Abstract
Abstract. Multi-scale interactions between the glacier surface, the overlying atmosphere, and the surrounding alpine terrain are highly complex and force temporally and spatially variable local glacier energy fluxes and melt rates. A comprehensive measurement campaign (Hintereisferner Experiment, HEFEX) was conducted during August 2018 with the aim to investigate spatial and temporal dynamics of the near-surface boundary layer and associated heat exchange processes close to the glacier surface during the melting season. The experimental set-up of five meteorological stations was designed to capture the spatial and temporal characteristics of the local wind system on the glacier and to quantify the contribution of horizontal heat advection from surrounding ice-free areas to the local energy flux variability at the glacier. Turbulence data suggest that temporal changes in the local wind system strongly affect the micrometeorology at the glacier surface. Persistent low-level katabatic flows during both night and daytime cause consistently low near-surface air temperatures with only small spatial variability. However, strong changes in the local thermodynamic characteristics occur when westerly flows disturbed this prevailing katabatic flow, forming across-glacier flows and facilitating warm-air advection from the surrounding ice-free areas. Such heat advection significantly increased near-surface air temperatures at the glacier, resulting in strong horizontal temperature gradients from the peripheral zones towards the centre line of the glacier. Despite generally lower near-surface wind speeds during across-glacier flow, peak horizontal heat advection from the peripheral zones towards the centre line and strong transport of turbulence from higher atmospheric layers downward resulted in enhanced turbulent heat exchange towards the glacier surface at the glacier centre line. Thus, at the centre line of the glacier, exposure to strong larger-scale westerly winds promoted heat exchange processes, potentially contributing to ice melt, while at the peripheral zones of the glacier, stronger sheltering from larger-scale flows allowed the preservation of a katabatic jet, which suppressed the efficiency of the across-glacier flow to drive heat exchange towards the glacier surface by decoupling low-level atmospheric layers from the flow aloft. A fuller explanation of the origin and structure of the across-glacier flow would require large-eddy simulations.
Highlights
Mountain glaciers are important contributors to the regional and global hydrological cycle (e.g. Bahr and Radic, 2012) as well as sea-level rise (e.g. Radicand Hock, 2011)
As we are interested in the interplay of katabatic flow with other local circulation patterns, in this study we focus on 5 d in August 2018 that meet three criteria: (1) good data quality at all the stations, (2) predominantly clear-sky conditions, and (3) flow characterized by a significant shift in wind direction from katabatic down-glacier flow direction to a westerly or north-westerly flow during the day
In order to assess the effect of heat advection on the heat exchange processes during disturbed conditions, we focus our analysis on flow characteristics during those conditions (Figs. 10, 12, 13)
Summary
Mountain glaciers are important contributors to the regional and global hydrological cycle (e.g. Bahr and Radic, 2012) as well as sea-level rise (e.g. Radicand Hock, 2011). Mölg et al, 2009; Klok and Oerlemans, 2002) have been used to analyse the climatic drivers of prevalent rapid mass losses of mountain glaciers, showing that shortwave radiation is the main driver for snowmelt and ice melt, the sensitivity of the melt rate to temperature is strongly affected by the net longwave radiation and the turbulent heat fluxes (e.g. Oerlemans, 2001; Cullen and Conway, 2015). Some studies highlighted the effect of katabatic flows in laterally decoupling the local atmosphere from its surroundings, lowering the climatic sensitivity of glaciers to external temperature changes (Shea and Moore, 2010; Sauter and Galos, 2016; Mott et al, 2019)
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