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Turbulent fluxes make a substantial and growing contribution to the energy balance of ice surfaces globally, but are poorly constrained owing to challenges in estimating the aerodynamic roughness length (z0). Here, we used structure from motion (SfM) photogrammetry and terrestrial laser scanning (TLS) surveys to make plot-scale 2-D and 3-D microtopographic estimations ofz0and upscale these to mapz0across an ablating mountain glacier. At plot scales, we found spatial variability inz0estimates of over two orders of magnitude with unpredictablez0trajectories, even when classified into ice surface types. TLS-derived surface roughness exhibited strong relationships with plot-scale SfMz0estimates. At the glacier scale, a consistent increase inz0of ~0.1 mm d−1was observed. Space-for-time substitution based on time since surface ice was exposed by snow melt confirmed this gradual increase inz0over 60 d. These measurements permit us to propose a scale-dependent temporalz0evolution model where unpredictable variability at the plot scale gives way to more predictable changes ofz0at the glacier scale. This model provides a critical step towards deriving spatially and temporally distributed representations ofz0that are currently lacking in the parameterisation of distributed glacier surface energy balance models.

Multi-scale interactions between the glacier surface, the overlying atmosphere and the surrounding alpine terrain are highly complex. The high heterogeneity of boundary layer processes that couple these systems drives temporally and spatially variable energy fluxes and melt rates. A comprehensive measurement campaign, the HEFEX (Hintereisferner Experiment), was conducted during the summer of 2018. The aim of this experiment was 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 setup 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 the temporal change in the local wind system strongly affect the micrometeorology at the glacier. Low-level katabatic flows were persistently measured during both night time and daytime and were responsible for consistently low near-surface air temperatures with small spatial variations at the glacier. On the contrary, local turbulence profiles of momentum and heat revealed strong changes of the local thermodynamic characteristics at the glacier when larger-scale westerly flows disturbed the prevailing katabatic flow forming low-level across-glacier flows. Warm air advection from the surrounding ice-free areas significantly increased near-surface air-temperatures at the glacier, with strong horizontal temperature gradients from the peripheral zones towards the centerline of the glacier. Despite generally lower near-surface wind speeds during the across-glacier flow, peak horizontal heat advection from the peripheral zones towards the centerline and strong transport of turbulence from higher atmospheric layers downward resulted in enhanced turbulent heat exchange towards the glacier surface at the glacier centerline. Thus, at the centerline of the glacier the exposure to strong larger-scale westerly winds promoted heat exchange processes at the glacier surface potentially contributing to ice melt. On the contrary, at the peripheral zones of the glacier turbulence data indicate that stronger sheltering from the 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. To explain the origin of the across-glacier flow would however require large eddy simulations.

Increased resolution and availability of remote sensing products, and advancements in small‐scale aerial drone systems, allows observations of glacial changes at unprecedented levels of detail. Software developments, such as structure‐from‐motion (SfM), now allow users an easy and efficient method to generate three‐dimensional (3D) models and orthoimages from aerial or terrestrial datasets. While these advancements show promise for current and future glacier monitoring, many regions still suffer a lack of observations from earlier time periods. We report on the use of SfM to extract spatial information from various historic imagery sources. We focus on three geographic regions, the European Alps, high Arctic Norway and the Nepal Himalayas. We used terrestrial field photographs from 1896, high oblique aerial photographs from 1936 and aerial handheld photographs from 1978 to generate digital elevation models (DEMs) and orthophotos of the Rhone glacier, Broggerhalvoya and the lower Khumbu glacier, respectively. Our analysis shows that applying SfM to historic imagery can generate high quality models using only ground control points. Limited camera/orientation information was largely reproduced using self‐calibrated model data. Using these data, we calculated mean ground sampling distances across each site which demonstrates the high potential resolution of resulting models. Vertical errors for our models are ±5.4 m, ±5.2 m and ±3.3 m. Differencing shows similar patterns of thinning at lower Rhone (European Alps) and Broggerhalvoya (Norway) glaciers, which have mean thinning rates of 0.31 m a−1 (1896–2010) to 0.86 m a−1 (1936–2010) respectively. On these clean ice glaciers thinning is highest in the terminus region and decreasing up‐glacier. In contrast to these glaciers, uneven topography, exposed ice‐cliffs and debris cover on the Khumbu glacier create a highly variable spatial distribution of thinning. The mean thinning rate for the Khumbu study area was found to be 0.54 ± 0.9 m a−1 (1978–2015).

The large spatial variability in Arctic tundra complicates the representative assessment of CO2 budgets. Accurate measurements of these heterogeneous landscapes are, however, essential to understanding their vulnerability to climate change. We surveyed a polygonal tundra lowland on Svalbard with an unmanned aerial vehicle (UAV) that mapped ice-wedge morphology to complement eddy covariance (EC) flux measurements of CO2. The analysis of spectral distributions showed that conventional EC methods do not accurately capture the turbulent CO2 exchange with a spatially heterogeneous surface that typically features small flux magnitudes. Nonlocal (low-frequency) flux contributions were especially pronounced during snowmelt and introduced a large bias of −46 gC m−2 to the annual CO2 budget in conventional methods (the minus sign indicates a higher uptake by the ecosystem). Our improved flux calculations with the ogive optimization method indicated that the site was a strong sink for CO2 in 2015 (−82 gC m−2). Due to differences in light-use efficiency, wetter areas with low-centered polygons sequestered 47 % more CO2 than drier areas with flat-centered polygons. While Svalbard has experienced a strong increase in mean annual air temperature of more than 2 K in the last few decades, historical aerial photographs from the site indicated stable ice-wedge morphology over the last 7 decades. Apparently, warming has thus far not been sufficient to initiate strong ice-wedge degradation, possibly due to the absence of extreme heat episodes in the maritime climate on Svalbard. However, in Arctic regions where ice-wedge degradation has already initiated the associated drying of landscapes, our results suggest a weakening of the CO2 sink in polygonal tundra.

Supraglacial lakes and ponds can create hotspots of mass loss on debris-covered glaciers. While much research has been directed at understanding lateral lake expansion, little is known about the rates or processes governing lake deepening. To a large degree, this knowledge gap persists due to sparse observations of lake beds. Here we report on the novel use of ground penetrating radar (GPR) surveys to simultaneously collect supraglacial lake bathymetry and bottom composition data from Spillway Lake (surface area of 2.4 x 105 m2; volume of 9.5 x104 m3), which is located in the terminus region of the Ngozumpa Glacier in the Khumbu region of the Nepal Himalaya. We identified two GPR bottom signals corresponding to two sedimentary facies of (1) sub-horizontal layered fine sediment drape and (2) coarse blocky diamict. We provide an understanding of the changes in subaqueous debris distribution that occur through stages of lake expansion by combining the GPR results with in situ observations of shoreline deposits matching the interpreted facies. From this, we present an updated conceptual model of supraglacial lake evolution, with the addition of data on the evolving debris environment, showing how dominant depositional processes can change as lakes evolve from perched lakes to multi-basin base-level lakes and finally onto large moraine-dammed lakes. Throughout lake evolution, processes such as shoreline steepening, lakebed collapse into voids and conduit interception, subaerial and subaqueous calving and rapid areal expansion alter the spatial distribution and makeup of lakebed debris and sediments forcing a number of positive and negative feedbacks on lake expansion.

Funding for Sarah Thompson as provided by the European Commission FP7-MC-IEF grant PIEF-GA-2012-330805, and for Lindsey Nicholson by the Austrian Science Fund (FWF) Elise Richter Grant (V309-N26). Financial support for fieldwork in 2009 was provided by the University Centre in Svalbard and a Royal Geographical Society fieldwork grant to Sarah Thompson. TerraSAR-X data were kindly provided by the German Aerospace Center (DLR) under project HYD0178. The meteorological data were collected within the Ev-K2-CNR SHARE Project, funded by contributions from the Italian National Research Council and the Italian Ministry of Foreign Affairs. We provide the first synoptic view of the drainage system of a Himalayan debris-covered glacier and its evolution through time, based on speleological exploration and satellite image analysis of Ngozumpa Glacier, Nepal. The drainage system has several linked components: 1) a seasonal subglacial drainage system below the upper ablation zone; 2) supraglacial channels allowing efficient meltwater transport across parts of the upper ablation zone; 3) sub-marginal channels, allowing long-distance transport of meltwater; 4) perched ponds, which intermittently store meltwater prior to evacuation via the englacial drainage system; 5) englacial cut-and-closure conduits, which may undergo repeated cycles of abandonment and reactivation; 6) a 'base-level' lake system (Spillway Lake) dammed behind the terminal moraine. The distribution and relative importance of these elements has evolved through time, in response to sustained negative mass balance. The area occupied by perched ponds has expanded upglacier at the expense of supraglacial channels, and Spillway Lake has grown as more of the glacier surface ablates to base level. Subsurface processes play a governing role in creating, maintaining and shutting down exposures of ice at the glacier surface, with a major impact on spatial patterns and rates of surface mass loss. Comparison of our results with observations on other glaciers indicate that englacial drainage systems play a key role in the response of debris-covered glaciers to sustained periods of negative mass balance. Publisher PDF

This research was supported financially by the European Commission FP7-MC-IEF (PIEF-GA-2012-330805), the University Centre in Svalbard (UNIS), National Geographic Society GRANT #W135-10. Moraine-dammed lakes at debris-covered glaciers are becoming increasingly common and pose significant outburst flood hazards if the dam is breached. While moraine subsurface structure and internal processes are likely to influence dam stability, only few sites have so far been investigated. We conducted electrical resistivity tomography (ERT) surveys at two sites on the terminal moraine complex of the Ngozumpa Glacier, Nepal, to aid assessment of future terminus stability. The resistivity signature of glacier ice at the site (100-15 kΩ m) is more consistent with values measured from cold glacier ice and while this may be feasible, uncertainties in the data inversion introduce ambiguity to this thermal interpretation. However, the ERT data does provide a significant improvement to our knowledge of the subsurface characteristics at these sites, clearly showing the presence (or absence) of glacier ice. Our interpretation is that of a highly complex latero-terminal moraine, resulting from interaction between previous glacier advance, recession and outburst flooding. If the base-level Spillway Lake continues to expand to a fully formed moraine-dammed glacial lake, the degradation of the ice core could have implications for glacial lake outburst risk. Publisher PDF

The large spatial variability in Arctic tundra complicates the representative assessment of CO2 budgets. Accurate measurements of these heterogeneous landscapes are, however, essential to understand their vulnerability to climate change. We surveyed a polygonal tundra lowland on Svalbard with a UAV, mapping ice-wedge morphology to complement eddy covariance (EC) flux measurements of CO2. The analysis of spectral distributions showed that conventional EC methods do not accurately capture the turbulent CO2 exchange with the spatially heterogeneous surface which typically features small flux magnitudes. Non-local (low frequency) flux contributions were especially pronounced during snowmelt and introduced a bias to the annual CO2 budget of −46 gC m−2 in conventional methods. Our improved flux calculations with the ogive optimization method indicated that the site was a sink for CO2 of −82 gC m−2 in 2015 and due to differences in light-use efficiency, wetter areas with low-centered polygons sequestered 47 % more CO2 than drier areas with flat-centered polygons. While Svalbard has experienced a strong increase in mean annual air temperature of more than 2 °C in the last few decades, historical aerial photographs from the site indicated stable ice-wedge morphology over the last seven decades. Apparently, warming has thus far not been sufficient to initiate strong ice-wedge degradation, possibly due to the absence of extreme heat episodes in the maritime climate on Svalbard. However, in Arctic regions where ice-wedge degradation has already initiated the associated drying of landscapes, our results suggest a weakening of the CO2 sink of polygonal tundra.