefore suggest a strong role of local effects in modulating temperature sensitivity close to the glacier terminus, although further work is still required to explain the variability of these effects for different glaciers.
Highlights
Near-surface air temperature (Ta) is one of the dominant controls on glacier energy and mass balance during the ablation season (Petersen et al, 2013; Gabbi et al, 2014; Sauter and Galos, 2016; Maurer et al, 2019; Wang et al, 2019), though modelling its spatio-temporal behaviour above melting ice surfaces remains a challenge
For a comparison to previous studies (Petersen and Pellicciotti, 2011; Shaw et al, 2017), we report the equivalent on-glacier lapse rate that would be calculated for the above conditions
Following Carturan et al (2015), we suggest a potential non-linear behaviour of lapse rates between AWS_Off and the top of the flowline for Parlung390, though we lack the off-glacier observations above the flowline origin to test this (Fig. 4b)
Summary
Near-surface air temperature (Ta) is one of the dominant controls on glacier energy and mass balance during the ablation season (Petersen et al, 2013; Gabbi et al, 2014; Sauter and Galos, 2016; Maurer et al, 2019; Wang et al, 2019), though modelling its spatio-temporal behaviour above melting ice surfaces remains a challenge. The absence of distributed information regarding Ta has favoured the use of simple, space–time invariant relationships of Ta with elevation, typically that of the free-air environmental lapse rate (ELR). Any extrapolation of Ta observations from an off-glacier location, those at lower elevations, are likely to lead to an overestimation of snow and ice ablation in energy balance and enhanced temperature index melt simulations While models applying the degree day approach can make use of off-glacier temperatures as forcing because they are heavily reliant on calibration, for energy balance models and models of intermediate complexity (Pellicciotti et al, 2005; Ragettli et al, 2016) it is key to resolve the air temperature distribution over glaciers, especially for turbulent flux calculations and typical parameterisations of incoming longwave radiation
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