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Properties of natural supraglacial debris in relation to modelling sub-debris ice ablation
Abstract
As debris-covered glaciers become a more prominent feature of a shrinking mountain cryosphere, there is increasing need to successfully model the surface energy and mass balance of debris-covered glaciers, yet measurements of the processes operating in natural supraglacial debris covers are sparse. We report measurements of vertical temperature profiles in debris on the Ngozumpa glacier in Nepal, that show: (i) conductive processes dominate during the ablation season in matrix-supported diamict; (ii) ventilation may be possible in coarse surface layers; (iii) phase changes associated with seasonal change have a marked effect on the effective thermal diffusivity of the debris. Effective thermal conductivity determined from vertical temperature profiles in the debris is generally ~30% higher in summer than in winter, but values depend on the volume and phase of water in the debris. Surface albedo can vary widely over small spatial scales, as does the debris thickness. Measurements indicate that debris thickness is best represented as a probability density function with the peak debris thickness increasing down-glacier. The findings from Ngozumpa glacier indicate that the probability distribution of debris thickness changes from positively skewed in the upper glacier towards a more normal distribution nearer the terminus. Although many of these effects remain to be quantified, our observations highlight aspects of spatial and temporal variability in supraglacial debris that may require consideration in annual or multi-annual distributed modelling of debris-covered glacier surface energy and mass balance. Copyright © 2012 John Wiley & Sons, Ltd.
... The presence of this surface debris substantially alters the glacier mass balance gradient, in comparison to cleanice glaciers: melt is supressed close to the termini, due to the presence of thick debris which insulates the underlying ice, and enhanced up glacier, where debris is thinner and increases melt (e.g. Quincey et al., 2009;Nicholson and Benn, 2013). As a 45 result, the glaciers are characterised by high elevation accumulation areas, and lower-elevation ice tongues, which have low surface slopes and are near-stagnant, due to the low driving stresses (e.g. ...
... We suggest this is because of the inverted mass balance gradient observed on debris-covered glaciers in the Everest region: thick debris at the terminus supresses melt, whereas thinner debris further up glacier enhances melt (Bolch et al., 2008;Quincey et al., 2009;King et al., 2018). This pattern of mass balance 365 results in near-stagnant ice velocities over much of the tongue and rapid down-wasting at mid-elevations (Quincey et al., 2007(Quincey et al., , 2009Nicholson and Benn, 2013;Juen et al., 2014;King et al., 2017), which produces uneven 'ablation topography' (Nicholson and Benn, 2013). Together, this creates ideal conditions for pond formation in mid-elevation areas: the uneven topography allows water to collect; melt rates are at their maximum, due to the comparatively thin debris cover enhancing ice melt; and low ice velocities inhibit drainage via crevasse opening. ...
... We suggest this is because of the inverted mass balance gradient observed on debris-covered glaciers in the Everest region: thick debris at the terminus supresses melt, whereas thinner debris further up glacier enhances melt (Bolch et al., 2008;Quincey et al., 2009;King et al., 2018). This pattern of mass balance 365 results in near-stagnant ice velocities over much of the tongue and rapid down-wasting at mid-elevations (Quincey et al., 2007(Quincey et al., , 2009Nicholson and Benn, 2013;Juen et al., 2014;King et al., 2017), which produces uneven 'ablation topography' (Nicholson and Benn, 2013). Together, this creates ideal conditions for pond formation in mid-elevation areas: the uneven topography allows water to collect; melt rates are at their maximum, due to the comparatively thin debris cover enhancing ice melt; and low ice velocities inhibit drainage via crevasse opening. ...
Supraglacial ponds are characteristic of debris-covered glaciers and greatly enhance local melt rates. They can grow rapidly and coalesce to form proglacial lakes, which represent a major hazard. Here, we use Sentinel-2A satellite imagery (10 m) to quantify the spatiotemporal changes of 6,425 supraglacial ponds for 10 glaciers in the Everest region, Nepal, between 2015 and 2018. During the study period, ponded area increased on all glaciers, but showed substantial temporal and spatial variation, and the rate of pond growth increased substantially relative to 2000–2015 (Watson et al., 2016). Both Imja and Spillway Lake expanded and Khumbu Glacier developed a chain of connected ponds. 54 % of ponds were associated with an ice-cliff, but the proportion of ponds with cliffs decreased during the study period. Pond location generally corresponded to lower surface velocity, but this relationship was not ubiquitous. Ponds are now predominantly found at mid-elevations on our study glaciers, suggesting that conditions conducive to pond formation have advanced up-glacier compared to general theory. Results demonstrate the need to utilize high-resolution imagery (
... Field measurements of debris thickness are scarce and challenging to collect: manual excavations (e.g. others, 2008a, 2008b;Reid and others, 2012;Rounce and McKinney, 2014), extrapolations from ice-cliff surveying (Nicholson and Benn, 2013;Nicholson and Mertes, 2017) and ground-penetrating radar measurements (McCarthy and others, 2017;Nicholson and Mertes, 2017) are the most commonly used methods for measuring or estimating debris thicknesses. However, time and labour constraints limit the spatial distribution of measurements and these methods rely on the assumption that the area sampled is representative of the entire debris-covered area (Rounce and McKinney, 2014). ...
... However, time and labour constraints limit the spatial distribution of measurements and these methods rely on the assumption that the area sampled is representative of the entire debris-covered area (Rounce and McKinney, 2014). The thickness of a surface debris layer influences its surface temperature, such that thicker debris layers generally have higher surface temperatures than thin debris layers under the same meteorological forcing, because of longer distances for conduction that slowdown transport of the heat absorbed at the surface into the underlying ice (Nicholson and Benn, 2013). ...
... In contrast, it showed a minimal sensitivity to albedo, incoming shortwave radiation and finally surface temperature. As the model is relatively insensitive to changes in albedo, following the method of Rounce and McKinney (2014), we used an albedo value of 0.3 (Nicholson and Benn, 2013). The model was moderately sensitive to variations in the non-linear temperature gradient factor -G ratio (for a ±14% change in G ratio , the debris thickness varies by +0.08 m; Table 3). ...
Surface energy-balance models are commonly used in conjunction with satellite thermal imagery to estimate supraglacial debris thickness. Removing the need for local meteorological data in the debris thickness estimation workflow could improve the versatility and spatiotemporal application of debris thickness estimation. We evaluate the use of regional reanalysis data to derive debris thickness for two mountain glaciers using a surface energy-balance model. Results forced using ERA-5 agree with AWS-derived estimates to within 0.01 ± 0.05 m for Miage Glacier, Italy, and 0.01 ± 0.02 m for Khumbu Glacier, Nepal. ERA-5 data were then used to estimate spatiotemporal changes in debris thickness over a ~20-year period for Miage Glacier, Khumbu Glacier and Haut Glacier d'Arolla, Switzerland. We observe significant increases in debris thickness at the terminus for Haut Glacier d'Arolla and at the margins of the expanding debris cover at all glaciers. While simulated debris thickness was underestimated compared to point measurements in areas of thick debris, our approach can reconstruct glacier-scale debris thickness distribution and its temporal evolution over multiple decades. We find significant changes in debris thickness over areas of thin debris, areas susceptible to high ablation rates, where current knowledge of debris evolution is limited.
... The influence of supraglacial debris on ablation depends on its thickness and thermal properties (Mihalcea and others, 2006;Reid and others, 2012;Fyffe and others, 2020). While field observations of debris properties in HKKH are scarce (Conway and Rasmussen, 2000;Nicholson and Benn, 2013;Rounce and others, 2015;Chand and Kayastha, 2018;Rowan and others, 2021), they indicate that debris properties vary within and between glaciers at a range of scales, and through time (Mihalcea and others, 2008;Nicholson and others, 2018;Shah and others, 2019). Although the physical theory is understood, the actual variability of the debris layer properties is not currently well characterised, and the effects on ablation are poorly constrained (Nicholson and others, 2018). ...
... The alternate method of estimating sub-debris ablation from debris-temperature profiles recorded at auto-logging temperature sensors, which we focus on in this work, is much less labour-intensive. Only a couple of field visits per year are required to obtain both debris thermal properties and sub-debris ablation at up to daily temporal resolution (Nicholson and Benn, 2013). Conway and Rasmussen (2000) pioneered a method to compute the effective thermal diffusivity of a debris layer and estimate the subdebris ablation using observed vertical temperature profiles. ...
... Conway and Rasmussen (2000) pioneered a method to compute the effective thermal diffusivity of a debris layer and estimate the subdebris ablation using observed vertical temperature profiles. This approach, hereinafter referred to as the CR method (Conway and Rasmussen, 2000), has been adapted in several studies in HKKH (e.g., Haidong and others, 2006;Nicholson and Benn, 2013;Chand and Kayastha, 2018;Rowan and others, 2021) and elsewhere (e.g., Nicholson and Benn, 2006;Anderson and others, 2021). ...
A supraglacial debris layer controls energy transfer to the ice surface and moderates ice ablation on debris-covered glaciers. Measurements of vertical temperature profiles within the debris enables the estimation of thermal diffusivities and sub-debris ablation rates. We have measured the debris-layer temperature profiles at 16 locations on Satopanth Glacier (central Himalaya) during the ablation seasons of 2016 and 2017. Debris temperature profile data are typically analysed using a finite-difference method, assuming that the debris layer is a homogeneous one-dimensional thermal conductor. We introduce three more methods for analysing such data that approximate the debris layer as either a single or a two-layered conductor. We analyse the performance of all four methods using synthetic experiments and by comparing the estimated ablation rates with in situ glaciological observations. Our analysis shows that the temperature measurements obtained at equispaced sensors and analysed with a two-layered model improve the accuracy of the estimated thermal diffusivity and sub-debris ablation rate. The accuracy of the ablation rate estimates is comparable to that of the in situ observations. We argue that measuring the temperature profile is a convenient and reliable method to estimate seasonal to sub-seasonal variations of ablation rates in the thickly debris-covered parts of glaciers.
... After englacial transport, material deposited in the accumulation zone resurfaces in the ablation area (Rowan et al., 2015); clast shapes and degrees of angularity vary widely, and abrupt mineralogy transitions may occur given a range of rock sources. Exposed glacial debris is loosely packed, highly porous, 15 erratically bedded, and variable in thickness over meter to decimeter spatial scales. At any location there is a random distribution of sizes and a lack of sorting beyond that which occurs during supraglacial resedimentation (Lawson, 1979;Menzies and van der Meer, 2017). ...
... Zhang et al., 2011). Other approaches include 30 geometrical scaling estimations of exposed debris (Nicholson and Benn, 2012;Nicholson and Mertes, 2017) and calculations from energy balance models in concert with remote thermal imagery (e.g. Foster et al., 2012;Rounce and McKinney, 2014;Schauwecker et al., 2015) or surface height changes (Ragettli et al., 2015). ...
... Changri Nup Glacier analog in terms of the clast mineralogy and shape, and sub-debris interface reflectivity. Freshly broken rocks sourced from a local quarry were angular and faceted quartzo-feldspathic gray gneiss, similar to the debris layer on 15 Changri Nup Glacier. The rocks were placed in a trough ( Figure 4) in three clast size groups, small (1 − 4 cm), medium (5 − 9 cm), and large (10 − 20 cm), over 4 cm thick dry pine ( = 1.9 − 2.0) boards above densely packed pine shavings ( = 1.4). ...
The thickness of supraglacial debris affects the surface energy balance and retreat patterns of mountain glaciers. Therefore, knowing a debris layer’s thickness is crucial for understanding the magnitude and timeframe of glacier melt. Field- based ground-penetrating radar (GPR) has recently gained attention as a possible method for measuring debris thickness. Airborne assessments achieve extensive coverage and characterization, but the use of GPR for such platforms remains relatively unexplored. We investigated the performance of 960 MHz and 2.6 GHz GPR signals through dry laboratory rock debris, and of 960 MHz over ∼ 2 km of transects on the debris cover of Changri Nup Glacier, Nepal Himalaya. On the glacier, 960 MHz profiles were characterized by no clear reflection from the ice interface and volumetric backscatter from within ∼ 10–40 cm, a depth that corresponds to approximate ground-truth debris thicknesses on all transects. The laboratory results show that the lack of an ice-debris interface return in field data was likely caused by a weak dielectric contrast between solid ice and porous dry debris and that surface scatter is coherent but weak. This suggests that the debris-ice interface reflection was also likely coherent, supporting our conclusion of a weak dielectric contrast. The laboratory 2.6 GHz results show significant penetration for only smaller clast sizes up to 4 cm. We used a statistical approach to estimate ice depth from volumetric scatter, which gave reasonable agreement with ground-truth depth measurements. We conclude that a remote system operating near 1 GHz could successfully estimate dry debris cover thicknesses based on depth of volumetric backscatter.
... Field (e.g. Mattson et al., 1993;Nicholson and Benn, 2013;Rowan et al., 2021), laboratory (Reznichenko et al., 2010) and modeling (e.g. Nakawo and Young, 1981;Nicholson and Benn, 2006;Reid and Brock, 2010;Evatt et al., 2015) studies demonstrate that debris thickness is the primary determinant of how sub-debris ice ablation rate differs to that of clean ice, with other properties of the debris layer, such as lithology, porosity and moisture content playing only secondary roles (e.g. ...
... Nakawo and Young, 1981;Nicholson and Benn, 2006;Reid and Brock, 2010;Evatt et al., 2015) studies demonstrate that debris thickness is the primary determinant of how sub-debris ice ablation rate differs to that of clean ice, with other properties of the debris layer, such as lithology, porosity and moisture content playing only secondary roles (e.g. Reznichenko et al., 2010;Nicholson and Benn, 2013;Collier et al., 2014). ...
... There is a systematic tendency for supraglacial debris cover thickness to increase downglacier (e.g. Zhang et al., 2011;Nicholson and Benn, 2013). This is because melt out of englacial material can only occur in the ablation zone of the glacier, and debris is continually conveyed downglacier with ice flow (Kirkbride, 2000), and is concentrated towards the glacier terminus by ice flow deceleration towards the glacier front . ...
Ongoing changes in mountain glaciers affect local water resources, hazard potential and global sea level. An increasing proportion of remaining mountain glaciers are affected by the presence of a surface cover of rock debris, and the response of these debris-covered glaciers to climate forcing is different to that of glaciers without a debris cover. Here we take a back-to-basics look at the fundamental terms that control the processes of debris evolution at the glacier surface, to illustrate how the trajectory of debris cover development is partially decoupled from prevailing climate conditions, and that the development of a debris cover over time should prevent the glacier from achieving steady state. We discuss the approaches and limitations of how this has been treated in existing modeling efforts and propose that “surrogate world” numerical representations of debris-covered glaciers would facilitate the development of well-validated parameterizations of surface debris cover that can be used in regional and global glacier models. Finally, we highlight some key research targets that would need to be addressed in order to enable a full representation of debris-covered glacier system response to climate forcing.
... Table 1 contrasts bulk thermal conductivity, heat capacity, and density of dry debris with debris of the same porosity (φ = 0.39) that has water-filled and ice-filled interstitial spaces. A number of studies (e.g., Conway and Rasmussen, 2000;Reznichenko et al., 2010;Nicholson and Benn, 2012; have emphasized the importance of moisture to the thermal properties of debris, particularly in transition seasons. Rounce and McKinney (2014) found a dramatic increase in conductivity from the top 10 cm of debris on Khumbu region glaciers to the deeper depths; they attribute this difference to water content, noting that Nicholson and Benn (2006) found the conductivity of fully saturated debris to be a factor of 2-3 greater than that of dry debris. ...
... Observations suggest that moisture transport in glacier debris is neither completely reservoir-like (as parameterized in nor fully governed by Darcy's law (as in the original ISBA for soil) but rather some of both simultaneously. A number of studies (e.g., Nicholson and Benn, 2012) mention a saturated basal layer of debris, and Rounce and McKinney (2014) discuss deeper, wet debris overlain by dry debris; our own field observations are consistent. The concentration of wetness at the debris base is due both to the fact that debris coarsens upward (Reid and Brock, 2010) and to the fact that the permeability of the overlying debris is greater than that of the debris at the interface (precipitation quickly moves through the debris until it reaches the impermeable ice surface). ...
... Since it takes a saturated sandy soil 24-48 h to drain to its field capacity, 48 h for τ max is consistent with measurements of the kinds of particles at the base of a debris layer. A shape factor of 30 is consistent with observations of wetted debris right at the debris-ice interface (Nakawo and Young, 1981;Conway and Rasmussen, 2000;Nicholson and Benn, 2012). ...
Few surface energy balance models for debris-covered glaciers account for the presence of moisture in the debris, which invariably affects the debris layer's thermal properties and, in turn, the surface energy balance and sub-debris melt of a debris-covered glacier. We adapted the interactions between soil, biosphere, and atmosphere (ISBA) land surface model within the SURFace EXternalisée (SURFEX) platform to represent glacier debris rather than soil (referred to hereafter as ISBA-DEB). The new ISBA-DEB model includes the varying content, transport, and state of moisture in debris with depth and through time. It robustly simulates not only the thermal evolution of the glacier–debris–snow column but also moisture transport and phase changes within the debris – and how these, in turn, affect conductive and latent heat fluxes. We discuss the key developments in the adapted ISBA-DEB and demonstrate the capabilities of the model, including how the time- and depth-varying thermal conductivity and specific heat capacity depend on evolving temperature and moisture. Sensitivity tests emphasize the importance of accurately constraining the roughness lengths and surface slope. Emissivity, in comparison to other tested parameters, has less of an effect on melt. ISBA-DEB builds on existing work to represent the energy balance of a supraglacial debris layer through time in its novel application of a land surface model to debris-covered glaciers. Comparison of measured and simulated debris temperatures suggests that ISBA-DEB includes some – but not all – processes relevant to melt under highly permeable debris. Future work, informed by further observations, should explore the importance of advection and vapor transfer in the energy balance.
... The magnitude of melt of the underlying ice interface is strongly modulated by debris thickness because the gradient of the temperature profile at the debris-ice interface determines the conductive heat flux to or from the ice surface (Østrem, 1959;Nicholson and Benn, 2006;Reid and Brock, 2010). Supraglacial debris in the Central Himalaya is typically tens of centimetres thick and commonly exceeds 2.0 m in thickness (Nicholson and Benn, 2013;McCarthy and others, 2017;Nicholson and Mertes, 2017). Sub-debris ablation is therefore expected to be strongly reduced compared to that of a clean-ice surface (Mihalcea and others, 2006;Nicholson and Benn, 2006;Brock and others, 2010;Reid and others, 2012). ...
... Debris layers thicker than a few centimetres heat up rapidly due to warm daytime air temperatures and incoming shortwave radiation, but due to thermal inertia of the debris, a portion of the energy absorbed during the day is reemitted to the atmosphere instead of being transmitted to the underlying ice (Reznichenko and others, 2010). The diurnal cycling of surface energy receipts ensures that the debris cover can rarely reach instantaneous thermal equilibrium with air temperature (Nicholson and Benn, 2013). Vertical profiles of debris temperature demonstrate thermally unstable behaviour at sub-diurnal timescales (Conway and Rasmussen, 2000) and therefore the temperature change within the debris with depth from the surface, hereafter the debris temperature gradient, is likely to be non-linear due to variable meteorological conditions (Reid and Brock, 2010;Foster and others, 2012;Rounce and McKinney, 2014;Schauwecker and others, 2015). ...
... To date, studies have yet to (conclusively) demonstrate how the relationship between debris temperature and thickness varies through the year in the monsoon-influenced Himalaya, or if it can be approximated at a suitable timescale for calculating annual ablation. Previous studies have suggested that, in the absence of snowfall and phase changes within the debris, quasi-linear temperature profiles can be expected over the core ablation season in the Himalaya (Nicholson and Benn, 2013), which potentially simplifies the calculation of annual ablation from debris-covered glaciers. Furthermore, sub-debris melt appears to be more strongly controlled by debris thickness rather than glacier elevation (Shah and others, 2019). ...
Rock debris covers about 30% of glacier ablation areas in the Central Himalaya and modifies the impact of atmospheric conditions on mass balance. The thermal properties of supraglacial debris are diurnally variable but remain poorly constrained for monsoon-influenced glaciers over the timescale of the ablation season. We measured vertical debris profile temperatures at 12 sites on four glaciers in the Everest region with debris thickness ranging from 0.08–2.8 m. Typically, the length of the ice ablation season beneath supraglacial debris was 160 days (15 May to 22 October)—a month longer than the monsoon season. Debris temperature gradients were approximately linear (r2 > 0.83), measured as –40°C m–1 where debris was up to 0.1 m thick, –20°C m–1 for debris 0.1–0.5 m thick, and –4°C m–1 for debris greater than 0.5 m thick. Our results demonstrate that the influence of supraglacial debris on the temperature of the underlying ice surface, and therefore melt, is stable at a seasonal timescale and can be estimated from near-surface temperature. These results have the potential to greatly improve the representation of ablation in calculations of debris-covered glacier mass balance and projections of their response to climate change.
... Debris-covered glaciers (DCGs) respond differently to clean ice glaciers under the same climatic forcing (Nicholson and Benn, 2013). The empirical relationship between debris thickness and ablation rates is well established (Östrem, 1959;Nakawo and Young, 1981;Mattson et al., 1993;Nicholson and Benn, 2006). ...
... The combined surface temperature/debris thickness dataset from the six glaciers has a mean debris thickness of 2.02 m, a median of 1.64 m, a standard deviation of 1.33 m, and a range spanning 0-7.34 m. This would appear to be representative of the debris thickness distribution we might expect on DCGs in the HMA region (Nicholson and Benn, 2013;Juen et al., 2014;Rounce and McKinney, 2014;Rounce et al., 2015). For the collated dataset (n 151,821), the nonparametric Spearman's Rank Correlation Coefficient between surface temperature and debris thickness was 0.30 (99% confidence). ...
... Only a single debris thickness value can be derived for a pixel area represented by a single surface temperature value. However, debris thickness varies on a scale smaller than a 30 m × 30 m area (Nicholson and Benn, 2013). In the datasets used to derive the empirical relationships, there is often a range of debris thickness measurements associated with a single surface temperature (Figure 4). ...
Mapping patterns of supraglacial debris thickness and understanding their controls are important for quantifying the energy balance and melt of debris-covered glaciers and building process understanding into predictive models. Here, we find empirical relationships between measured debris thickness and satellite-derived surface temperature in the form of a rational curve and a linear relationship consistently outperform two different exponential relationships, for five glaciers in High Mountain Asia (HMA). Across these five glaciers, we demonstrate the covariance of velocity and elevation, and of slope and aspect using principal component analysis, and we show that the former two variables provide stronger predictors of debris thickness distribution than the latter two. Although the relationship between debris thickness and slope/aspect varies between glaciers, thicker debris occurs at lower elevations, where ice flow is slower, in the majority of cases. We also find the first empirical evidence for a statistical correlation between curvature and debris thickness, with thicker debris on concave slopes in some settings and convex slopes in others. Finally, debris thickness and surface temperature data are collated for the five glaciers, and supplemented with data from one more, to produce an empirical relationship, which we apply to all glaciers across the entire HMA region. This rational curve: 1) for the six glaciers studied has a similar accuracy to but greater precision than that of an exponential relationship widely quoted in the literature; and 2) produces qualitatively similar debris thickness distributions to those that exist in the literature for three other glaciers. Despite the encouraging results, they should be treated with caution given our relationship is extrapolated using data from only six glaciers and validated only qualitatively. More (freely available) data on debris thickness distribution of HMA glaciers are required.
... Supraglacial debris has a complex effect on ablation that causes debris-covered glaciers to interact with the atmosphere differently than debris-free glaciers (Anderson and Anderson, 2016;Nakawo and Young 1981;Nicholson and Benn 2013;Østrem 1959;Scherler, Bookhagen, and Strecker 2011). In alpine settings where many debriscovered glaciers are located, predicting the local response to climate change and glacial contributions to the water resources is of critical importance for water resource planning and hazard mitigation, among other things. ...
... In alpine settings where many debriscovered glaciers are located, predicting the local response to climate change and glacial contributions to the water resources is of critical importance for water resource planning and hazard mitigation, among other things. As a result, the impacts of debris on energy budgets and mass balance have received increasing attention from geoscientists (e.g., Nicholson and Benn 2013;Reznichenko, Davies, and Alexander 2011;Richardson and Reynolds 2000;Scherler, Bookhagen, and Strecker 2011;Thompson et al. 2016). ...
The north and east slopes of Mount Rainier, Washington, are host to three of the largest glaciers in the contiguous United States: Carbon Glacier, Winthrop Glacier, and Emmons Glacier. Each has an extensive blanket of supraglacial debris on its terminus, but recent work indicates that each has responded to late twentieth- and early twenty-first-century climate changes in a different way. While Carbon Glacier has thinned and retreated since 1970, Winthrop Glacier has remained steady and Emmons Glacier has thickened and advanced. There are several possible climatic and dynamic factors that can account for some of these disparities, but differences in supraglacial debris properties and distribution have not been systematically evaluated. We combine field measurements and satellite remote sensing analysis from a 10-day period in the 2014 melt season to estimate both the debris thickness distribution and key debris thermal properties on Emmons Glacier. A simplified energy-balance model was then used with debris surface temperatures derived from Landsat 8 thermal infrared bands to estimate the distribution of debris across all three debris-covered termini. The results suggest that differences in summer balance among these glaciers can be partly explained by differences in the thermal resistance of their debris mantles.
... Since it takes a saturated sandy soil 24 -48 hours to drain to its field capacity, 48 hours for ⌧ max is consistent with measurements of the kinds of particles at the base of a debris layer. A shape factor of 30 is consistent with observations of wetted debris right at the debris-ice interface (Nakawo and Young, 1981;Conway and Rasmussen, 2000;Nicholson and Benn, 2012). ...
... In most cases, the tested ranges were informed by 5 literature. In the case of albedo, which has been found to vary up to 0.6 on debris-covered glaciers in the Everest region , we tested values ranging from 0.1 -0.5; Kayastha et al. (2000) claimed that most albedo values fall in the 0.2 -0.4 range, while Nicholson and Benn (2012) showed that 62% of their measurements fell between 0.1 and 0.3. A mid-day mean of the ratio of reflected to incoming shortwave radiation measured on West Changri Nup glacier gives an albedo of 0.2. ...
Few surface energy balance models for debris-covered glaciers account for the presence of moisture in the debris, which invariably affects the debris layer's thermal properties and, in turn, the surface energy balance and sub-debris melt of a debris-covered glacier. We adapted the Interactions between Soil, Biosphere, and Atmosphere (ISBA) land surface model within the SURFace EXternalisée (SURFEX) platform to represent glacier debris rather than soil. The new ISBA-DEBris model includes the varying content, transport, and state of moisture in debris with depth and through time. It robustly simulates not only the thermal evolution of the glacier-debris-snow column but also moisture transport and phase changes within the debris – and how these, in turn, affect conductive and latent heat fluxes. We discuss the key developments in the adapted ISBA-DEB and demonstrate the capabilities of the model, including how the time- and depth-varying thermal conductivity and specific heat capacity depend on evolving temperature and moisture. Sensitivity tests emphasize the importance of accurately constraining the roughness lengths and surface slope. Emissivity, in comparison to other tested parameters, has less of an effect on melt. ISBA-DEB builds on existing work to represent the energy balance of a supraglacial debris layer through time in its novel application of a land surface model to debris covered glaciers. Comparison of measured and simulated debris temperatures suggests that ISBA-DEB includes some – but not all – processes relevant to melt under highly permeable debris. Future work, informed by further observations, should explore the importance of advection and vapor transfer.
... This implies that beneath the critical value of the thermal resistance, the melt ratio is larger than 1.0 (Figure 8), which an acceleration effect of the debris cover, while above the critical value, the melt ratio is less than 1.0, which means an insulation effect of the debris cover. Heterogeneous distribution of debris thickness leads to strong contrasting ablation and reversed ablation gradients in the ablation zone [18,30,62], and then affects the morphology and evolution of the hydrological system of a glacier [16,18,63], with important consequences for the formation of catchment runoff. It is well known that debris-covered glaciers are prevalent in the Tibetan Plateau and its surroundings [10,12,13], as well as in many other high mountain ranges of the world [11,21]. ...
... As discussed above, our results highlight the importance of debris cover for better Heterogeneous distribution of debris thickness leads to strong contrasting ablation and reversed ablation gradients in the ablation zone [18,30,62], and then affects the morphology and evolution of the hydrological system of a glacier [16,18,63], with important consequences for the formation of catchment runoff. It is well known that debris-covered glaciers are prevalent in the Tibetan Plateau and its surroundings [10,12,13], as well as in many other high mountain ranges of the world [11,21]. ...
Runoff from high-elevation, debris-covered glaciers is a crucial water supply in the Tibetan Plateau (TP) and its surroundings, where insufficient debris thickness data make it difficult to analyze its influence. Here, we investigated the role of debris cover in runoff formation of the Hailuogou catchment in the south-eastern Tibetan Plateau for the 1988–2017 period, based on long-term observations combined with a physically based glacio-hydrological model. The catchment is characterized by extensive thin debris on the ablation zones of three debris-covered glaciers. An increasing trend in catchment runoff has been observed in the past three decades, more than 50% of which is attributed to glacier runoff in the last decade. With the exception of the influence of temperature rising and precipitation decreasing, our results underline the importance of debris cover and its spatial features in the glaciological and hydrological processes of the catchment, in which the acceleration effect of debris cover is dominant in the catchment. An experimental analysis indicated that the extraordinary excess meltwater in the catchment is generated from the debris-covered surface, especially the lower elevation region below 3600 m a.s.l.
... Other properties of supraglacial debris, such as moisture content, rock type, and grain size, can alter the thermal conductivity of the debris layer, consequently modifying the relationship between debris thickness and surface melt rates (e.g. [15][16][17]). The ablation rates on heavily debris-covered glaciers are extremely difficult to measure, due to the challenges that are associated with drilling stakes through the debris layer, as well as the large heterogeneity of local ablation rates (e.g. ...
... Differences in debris-thickness distribution, as well as local debris properties, such as lithology, grain size, and moisture content, may contribute towards the contrasting breakpoint elevations and magnitudes of reversed altitudinal SMB gradients that were observed within these regions (e.g. [15][16][17]. Supraglacial ice cliffs and ponds can also influence the SMB of debris-covered glaciers by creating localised areas of enhanced melting [24][25][26][27]. Therefore, it is possible that these features could also partially explain the heterogeneity that was observed between debris-covered glaciers. ...
Meltwater from the glaciers in High Mountain Asia plays a critical role in water availability and food security in central and southern Asia. However, observations of glacier ablation and accumulation rates are limited in spatial and temporal scale due to the challenges that are associated with fieldwork at the remote, high-altitude settings of these glaciers. Here, using a remote-sensing-based mass-continuity approach, we compute regional-scale surface mass balance of glaciers in five key regions across High Mountain Asia. After accounting for the role of ice flow, we find distinctively different altitudinal surface-mass-balance gradients between heavily debris-covered and relatively debris-free areas. In the region surrounding Mount Everest, where debris coverage is the most extensive, our results show a reversed mean surface-mass-balance gradient of -0.21 ± 0.18 m w.e. a−1 (100m)−1 on the low-elevation portions of glaciers, switching to a positive mean gradient of 1.21 ± 0.41 m w.e. a−1 (100m)−1 above an average elevation of 5520 ± 50 m. Meanwhile, in West Nepal, where the debris coverage is minimal, we find a continuously positive mean gradient of 1.18 ± 0.40 m w.e. a−1 (100m)−1. Equilibrium line altitude estimates, which are derived from our surface-mass-balance gradients, display a strong regional gradient, increasing from northwest (4490 ± 140 m) to southeast (5690 ± 130 m). Overall, our findings emphasise the importance of separating signals of surface mass balance and ice dynamics, in order to constrain better their contribution towards the ice thinning that is being observed across High Mountain Asia.
... Higher surface temperatures, and a larger amplitude in the diurnal cycle of near-surface temperature have been found over debris-covered glaciers, compared to clean-ice glaciers (Steiner and Pellicciotti, 2016;Yang et al., 2017;Nicholson and Stiperski, 2020). It has been shown that after heating during the day, debris cover greater than 0.05 m returns the energy to the atmosphere at night, rather than heating the ice below (Reznichenko et al., 2010;Nicholson and Benn, 2013). Differences have also been found in near-surface wind speed and incoming short wave radiation between debris-covered and clean-ice glaciers (Yang et al., 2017). ...
... For this simple representation of debris cover in the WRF model, the debris is assumed to occupy the full soil depth in the WRF model, which extends to 255 cm below the surface (i.e., there is no representation of the ice underneath). As such, this method best represents thick debris cover, as in general thick debris cover is thought to provide insulation between the air and the glacier ice below the debris cover (Reznichenko et al., 2010;Nicholson and Benn, 2013). ...
Abstract Many of the glaciers in the Nepalese Himalaya are partially covered in a layer of loose rock known as debris cover. In the Dudh Koshi River Basin, Nepal, approximately 25% of glaciers are debris‐covered. Debris‐covered glaciers have been shown to have a substantial impact on near‐surface meteorological variables and the surface energy balance, in comparison to clean‐ice glaciers. The Weather Research and Forecasting (WRF) model is often used for high‐resolution weather and climate modelling, however representation of debris‐covered glaciers is not included in the standard land cover and soil categories. Here we include a simple representation of thick debris‐covered glaciers in the WRF model, and investigate the impact on the near‐surface atmosphere over the Dudh Koshi River Basin for July 2013. Inclusion of this new category is found to improve the model representation of near‐surface temperature and relative humidity, in comparison with a simulation using the default category of clean‐ice glaciers, when compared to observations. The addition of the new debris‐cover category in the model warms the near‐surface air over the debris‐covered portion of the glacier, and the wind continues further up the valley, compared to the simulation using clean‐ice. This has consequent effects on water vapour and column‐integrated total water path, over both the portions of the glacier with and without debris cover. Correctly simulating meteorological variables such as these is vital for accurate precipitation forecasts over glacierized regions, and therefore estimating future glacier melt and river runoff in the Himalaya. These results highlight the need for debris cover to be included in high‐resolution regional climate models over debris‐covered glaciers.
... The observed glacier loss in the Himalaya would adversely impact the water supplies for irrigation and other dependent services and livelihoods in the region (Abdullah et al., 2020;Immerzeel et al., 2020;Nie et al., 2021). These concerns have led to a number of glacier studies in the region (Scherler et al., 2011;Nicholson and Benn, 2013;Kääb et al., 2015;Racoviteanu et al., 2015;Garg et al., 2017;Murtaza and Romshoo, 2017;Bhushan et al., 2018;Vijay and Braun., 2018;Muhammad et al., 2019;Kumar et al., 2019;Abdullah et al., 2020;Romshoo et al., 2020), which indicated that glaciers in the Himalaya are showing a significant area and mass loss and that the glacier behavior observed across the Himalaya is not uniform due to the varying influence of topographic, climatic and other driving factors. ...
... The slight deviation in the shrinkage rates between these studies and our observations could be because of the variation in the climatic conditions (Shrestha et al., 2012) and tectonic setting of the basins (DiPietro and Pogue, 2004). However, the physical, thermal and lithological properties of the debris-cover like thickness, composition, thermal conductivity, moisture content, porosity (Conway et al., 2000;Nicholson and Benn, 2013) depth to the critical thickness of debris (Nakawo and Young, 1981) and the regional variation in the distribution of debris-cover influence the recession rates (Scherler et al., 2011). ...
Out of 4530 glaciers covering ∼4770 km² in the Upper Indus Basin, viz., Chenab, Zanaskar and Suru sub-basins, we selected topographically and morphologically homogeneous set of 98 glaciers; 56 debris-covered and 42 clean to assess the impact of debris-cover on glacier melting. Satellite data analysis from 2000 to 2017 showed that the debris-covered glaciers have melted slower (2.49 ± 0.05%; 0.15%a⁻¹) than the clean glaciers (4.07 ± 0.03%; 0.24%a⁻¹). The intra-basin analyses revealed that clean and debris-covered glaciers in the Chenab have melted 2.34 ± 0.17% and 1.1 ± 0.30% (0.14%a⁻¹ and 0.06%a⁻¹) and retreated 8.82 ± 2.67ma⁻¹ and 5.7 ± 2.67ma⁻¹ respectively. Similarly, clean and debris-covered glaciers in the Zanaskar have melted at similar rates of 4.58 ± 0.07% and 4.55 ± 0.21% (0.27%a⁻¹ and 0.25%a⁻¹) and retreated 10.0 ± 2.67ma⁻¹ and 8.4 ± 2.67ma⁻¹ respectively. Clean and debris-covered glaciers in the Suru have melted 5.56 ± 0.09% and 4.53 ± 0.14% (0.33%a⁻¹ and 0.27%a⁻¹) and retreated 9.6 ± 2.67ma⁻¹ and 7.0 ± 2.67ma⁻¹ respectively. Extensive debris-cover of ∼32.3% modulates glacier loss in the Chenab and sparse debris-cover of 11.4% and 6.4% in the Suru and Zanaskar enhances the melting of the debris-covered glaciers. Overall, debris-covered glaciers melted slower than the clean glaciers, with inter- and intra-basin variations, suggesting that debris-cover plays a complex but significant role in modulating glacier melting under climate change.
... Zhang and others, 2011). Other approaches include geometrical scaling estimations of exposed debris (Nicholson and Benn, 2012;Nicholson and Mertes, 2017) and calculations from energy-balance models in concert with remote thermal imagery (e.g. Foster and others, 2012;Rounce and McKinney, 2014;Schauwecker and others, 2015) or surface height changes (Ragettli and others, 2015). ...
The thickness of a supraglacial layer is critical to the magnitude and time frame of glacier melt. Field-based, short pulse, ground-penetrating radar (GPR) has successfully measured debris thickness during a glacier's melt season, when there is a strong return from the ice–debris interface, but profiling with GPR in the absence of a highly reflective ice interface has not been explored. We investigated the performance of 960 MHz signals over 2 km of transects on Changri Nup Glacier, Nepal, during the post-monsoon. We also performed laboratory experiments to interpret the field data and investigate electromagnetic wave propagation into dry rocky debris. Laboratory tests confirmed wave penetration into the glacier ice and suggest that the ice–debris interface return was missing in field data because of a weak dielectric contrast between solid ice and porous dry debris. We developed a new method to estimate debris thicknesses by applying a statistical approach to volumetric backscatter, and our backscatter-based calculated thickness retrievals gave reasonable agreement with debris depths measured manually in the field (10–40 cm). We conclude that, when melt season profiling is not an option, a remote system near 1 GHz could allow dry debris thickness to be estimated based on volumetric backscatter.
... Also, Collier et al. (2015) linearly distributed the debris thickness from terminus to the upper debris-covered zone to model and compare melt rates between debris cover and debris free ice. The findings are useful for the impact assessment of debris cover on glacier-melt but for accurate model parametrization, estimation of field-based melt rate and spatial distribution of debris is necessary (Nicholson and Benn, 2013). ...
The assessment of meltwater sourcing from the clean and debris-covered glaciers is scarce in High Mountain Asia (HMA). The melting rate varies with the debris cover thickness and glacier orientation. The present study quantifies glacier melting rate attributed to varying thickness of debris cover in the Karakoram. We observed daily melting rates by installing ablation stakes over debris-free and debris-covered ice during a field expedition. The stakes were installed on glacier surface with debris cover thickness ranges between 0.5 and 40 cm at selected experimental sites during the ablation period (September and October 2018) and (July to August 2019). We selected three glaciers including Ghulkin, Hinarchi, and Hoper facing east, south, and north, respectively to assess the role of glacier orientation on melting rates. We observed that the debris-free ice melts faster than the debris-covered ice. Intriguingly, a thin debris layer of 0.5 cm does not enhance melting compared to the clean ice which is inconsistent with the earlier studies. The melting rate decreases as the thickness of debris cover increases at all the three selected glaciers. Furthermore, south-facing glacier featured the highest melting (on average ~ 25% more). However, the north and east-facing glaciers revealed almost same melting rates. We observed that the average degree-day factors (DDF) slightly varies within a range of 0.58–0.73 and 0.55–0.68 cm °C−1 day−1 for debris-free and 0.5 cm debris-covered ice, respectively, however, DDF largely reduces to 0.13–0.25 cm °C−1 day−1 for 40 cm debris-covered ice. We suggest continuous physical glacier ablation observations for various debris cover throughout the ablation zone to better understand the role of debris on melting.
... Supra-glacial debris thickness can also play a key role in changes of mass balance for individual glaciers. This is broadly confirmed that supra-glacial debris cover affects surface melt with increasing ablation in cases of thin debris cover (less than a few cm) or decreasing ablation under continuous thick debris cover [43][44][45]. It is also possible that ice cliffs and supra-glacial ponds play a key role by enhancing the total ablation of debris-covered glaciers in the Greater Caucasus. ...
Glaciers and snow in the Caucasus are major sources of runoff for populated places in many parts of this mountain region. These glaciers have shown a continuous area decrease; however, the magnitude of mass balance changes at the regional scale need to be further investigated. Here, we analyzed regional changes in surface elevation (or thickness) and geodetic mass balance for 1861 glaciers (1186.1 ± 53.3 km2) between 2000 and 2019 from existing dataset and outlines of the Caucasus glacier inventory. We used a debris-covered glacier dataset to compare the changes between debris-free and debris-covered glaciers. We also used 30 m resolution ASTER GDEM (2011) to determine topographic details, such as aspect, slope, and elevation distribution of glaciers. Results indicate that the mean rate of glacier mass loss has accelerated from 0.42 ± 0.61 m of water equivalent per year (m w.e. a−1) over 2000–2010, to 0.64 ± 0.66 m w.e. a−1 over 2010–2019. This was 0.53 ± 0.38 m w.e. a−1 in 2000–2019. Mass loss rates differ between the western, central, and eastern Greater Caucasus, indicating the highest mean annual mass loss in the western section (0.65 ± 0.43 m w.e. a−1) in 2000–2019 and much lower in the central (0.48 ± 0.35 m w.e. a−1) and eastern (0.38 ± 0.37 m w.e. a−1) sections. No difference was found between the northern and southern slopes over the last twenty years corresponding 0.53 ± 0.38 m w.e. a−1. The observed decrease in mean annual geodetic mass balance is higher on debris-covered glaciers (0.66 ± 0.17 m w.e. a−1) than those on debris-free glaciers (0.49 ± 0.15 m w.e. a−1) between 2000 and 2019. Thickness change values in 2010–2019 were 1.5 times more negative (0.75 ± 0.70 m a−1) than those in 2000–2010 (0.50 ± 0.67 m a−1) in the entire region, suggesting an acceleration of ice thinning starting in 2010. A significant positive trend of May-September air temperatures at two selected meteorological stations (Terskol and Mestia) along with a negative trend of October-April precipitation might be responsible for the negative mass balances and thinning for all Caucasus glaciers over the study period. These results provide insight into the change processes of regional glaciers, which is key information to improve glaciological and hydrological projections in the Caucasus region.
... The smooth down-glacier increase in debris thickness, and the corresponding decline of the surface ablation rate as discussed above, provide only a first-order description of the debris effects (Benn and Lehmkuhl, 2000;Scherler and others, 2011b;Banerjee and Shankar, 2013). The role of several other complicating factors, e.g., the presence of numerous thermokarst ephemeral ponds and cliffs that increase local melt rate (Reynolds, 2000;Sakai and others, 2000;Miles and others, 2017), vertical and horizontal variations of the thermal properties of debris (Nicholson and Benn, 2013;Rowan and others, 2017), the random short-scale spatial variation of debris thickness (Mihalcea and others, 2006;Zhang and others, 2011;Nicholson and Mertes, 2017;Rounce and others, 2018) and the accumulation contribution from avalanches (Laha and others, 2017) need to be quantified for accurate surface mass-balance estimates on any typical debris-covered Himalayan glacier. The standard glaciological mass-balance measurement protocol (Kaser and others, 2003) may not be designed to handle the above issues. ...
This is the preprint of an article that is under review in the Journal of Glaciology. The abstract is as follows: Glaciological mass balance is computed from point-scale field data at a few ablation stakes that are regressed as a function of elevation, and averaged over the area-elevation distribution of the glacier. This method is contingent on a tight control of elevation on local ablation. On debris-covered glaciers, systematic and random spatial variations of debris thickness modify ablation rates. A method that takes into account the debris-thickness variability in extrapolating point-scale ablation data may be more accurate on these glaciers. We propose and test a methGlaciological mass balance is computed from point-scale field data at a few ablation stakes that are regressed as a function of elevation, and averaged over the area-elevation distribution of the glacier. This method is contingent on a tight control of elevation on local ablation. On debris-covered glaciers, systematic and random spatial variations of debris thickness modify ablation rates. A method that takes into account the debris-thickness variability in extrapolating point-scale ablation data may be more accurate on these glaciers. We propose and test a method where stake data are interpolated as a function of debris-thickness alone, and averaged over the observed debris-thickness distribution at different parts of the glacier. We apply this method to compute sub-debris ablation rate on Satopanth Glacier (Central Himalaya) utilising about a thousand ablation measurements at a network of up to 56 stakes during 2015--2017. We compare our results with that from the standard glaciological method. The uncertainties in both the estimates due to the corresponding uncertainties in measurement of ablation and debris-thickness distribution, and that due to interpolation procedures are estimated using Monte Carlo methods. Possible biases due to finite number of stakes used are investigated, and net specific balance of Satopanth glacier is computed.od where stake data are interpolated as a function of debris-thickness alone, and averaged over the observed debris-thickness distribution at different parts of the glacier. We apply this method to compute sub-debris ablation rate on Satopanth Glacier (Central Himalaya) utilising about a thousand ablation measurements at a network of up to 56 stakes during 2015--2017. We compare our results with that from the standard glaciological method. The uncertainties in both the estimates due to the corresponding uncertainties in measurement of ablation and debris-thickness distribution, and that due to interpolation procedures are estimated using Monte Carlo methods. Possible biases due to finite number of stakes used are investigated, and net specific balance of Satopanth glacier is computed.
... The smooth down-glacier increase in debris thickness, and the corresponding decline of the surface ablation rate as discussed above, provide only a first-order description of the debris effects (Benn and Lehmkuhl, 2000;Scherler and others, 2011b;Banerjee and Shankar, 2013). The role of several other complicating factors, e.g., the presence of numerous thermokarst ephemeral ponds and cliffs that increase local melt rate (Reynolds, 2000;Sakai and others, 2000;Miles and others, 2017), vertical and horizontal variations of the thermal properties of debris (Nicholson and Benn, 2013;Rowan and others, 2017), the random short-scale spatial variation of debris thickness (Mihalcea and others, 2006;Zhang and others, 2011;Nicholson and Mertes, 2017;Rounce and others, 2018) and the accumulation contribution from avalanches (Laha and others, 2017) need to be quantified for accurate surface mass-balance estimates on any typical debris-covered Himalayan glacier. The standard glaciological mass-balance measurement protocol (Kaser and others, 2003) may not be designed to handle the above issues. ...
Glaciological ablation is computed from point-scale data at a few ablation stakes that are usually regressed as a function of elevation and averaged over the area-elevation distribution of a glacier. This method is contingent on a tight control of elevation on local ablation. However, in debris-covered glaciers, systematic and random spatial variations of debris thickness modify the ablation rates. We propose and test a method to compute sub-debris ablation where stake data are interpolated as a function of debris-thickness alone and averaged over the debris-thickness distribution at different parts of the glacier. We apply this method on Satopanth Glacier located in Central Himalaya utilising ~1000 ablation measurements obtained from a network of up to 56 stakes during 2015–2017. The estimated mean sub-debris ablation ranges between 1.5±0.2 to 1.7±0.3 cm d ⁻¹ . We show that the debris-thickness-dependent regression describes the spatial variability of the sub-debris ablation better than the elevation dependent regression. The uncertainties in ablation estimates due to the corresponding uncertainties in the measurement of ablation and debris-thickness distribution, and those due to interpolation procedures are estimated using Monte Carlo methods. Possible biases due to a finite number of stakes used are also investigated.
... 川表碛厚度的精确测量或反演在冰川消融率估算、物质平衡模拟、区域水资源评估乃至 冰川灾害演化研究等方面具有重要意义 [12] 。 确定冰川表碛厚度有两类方式：① 传统的现场实地测量，这种方法工作量大且无法 覆盖整个表碛范围，对于无法到达的冰川，获得表碛厚度值更为困难 [13] ；② 利用遥感技 术反演区域冰川表碛厚度 [14] 。随着遥感科学的快速发展，利用遥感影像反演冰川表碛厚 度已经较为成熟，但对于如何提高反演精度及结果验证仍面临较大困难。在具体应用 中，地表温度-表碛厚度关系法、雷达影像反演表碛厚度、基于能量平衡和表碛热阻系 数估算表碛厚度等方法运用最为广泛 [6] 。Zhang 等 [15] 实地测量了海螺沟冰川消融区的表碛 厚度，发现表碛厚度随海拔上升而降低且与基于热红外影像提取的表碛层热阻系数相关 性很强。Mihalcea 等 [16] 基于地表能量平衡方程反演 Baltoro 冰川表碛厚度并估算了不同表 碛厚度下的消融率，这种方法的优点在于不需要太多观测数据就能对表碛厚度分布进行 制图。Mihalcea 等 [17] 通过建立冰川表面温度与表碛厚度之间的关系模拟了 Miage 冰川表碛 的空间分布，发现地表测量和遥感反演均能很好地估算消融区的表碛厚度。此外，也有 学者利用合成孔径雷达 (SAR) 影像的 L 波段估算了天山托木尔峰地区冰川表碛厚度 [18] 。 冰川表碛分布与冰面湖、冰崖等冰面地貌演化过程密切相关。总结国内外相关文 献，利用遥感影像反演冰川表碛厚度已经相对较为成熟，但通过多时相分析表碛厚度变 化研究较少。在气候变暖冰川消融加剧背景下，冰川表碛覆盖存在分布范围向上游扩张 和厚度向下游区增厚的趋势。贡嘎山位于青藏高原东南缘，是典型地海洋型冰川作用 区，这种趋势在贡嘎山地区表现得更为明显。目前，针对贡嘎山地区冰川表碛的研究还 只停留在实地测量阶段，缺乏全面系统的时空对比研究。本文将利用 Landsat TM/TIRS 影像的热红外波段反演贡嘎山地区冰川表碛厚度，揭示其典型冰川在 1990-2019 年间表 碛覆盖范围和厚度变化特征以及贡嘎山东西坡差异，并在此基础上进行分析讨论。 2 贡嘎山位于青藏高原东南缘，是中国重要的海洋型冰川作用区 [19] 。针对贡嘎山地区 冰川的研究最早起源于 20 世纪 30 年代，之后国内外学者先后多次对该区域内的冰川进行 考察和监测。1990 年中苏联合贡嘎山冰川考察，为后来山地冰川研究积累了大量宝贵研 究资料。中国科学院四川贡嘎山森林生态系统野外科学观测研究站于 1987 年建站，为该 区域内冰川的监测与研究提供了重要平台 [20] 。 贡嘎山最高峰达 7514 m，区内有现代冰川 74 条，冰川总面积 255.1 km 2 [21] 。基于自 动气象站连续观测表明，贡嘎山东坡海螺沟冰川区附近 (3000 m) 年平均气温 (5 ℃) 高于西坡贡巴冰川区附近 (3700 m) 的贡嘎寺 (3 ℃) ，对应站点东坡多年平均年降水量 约为 1700 mm，然而西坡仅约 900 mm。受东亚夏季风的影响，贡嘎山东坡冰川较为发 育，典型的较大型山岳冰川有海螺沟冰川、磨子沟冰川、燕子沟冰川和南门关沟冰川； 西坡发育冰川数量多，但规模小，其中典型冰川有大贡巴冰川和小贡巴冰川 (图 1) [22] 。 以上 6 条冰川除小贡巴冰川外长度均超过了 10 km，面积占据了整个贡嘎山冰川的 47.6%，冰川消融区均有大范围表碛覆盖 [23] 。Zhang 等 [15] [32] ...
表碛覆盖型冰川是中国西部较为常见的冰川类型。表碛层存在于大气—冰川冰界面， 强烈影响大气圈与冰冻圈之间的热交换。表碛厚度的空间异质性可极大地改变冰川的消融率 和物质平衡过程，进而影响冰川径流过程和下游水资源。基于Landsat TM/TIRS数据，运用能 量平衡方程反演了贡嘎山地区冰川表碛厚度，研究了贡嘎山地区冰川在1990—2019年间表碛 覆盖范围及厚度变化情况，同时对比了东西坡差异。结果表明：①贡嘎山地区冰川表碛扩张总 面积达43.824 km2。其中，海螺沟冰川扩张2.606 km2、磨子沟冰川1.959 km2、燕子沟冰川1.243 km2、大贡巴冰川0.896 km2、小贡巴冰川0.509 km2、南门关沟冰川2.264 km2，年均扩张率分别为 3.2%、11.1%、1.5%、0.9%、1.0%和6.5%；②海螺沟冰川、磨子沟冰川、燕子沟冰川、大贡巴冰川、 小贡巴冰川、南门关沟冰川表碛平均增厚分别为5.2 cm、3.1 cm、3.7 cm、6.8 cm、7.3 cm和13.1 cm；③西坡冰川表碛覆盖度高，表碛覆盖年均扩张率低，冰川末端退缩量小；东坡冰川表碛覆 盖年均扩张率高，但表碛覆盖度总体低于西坡，冰川末端退缩量大。
... Temperature is one of the most important meteorological variables for numerous societal and environmental applications, such as in climatology, glaciology, hydrology, agriculture and ecology. The quantification of the elevational temperature distribution is particularly essential for glacio-hydrological modeling approaches, which investigate the form of precipitation at different elevations in order to estimate present and future water availability in mountain environments (Minder et al. 2010;Nicholson and Benn 2013;Nepal et al. 2014;Soncini et al. 2016;Eeckman et al. 2017). In this regard, elevation dependent warming is of particular interest for regional climate change impact scenarios (Shrestha and Aryal 2011;Benn et al. 2012;Gerlitz et al. 2014;Pepin et al. 2015;Hasson et al. 2016a, Minder et al. 2018. ...
Various environmental processes are strongly controlled by spatio-temporal variations of surface air temperature (hereafter temperature) in complex terrain. However, the usually scarce network of observations in high mountain regions does not allow for an investigation of the relevant micro-meteorological processes that result in complex temperature fields. Climate impact studies often utilize a constant lapse rate of temperature (LRT hereafter) in order to generate spatially distributed temperature data, although it is well acknowledged that LRTs feature a pronounced variability at spatial, seasonal, and diurnal scales. In this study, the Weather Research and Forecasting (WRF) model is used to understand the factors and processes influencing temperature and LRT in the Khumbu and Rolwaling regions of the central Himalayas. A high resolution simulation is performed for one complete year (June 2014–May 2015) in order to capture the entire seasonal cycle. To test the model response to land cover and terrain characteristics, additional simulations with adjusted surface conditions are conducted. Our results demonstrate the capability of WRF to reproduce the processes controlling LRT, although an LRT bias is detected during non-monsoon seasons. The simulated temperature fields feature two LRT minima (i.e. low temperature decrease with elevation) during Nov–Dec and monsoon season, and two LRT maxima (strong temperature decrease with elevation) during the early post-monsoon and pre-monsoon seasons. A steeper LRT (i.e. a rapid decrease of temperature with elevation) is found at high elevations (> 4500 m) while shallower LRT values (i.e. a slower decrease or even increase of temperature with elevation) are apparent at lower elevations. During the pre-monsoon season, high net insolation rates and a reduced latent heat loss from snow free surfaces cause strong sensible heating at low elevations, while the presence of snow at high elevations leads to reduced sensible heating. This strong contrast results in steeper LRT values. Early post-monsoon shows similar characteristics but with a reduced magnitude. The shallow LRT during monsoon season is shown to be caused by the large-scale moisture supply and the associated latent heat release at the Himalayan slopes. This effect is further intensified due to strong up-valley winds which contribute to a well-mixed troposphere. Temperature inversions associated with cold air pooling cause shallow LRT values in lower river valleys during Nov–Dec. The results suggest that the identified processes should be considered for downscaling applications, particularly if distributed temperature fields are required for climate impact investigations.
... They were attached with bamboo stakes, and they continuously logged temperature data at 30 min intervals between September 2016 and October 2017 ( Table 1). Many studies have used the same thermistor probes and dataloggers for temperature profiling for similar investigations of debris-covered glaciers in the Nepal Himalaya (Mihalcea et al., 2006;Brock et al., 2010;Nicholson and Benn, 2013;Rowan et al., 2021). The Tinytag datalogger (TGP 4520) has an accuracy of ± 0.4°C at 0°C, and two thermal probes can be connected into the logger unit. ...
A large number of glaciers in the Hindu-Kush Himalaya are covered with debris in the lower part of the ablation zone, which is continuously expanding due to enhanced glacier mass loss. The supraglacial debris transported over the melting glacier surface acts as an insulating barrier between the ice and atmospheric conditions and has a strong influence on the spatial distribution of surface ice melt. We conducted in-situ field measurements of point-wise ablation rate, supraglacial debris thickness, and debris temperature to examine the thermal resistivity of the debris pack and its influence on ablation over three glaciers (Bara Shigri, Batal, and Kunzam) in Chandra Basin of Western Himalaya during 2016-2017. Satellite-based supraglacial debris cover assessment shows an overall debris covered area of 15% for Chandra basin. The field data revealed that the debris thickness varied between 0.5 and 326 cm, following a spatially distributed pattern in the Chandra basin. The studied glaciers have up to 90% debris cover within the ablation area, and together represent ∼33.5% of the total debris-covered area in the basin. The supraglacial debris surface temperature and near-surface air temperature shows a significant correlation (r > 0.88, p < 0.05), which reflects the effective control of energy balance over the debris surface. The thermal resistivity measurements revealed low resistance (0.009 ± 0.01 m 2°C W −1) under thin debris pack and high resistance (0.55 ± 0.09 m 2°C W −1) under thick debris. Our study revealed that the increased thickness of supraglacial debris significantly retards the glacier ablation due to its high thermal resistivity.
... When calving into a proglacial lake is involved, mass loss is shifted dramatically to the calving front (Bolch et al., 2011;Thakuri et al., 2016;King et al., 2018). However, rates of mass loss are spatially variable on both calving and non-calving glaciers due to variable debris thickness and the distribution of supraglacial ponds and ice cliffs (Nicholson and Benn, 2013;Immerzeel et al., 2014;Buri et al., 2016;Miles et al., 2017;Watson et al., 2017b), and variations in microclimate parameters, such as due to radiative and climatic orography (Heynen et al., 2016). Recently, it was shown that Himalayan glaciers connecting with larger proglacial lakes are shrinking more rapidly than glaciers lacking them (Maurer et al., 2019). ...
The formation and expansion of Himalayan glacial lakes has implications for glacier dynamics, mass balance and glacial lake outburst floods (GLOFs). Subaerial and subaqueous calving is an important component of glacier mass loss but they have been difficult to track due to spatiotemporal resolution limitations in remote sensing data and few field observations. In this study, we used near-daily 3 m resolution PlanetScope imagery in conjunction with an uncrewed aerial vehicle (UAV) survey to quantify calving events and derive an empirical area–volume relationship to estimate calved glacier volume from planimetric iceberg areas. A calving event at Thulagi Glacier in 2017 was observed by satellite from before and during the event to nearly complete melting of the icebergs, and was observed in situ midway through the melting period, thus giving insights into the melting processes. In situ measurements of Thulagi Lake’s surface and water column indicate that daytime sunlight absorption heats mainly just the top metre of water, but this heat is efficiently mixed downwards through the top tens of metres due to forced convection by wind-blown icebergs; this heat then is retained by the lake and is available to melt the icebergs. Using satellite data, we assess seasonal glacier velocities, lake thermal regime and glacier surface elevation change for Thulagi, Lower Barun and Lhotse Shar glaciers and their associated lakes. The data reveal widely varying trends, likely signifying divergent future evolution. Glacier velocities derived from 1960/70s declassified Corona satellite imagery revealed evidence of glacier deceleration for Thulagi and Lhotse Shar glaciers, but acceleration at Lower Barun Glacier following lake development. We used published modelled ice thickness data to show that upon reaching their maximum extents, Imja, Lower Barun and Thulagi lakes will contain, respectively, about 90 × 106, 62 × 106 and 5 × 106 m3 of additional water compared to their 2018 volumes. Understanding lake–glacier interactions is essential to predict future glacier mass loss, lake formation and associated hazards.
... The second is the thermal resistance effect, which reduces ablation due to the decrease of heat conducted from the air to the glacier surface. Thermal resistance increases with deposition thickness and it can be quantified either by empirical measurements or by physics-based considerations of the thermal properties of the deposit (Möller et al. 2016;Nicholson and Benn 2013). While deposition thickness is process-related, the thermal properties and, in particular, thermal conductivity are strongly related to the petrographical characteristics of the volcanic material and to the water content of the deposit (Clauser and Huenges 1995;Kuznetsova 2017). ...
Supraglacial deposits of tephra or volcaniclastics have the potential to cause significant anomalies of glacier ablation and runoff. The intensity of these anomalies is governed by the thermal resistivity of the covering layer and hence the thermal conductivity of the deposited grains. This study concentrates on causal and quantitative relationships between density, geochemical composition and thermal conductivity of volcanic materials based on the analysis of 43 samples from locations across Iceland. Thermal conductivity is primarily influenced by density, whereas geochemical composition has been proved to be of subsidiary importance. Four different multiple regression models were calibrated that calculate the grain thermal conductivity of a volcanic material based on rock properties and geochemical composition. In a subsequent step, the bulk thermal conductivity of the respective deposit is calculated as a function of porosity and degree of water saturation. Examples using volcanic material from the Eyjafjallajökull 2010 and Grímsvötn 2011 eruptions confirm that the presented calculation scheme can be executed using only limited geochemical data as input. This facilitates an easy application of the modeling scheme immediately after a volcanic eruption.
... It also reveals that at least small amounts of moisture are likely deposited onto the debris cover during the night. This is potentially significant as this moisture, along with moisture from precipitation events, can impact the bulk thermal properties of the layer (Nicholson and Benn, 2012). Latent heat flux measured at other debris-covered glaciers shows a stronger and more consistent diurnal variability than is seen at Suldenferner, and typically remains negative during ablation season conditions (Collier and others, 2014;Yao and others, 2014;Steiner and others, 2018). ...
We present the first direct comparison of turbulence conditions measured simultaneously over exposed ice and a 0.08 m thick supraglacial debris cover on Suldenferner, a small glacier in the Italian Alps. Surface roughness, sensible heat fluxes (~20–50 W m ⁻² ), latent heat fluxes (~2–10 W m ⁻² ), topology and scale of turbulence are similar over both glacier surface types during katabatic and synoptically disturbed conditions. Exceptions are sunny days when buoyant convection becomes significant over debris-covered ice (sensible heat flux ~ −100 W m ⁻² ; latent heat flux ~ −30 W m ⁻² ) and prevailing katabatic conditions are rapidly broken down even over this thin debris cover. The similarity in turbulent properties implies that both surface types can be treated the same in terms of boundary layer similarity theory. The differences in turbulence between the two surface types on this glacier are dominated by the radiative and thermal contrasts, thus during sunny days debris cover alters both the local surface turbulent energy fluxes and the glacier component of valley circulation. These variations under different flow conditions should be accounted for when distributing temperature fields for modeling applications over partially debris-covered glaciers.
... We calculated the sensible and latent heat fluxes following e.g. [82]: ...
Supraglacial debris strongly modulates glacier melt rates and can be decisive for ice dynamics and mountain hydrology. It is ubiquitous in High-Mountain Asia (HMA), yet because its thickness and supply rate from local topography are poorly known, our ability to forecast regional glacier change and streamflow is limited. Here we resolved the spatial distribution of supraglacial debris thickness (SDT) for 4401 glaciers in HMA for 2000-2016, via an inverse approach using a new dataset of glacier mass balance. We then determined debris-supply rate (DSR) to 3843 of those glaciers using a debris mass-balance model. Our results reveal high spatial variability in both SDT and DSR, with supraglacial debris most concentrated around Everest, and DSR highest in the Pamir-Alai. We demonstrate that DSR and, by extension, SDT increase with the temperature and slope of debris-supply slopes regionally and that SDT increases as ice flow decreases locally. Our centennial-scale estimates of DSR are an order of magnitude lower than millennial-scale estimates of headwall-erosion rate from 10Be cosmogenic nuclides, indicating that debris supply to the region's glaciers is highly episodic. We anticipate that our datasets will enable improved representation of the complex response of HMA's glaciers to climatic warming in future modelling efforts.
... Measurements of near-surface ice temperatures are important for modelling the surface energy balance and projecting the future mass-balance response of glaciers to anticipated climate change. This is especially the case for glaciers with a thick supraglacial debris layer that insulates the underlying ice (according to debris layer thickness and lithology), reducing ablation and potentially extending glacier longevity (Nicholson and Benn, 2006;Nicholson and Benn, 2013;Anderson and Anderson, 2016). ...
Meltwater from high-elevation debris-covered glaciers—particularly those located in the greater Himalaya and Andes—shapes the water supply of major rivers and nourishes substantial terrestrial, estuarine, and marine habitats (Kraaijenbrink et al., 2017; Immerzeel et al., 2020). However, the relative inaccessibility and high elevation of such glaciers results in a paucity of data relating to their fundamental physical properties and processes, limiting the information available to constrain and evaluate numerical models of their behaviour and project future change. Knowledge of the subsurface properties of such glaciers is particularly deficient because it is largely obscured to satellite and airborne remote sensing; englacial investigations therefore commonly require direct access (Miles et al., 2020). Of the physical properties of glaciers, ice temperature exerts an important control over glaciological processes, such as glacier motion, and their modelled behaviour. For example, ice viscosity is sensitive to temperature such that, under the same stress, ice approaching the melting point deforms 5–10 times more rapidly than it would at −10°C (Deeley and Woodward, 1908; Cuffey and Paterson, 2010). Basal motion depends on lubrication facilitated by the presence of meltwater at the ice-bed interface and/or within the pore space of a subglacial sediment layer. Measurements of near-surface ice temperatures are important for modelling the surface energy balance and projecting the future mass-balance response of glaciers to anticipated climate change. This is especially the case for glaciers with a thick supraglacial debris layer that insulates the underlying ice (according to debris layer thickness and lithology), reducing ablation and potentially extending glacier longevity (Nicholson and Benn, 2006; Nicholson and Benn, 2013; Anderson and Anderson, 2016). Here, we present a one-year time series of near-surface ice temperatures, measured between 1.5 and 7.0 m below the ice surface, in a borehole drilled by hot water into the debris-covered tongue of Khumbu Glacier, Nepal.
... However, the subtle weathering rind (Fig. 6) and rounded margins ( Fig. 1) do suggest that, at some time, the boulder was exposed to cold-climate, physical weathering and that stream-related processes may have transported the boulder onto the glacier. Because supraglacial debris tends to become more abundant and smaller in particle size down-glacier (Nakawo, 1979;Nicholson and Benn, 2012), the apparent lack of any other debris in the Cleveland Shale and the apparently isolated nature of the large boulder imply that the boulder may have originated in the shallow reaches of the glacier away from terminal areas. ...
... The Astore and Hunza sub-basins range from 1504 to 8069 and 1420 to 7809 m.a.s.l. with around 14% and 28% of the total area, respectively covered by glaciers and permanent ice cover [13,30]. Importantly, it may also be necessary to consider that glacier cover melt-rates vary depending on whether a glacier consideration is debris-covered or clean-ice [31][32][33]. Similarly, the most recent studies based on snow cover assessment and hydrological modelling were carried out in the Gilgit River Basin (GRB) and neighboring river basins [27][28][29]34]. ...
In contrast to widespread glacier retreat evidenced globally, glaciers in the Karakoram region have exhibited positive mass balances and general glacier stability over the past decade. Snow and glacier meltwater from the Karakoram and the western Himalayas, which supplies the Indus River Basin, provide an essential source of water to more than 215 million people, either directly, as potable water, or indirectly, through hydroelectric generation and irrigation for crops. This study focuses on water resources in the Upper Indus Basin (UIB) which combines the ranges of the Hindukush, Karakoram and Himalaya (HKH). Specifically, we focus on the Gilgit River Basin (GRB) to inform more sustainable water use policy at the sub-basin scale. We employ two degree-day approaches, the Spatial Processes in Hydrology (SPHY) and Snowmelt Runoff Model (SRM), to simulate runoff in the GRB during 2001-2012. The performance of SRM was poor during July and August, the period when glacier melt contribution typically dominates runoff. Consequently, SPHY outperformed SRM, likely attributable to SPHY's ability to discriminate between glacier, snow, and rainfall contributions to runoff during the ablation period. The average simulated runoff revealed the prevalent snowmelt contribution as 62%, followed by the glacier melt 28% and rainfall 10% in GRB. We also assessed the potential impact of climate change on future water resources, based on two Representative Concentration Pathways (RCP) (RCP 4.5 and RCP 8.5). We estimate that summer flows are projected to increase by between 5.6% and 19.8% due to increased temperatures of between 0.7 and 2.6 °C over the period 2039-2070. If realized, increased summer flows in the region could prove beneficial for a range of sectors, but only over the short to medium term and if not associated with extreme events. Long-term projections indicate declining water resources in the region in terms of snow and glacier melt.
... In turn, a debris cover may develop and expand. Ice ablation is retarded once the debris reaches a critical thickness (Østrem, 1959;Mattson et al., 1993;Kayastha et al., 2000;Kirkbride & Dugmore, 2003;Nicholson & Benn, 2013), triggering an increase in relief relative to adjacent debris-free surfaces. Thereafter, gravity-driven secondary dispersal mechanisms redistribute debris across the glacier surface to form a continuous debris mantle. ...
There exists a need to advance our understanding of debris‐covered glacier surfaces over relatively short timescales due to rapid, climatically induced areal expansion of debris cover at the global scale, and the impact debris has on mass balance. We applied UAV‐SfM and DEM differencing with debris thickness and debris stability modelling to unravel the evolution of a 0.15 km2 region of the debris‐covered Miage Glacier, Italy, between June 2015 and July 2018. DEM differencing revealed widespread surface lowering (mean 4.1 ± 1.0 m a‐1; max 13.3 m a‐1). We combined elevation change data with local meteorological data and a sub‐debris melt model, and used these relationships to produce high resolution, spatially distributed maps of debris thickness. These maps were differenced to explore patterns and mechanisms of debris redistribution. Median debris thicknesses ranged from 0.12 – 0.17 m and were spatially variable. We observed localised debris thinning across ice cliff faces, except those which were decaying, where debris thickened. We observed pervasive debris thinning across larger, backwasting slopes, including those bordered by supraglacial streams, as well as ingestion of debris by a newly exposed englacial conduit. Debris stability mapping showed that 18.2‐26.4% of the survey area was theoretically subject to debris remobilisation. By linking changes in stability to changes in debris thickness, we observed that slopes that remain stable, stabilise, or remain unstable between periods almost exclusively show net debris thickening (mean 0.07 m a‐1) whilst those which become newly unstable exhibit both debris thinning and thickening. We observe a systematic downslope increase in the rate at which debris cover thickens which can be described as a function of the topographic position index and slope gradient. Our data provide quantifiable insights into mechanisms of debris remobilisation on glacier surfaces over sub‐decadal timescales, and open avenues for future research to explore glacier‐scale spatiotemporal patterns of debris remobilisation.
... However, the subtle weathering rind (Fig. 6) and rounded margins ( Fig. 1) do suggest that, at some time, the boulder was exposed to cold-climate, physical weathering and that stream-related processes may have transported the boulder onto the glacier. Because supraglacial debris tends to become more abundant and smaller in particle size down-glacier (Nakawo, 1979;Nicholson and Benn, 2012), the apparent lack of any other debris in the Cleveland Shale and the apparently isolated nature of the large boulder imply that the boulder may have originated in the shallow reaches of the glacier away from terminal areas. ...
... The median conductivity of 1.29 J m −1 K −1 matches well with the few available direct measurements from our field site. Reported conductivity values are rare for specific seasons (Nicholson and Benn 2006;Nicholson and Benn 2013), but do exist for unspecified conditions (Conway and Rasmussen 2000;Reid and Brock 2010;Juen et al., 2013;Rounce et al., 2015). These reported values correspond well with the range in conductivity values that we have derived ( Figure 5B). ...
Debris-covered glaciers, especially in high-mountain Asia, have received increased attention in recent years. So far, few field-based observations of distributed mass loss exist and both the properties of the debris layer as well as the atmospheric drivers of melt below debris remain poorly understood. Using multi-year observations of on-glacier atmospheric data, debris properties and spatial surface elevation changes from repeat flights with an unmanned aerial vehicle (UAV), we quantify the necessary variables to compute melt for the Lirung Glacier in the Himalaya. By applying an energy balance model we reproduce observed mass loss during one monsoon season in 2013. We show that melt is especially sensitive to thermal conductivity and thickness of debris. Our observations show that previously used values in literature for the thermal conductivity through debris are valid but variability in space on a single glacier remains high. We also present a simple melt model, which is calibrated based on the results of energy balance model, that is only dependent on air temperature and debris thickness and is therefore applicable for larger scale studies. This simple melt model reproduces melt under thin debris (<0.5 m) well at an hourly resolution, but fails to represent melt under thicker debris accurately at this high temporal resolution. On the glacier scale and using only off-glacier forcing data we however are able to reproduce the total melt volume of a debris-covered tongue. This is a promising result for catchment scale studies, where quantifying melt from debris covered glaciers remains a challenge.
... a Thickness of the supraglacial debris layer that would result from melting the borehole length or full ice column (as indicated), assuming no debris redistribution at the surface and a bulk effective porosity of 0.33 (ref. 31 ). Calculations are provided the 'Methods' section. ...
Surface melting of High Mountain Asian debris-covered glaciers shapes the seasonal water supply to millions of people. This melt is strongly influenced by the spatially variable thickness of the supraglacial debris layer, which is itself partially controlled by englacial debris concentration and melt-out. Here, we present measurements of deep englacial debris concentrations from debris-covered Khumbu Glacier, Nepal, based on four borehole optical televiewer logs, each up to 150 m long. The mean borehole englacial debris content is ≤ 0.7% by volume in the glacier’s mid-to-upper ablation area, and increases to 6.4% by volume near the terminus. These concentrations are higher than those reported for other valley glaciers, although those measurements relate to discrete samples while our approach yields a continuous depth profile. The vertical distribution of englacial debris increases with depth, but is also highly variable, which will complicate predictions of future rates of surface melt and debris exhumation at such glaciers.
... Debris-covered ice represents 30% of the glacier mass in ablation areas in High Mountain Asia (Kraaijenbrink et al., 2017). Supraglacial debris in the Everest region is typically sufficiently thick to reduce ablation by insulating the underlying ice surface (Nicholson & Benn, 2013). As a result, these debris-covered glaciers have experienced lower sensitivity to atmospheric warming than would be expected for climatically equivalent clean-ice surfaces (Benn et al., 2012). ...
Sustained mass loss from Himalayan glaciers is causing supraglacial debris to expand and thicken, with the expectation that thicker debris will suppress ablation and extend glacier longevity. However, debris-covered glaciers are losing mass at similar rates to clean-ice glaciers in High Mountain Asia. This rapid mass loss is attributed to the combined effects of; (a) low or reversed mass balance gradients across debris-covered glacier tongues, (b) differential ablation processes that locally enhance ablation within the debris-covered section of the glacier, for example, at ice cliffs and supraglacial ponds, and (c) a decrease in ice flux from the accumulation area in response to climatic warming. Adding meter-scale spatial variations in supraglacial debris thickness to an ice-flow model of Khumbu Glacier, Nepal, increased mass loss by 47% relative to simulations assuming a continuous debris layer over a 31-year period (1984–2015 CE) but overestimated the reduction in ice flux. Therefore, we investigated if simulating the effects of dynamic detachment of the upper active glacier from the debris-covered tongue would give a better representation of glacier behavior, as suggested by observations of change in glacier dynamics and structure indicating that this process occurred during the last 100 years. Observed glacier change was reproduced more reliably in simulations of the active, rather than entire, glacier extent, indicating that Khumbu Glacier has passed a dynamic tipping point by dynamically detaching from the heavily debris-covered tongue that contains 20% of the former ice volume.
... A seasonal cycle of albedo has been demonstrated in previous observational studies and modelling efforts of broadband albedo, highlighting the importance of continuous measurements (e.g. Hoinkes and Wendler, 1968;Nicholson and Benn, 2013;Möller and Möller, 2017). ...
As Alpine glaciers become snow-free in summer, more dark, bare ice is exposed, decreasing local albedo and increasing surface melting. To include this feedback mechanism in models of future deglaciation, it is important to understand the processes governing broadband and spectral albedo at a local scale. However, few in situ reflectance data have been measured in the ablation zones of mountain glaciers. As a contribution to this knowledge gap, we present spectral reflectance data (hemispherical–conical–reflectance factor) from 325 to 1075 nm collected along several profile lines in the ablation zone of Jamtalferner, Austria. Measurements were timed to closely coincide with a Sentinel-2 and Landsat 8 overpass and are compared to the respective ground reflectance (bottom-of-atmosphere) products. The brightest spectra have a maximum reflectance of up to 0.7 and consist of clean, dry ice. In contrast, reflectance does not exceed 0.2 for dark spectra where liquid water and/or fine-grained debris are present. Spectra can roughly be grouped into dry ice, wet ice, and dirt or rocks, although gradations between these groups occur. Neither satellite captures the full range of in situ reflectance values. The difference between ground and satellite data is not uniform across satellite bands, between Landsat and Sentinel, and to some extent between ice surface types (underestimation of reflectance for bright surfaces, overestimation for dark surfaces). We highlight the need for further, systematic measurements of in situ spectral reflectance properties, their variability in time and space, and in-depth analysis of time-synchronous satellite data.
... For the case presented in the main text we assumed that C is uniform and does not vary in time, that porosity is 0.3 (Nicholson and Benn, 2013), and that the rock density is 2,200 kg m −3 (see MacKevett and Smith, 1972;Miles et al., 2021). Because we have no a priori knowledge of the pattern or magnitude of C, porosity, and rock density we ran 13 additional inversions in which 1) C increases downglacier linearly through the swath profile by factors of 2, 10, 100, and 1,000; 2) C decreases linearly downglacier through the swath profile by factors of 0.5, 0.1, 0.01, and 0.001; 3) porosity varies between 0.1 and 0.4; 4) rock density varies between 2000 and 2,700 kg m −3 ; and 5) use the remotely sensed debris pattern of Rounce et al. (2021) instead of the in situ debris thicknesses as the 2011 evaluating dataset (Supplementary Table S5). ...
The cause of debris-covered glacier thinning remains controversial. One hypothesis asserts that melt hotspots (ice cliffs, ponds, or thin debris) increase thinning, while the other posits that declining ice flow leads to dynamic thinning under thick debris. Alaska’s Kennicott Glacier is ideal for testing these hypotheses, as ice cliffs within the debris-covered tongue are abundant and surface velocities decline rapidly downglacier. To explore the cause of patterns in melt hotspots, ice flow, and thinning, we consider their evolution over several decades. We compile a wide range of ice dynamical and mass balance datasets which we cross-correlate and analyze in a step-by-step fashion. We show that an undulating bed that deepens upglacier controls ice flow in the lower 8.5 km of Kennicott Glacier. The imposed velocity pattern strongly affects debris thickness, which in turn leads to annual melt rates that decline towards the terminus. Ice cliff abundance correlates highly with the rate of surface compression, while pond occurrence is strongly negatively correlated with driving stress. A new positive feedback is identified between ice cliffs, streams and surface topography that leads to chaotic topography. As the glacier thinned between 1991 and 2015, surface melt in the study area decreased, despite generally rising air temperatures. Four additional feedbacks relating glacier thinning to melt changes are evident: the debris feedback (negative), the ice cliff feedback (negative), the pond feedback (positive), and the relief feedback (positive). The debris and ice cliff feedbacks, which are tied to the change in surface velocity in time, likely reduced melt rates in time. We show this using a new method to invert for debris thickness change and englacial debris content (∼0.017% by volume) while also revealing that declining speeds and compressive flow led to debris thickening. The expansion of debris on the glacier surface follows changes in flow direction. Ultimately, glacier thinning upvalley from the continuously debris-covered portion of Kennicott Glacier, caused by mass balance changes, led to the reduction of flow into the study area. This caused ice emergence rates to decline rapidly leading to the occurrence of maximum, glacier-wide thinning under thick, insulating debris.
... In 2010-2012, the values of k d increased significantly from June to a maximum in September, which is reflected in Figure 5b and the KM d 2 coefficients for month in Table 4. Short-term observations have found that heat storage in supraglacial debris is close to zero on the order of days (Brock and others, 2007) but may be higher at seasonal scale (Nicholson and Benn, 2013). The increase of values of k d between June and September at Dokriani Glacier suggests that heat accumulated in the debris layer after melting had begun, so that melt rates were increasingly sensitive to surface energy inputs. ...
Supraglacial debris is significant in many regions and complicates modeling of glacier melt, which is required for predicting glacier change and its influences on hydrology and sea-level rise. Temperature-index models are a popular alternative to energy-balance models when forcing data are limited, but their transferability among glaciers and inherent uncertainty have not been documented in application to debris-covered glaciers. Here, melt factors were compiled directly from published studies or computed from reported melt and MERRA-2 air temperature for 27 debris-covered glaciers around the world. Linear mixed-effects models were fit to predict melt factors from debris thickness and variables including debris lithology and MERRA-2 radiative exchange. The models were tested by leave-one-site-out cross-validation based on predicted melt rates. The best model included debris thickness (fixed effect) and glacier and year (random effects). Predictions were more accurate using MERRA-2 than on-site air temperature data, and pooling MERRA-2-derived and reported melt factors improved cross-validation accuracy more than including additional predictors such as shortwave or longwave radiation. At one glacier where monthly ablation was measured over 4 years, seasonal variation of melt factors suggested that heat storage significantly affected the relation between melt and energy exchange at the debris surface.
... The Himalayan glaciers are characterized by the presence of debris in their lower ablation zones and the thickness of debris varies from a few centimetres to meters (Benn et al., 2012;Chand et al., 2015;Chand and Kayastha, 2018;Fujii and Higuchi, 1977;Mattson and E., 1993;Östrem, 1959). The source of debris for these glaciers is due to the presence of large valleys and steep headwalls (Kraaijenbrink et al., 2017), which alter the surface energy balance and work as a barrier between the atmosphere and ice (Nicholson and Benn, 2013). The melt rate of the clean ice and debris-covered ice is different due to the difference in interaction between atmosphere and ice. ...
... Also, Collier et al. (2015) linearly distributed the debris thickness from terminus to the upper debris-covered zone to model and compare melt rates between debris cover and debris free ice. The findings are useful for the impact assessment of debris cover on glacier-melt but for accurate model parametrization, estimation of field-based melt rate and spatial distribution of debris is necessary (Nicholson and Benn, 2013). ...
... Also, Collier et al. (2015) linearly distributed the debris thickness from terminus to the upper debris-covered zone to model and compare melt rates between debris cover and debris free ice. The findings are useful for the impact assessment of debris cover on glacier-melt but for accurate model parametrization, estimation of field-based melt rate and spatial distribution of debris is necessary (Nicholson and Benn, 2013). ...
The assessment of meltwater sourcing from the clean and debris-covered glaciers is scarce in High Mountain Asia (HMA). The melting rate varies with the debris cover thickness and glacier orientation. The present
study quantifies glacier melting rate attributed to varying thickness of debris cover in the Karakoram. We
observed daily melting rates by installing ablation stakes over debris-free and debris-covered ice during a
field expedition. The stakes were installed on glacier surface with debris cover thickness ranges between
0.5 and 40 cm at selected experimental sites during the ablation period (September and October 2018)
and (July to August 2019). We selected three glaciers including Ghulkin, Hinarchi, and Hoper facing east,
south, and north, respectively to assess the role of glacier orientation on melting rates. We observed that
the debris-free ice melts faster than the debris-covered ice. Intriguingly, a thin debris layer of 0.5 cm does
not enhance melting compared to the clean ice which is inconsistent with the earlier studies. The melting
rate decreases as the thickness of debris cover increases at all the three selected glaciers. Furthermore,
south-facing glacier featured the highest melting (on average ~ 25% more). However, the north and eastfacing glaciers revealed almost same melting rates. We observed that the average degree-day factors
(DDF) slightly varies within a range of 0.58–0.73 and 0.55–0.68 cm °C−1 day−1 for debris-free and 0.5 cm
debris-covered ice, respectively, however, DDF largely reduces to 0.13–0.25 cm °C−1 day−1 for 40 cm
Abstract. The interaction between a glacier and its glacial lake plays an increasingly important role in glacier shrinkage and proglacial lake expansion, and it increases the risk of glacial lake outburst floods (GLOFs). Longbasaba Glacier is directly contacted by a moraine-dammed lake with a high outburst risk in the central Himalayas, and has drawn a great deal of attention from scientists and local governments. Based on Landsat images and in-situ measurements, the evolution records of the shrinkage of Longbasaba Glacier and the corresponding expansion of its proglacial lake were determined for 1988–2018, and the mass contributions of glacier shrinkage to the increase in lake water volume were assessed. During the past three decades, Longbasaba Glacier has experienced a continuous and accelerating recession in glacier area and length but accompanied by the decelerating surface lowing and ice flow. Consequently, Longbasaba Lake has expanded significantly at an accelerating rate. The glacier surface lowering played a predominant role in the mass contribution of glacier shrinkage to the increase in lake water volume, while ice avalanches were the main potential trigger for failure of moraine dams and subsequent GLOF events. Due to the areal expansion, decreasing mass contributions from parent glacier shrinkage, and some mitigation measures by local governments to improve the drainage systems, the potential risk of outburst for Longbasaba Lake has continuously decreased during the last decade.
Thaw slumps in ice‐rich permafrost can retreat tens of metres per summer, driven by the melt of subaerially exposed ground ice. However, some slumps retain an ice‐veneering debris cover as they retreat. A quantitative understanding of the thermal regime and geomorphic evolution of debris‐covered slumps in a warming climate is largely lacking. To characterize the thermal regime, we instrumented four debris‐covered slumps in the Canadian Low Arctic and developed a numerical conduction‐based model. The observed surface temperatures 20°C and steep thermal gradients indicate that debris insulates the ice by shifting the energy balance towards radiative and turbulent losses. After the model was calibrated and validated with field observations, it predicted sub‐debris ice melt to decrease four‐fold from 1.9 to 0.5 m as the thickness of the fine‐grained debris quadruples from 0.1 to 0.4 m. With warming temperatures, melt is predicted to increase most rapidly, in relative terms, for thick (~0.5‐1.0 m) debris covers. The morphology and evolution of the debris‐covered slumps were characterized using field and remote sensing observations, which revealed differences in association with morphology and debris composition. Two low‐angle slumps retreated continually despite their persistent fine‐grained debris covers. The observed elevation losses decreased from ~1.0 m/yr where debris thickness ~.2 m to 0.1 m/yr where thickness ~1.0 m. Conversely, a steep slump with a coarse‐grained debris veneer underwent short‐lived bursts of retreat, hinting at a complex interplay of positive and negative feedback processes. The insulative protection and behaviour of debris vary significantly with factors such as thickness, grain size and climate: debris thus exerts a fundamental, spatially variable influence on slump trajectories in a warming climate.
Abstract. Debris-covered glaciers account for 18 % of the total glacier ice volume in High Mountain Asia, however the drivers controlling the melt of these glaciers are still largely unknown and their total contribution to the total glacier melt remains uncertain. Debris influences the surface energy balance and therefore glacier melt by influencing the thermal properties (e.g. albedo, thermal conductivity, roughness) of the glacier surface. In this study, the impact of surface properties of debris on the spatial distribution of micro meteorological variables, such as the turbulent fluxes, wind fields, moisture and temperature and eventually the conductive heat flux for a debris-covered glacier is investigated.
We simulated a debris-covered glacier (Lirung Glacier, Nepal) at a high-resolution of 1 m with the MicroHH model with boundary conditions retrieved from an automatic weather station (temperature, wind and specific humidity) and UAV flights (digital elevation map and surface temperature), and the model is validated with eddy covariance data. Subsequently, a sensitivity analysis was performed to ascertain how heterogeneous surface variables control the glacier micro-climate. Additionally, we show ice cliffs are local melt hot spots and that turbulent fluxes and local heat advection amplify spatial heterogeneity on the surface. The high spatial variability of small-scale meteorological variables suggests that point based station observations cannot be simply extrapolated to an entire glacier and should be considered in future studies for a better estimation of glacier melt in High Mountain Asia.
The hydrological characteristics of debris-covered glaciers are known to be fundamentally different from those of clean-ice glaciers, even within the same climatological, geological and geomorphological setting. Understanding how these characteristics influence the timing and magnitude of meltwater discharge is particularly important for regions like High Mountain Asia, where downstream communities rely on this resource for sanitation, irrigation and hydropower. The hydrology of debris-covered glaciers is relatively complex: rugged surface topographies typically route meltwater through compound supraglacial-englacial systems involving both channels and ponds, as well as pathways that remain unknown. Low-gradient tongues that extend several kilometres retard water conveyance and promote englacial storage. Englacial channels are frequently abandoned and reactivated as water supply changes, new lines of permeability are exploited, and drainage is captured due to high rates of surface and subsurface change. Seasonal influences, such as the monsoon, are superimposed on these distinctive characteristics, reorganising surface and subsurface drainage rapidly from one season to the next. Recent advances in understanding have mostly come from studies aimed at quantifying and describing supraglacial processes; little is known about the subsurface hydrology, particularly the nature (or even existence) of subglacial drainage. In this review, we consider in turn the supraglacial, englacial, subglacial, and proglacial hydrological domains of debris-covered glaciers in High Mountain Asia. We summarise different lines of evidence to establish the current state of knowledge and, in doing so, identify major knowledge gaps. Finally, we use this information to suggest priorities for future hydrological research at High Mountain Asian debris-covered glaciers, and how they may influence our ability to be able to make long-term predictions of changes in the water they supply.
Known for their important role in locally enhancing surface melt, supraglacial ponds and ice cliffs are common features on debris-covered glaciers. We use high resolution satellite imagery to describe pond-cliff systems and surface velocity on Verde debris-covered glacier, Monte Tronador, and Southern Chile. Ponds and ice cliffs represent up to 0.4 and 2.7% of the glacier debris-covered area, respectively. Through the analyzed period and the available data, we found a seasonality in the number of detected ponds, with larger number of ponds at the beginning of the ablation season and less at the end of it. Using feature tracking, we determined glacier surface velocity, finding values up to 55 m/yr on the upper part of the debris-covered area, and decreasing almost to stagnation in the terminus. We found that larger ponds develop in glacier zones of low velocity, while zones of high velocity only contain smaller features. Meanwhile, ice cliffs appeared to be less controlled by surface velocity and gradient. Persistent ice cliffs were detected between 2009 and 2019 and backwasting up to 24 m/yr was measured, highlighting significant local glacier wastage.
The response of debris-covered glaciers to climate change is more heterogeneous and complex than that of debris-free glaciers. The debris has a significant effect on glacier dynamic characteristics, which causes uneven mass balance changes, and then affects the change of glacier geometry evolves in response to climate. In our study, both energy balance and a two-dimensional mass and momentum conservation equation were used to simulate the ice temperature and velocity respectively at the main stream line of Koxkar glacier. The influence of the debris on ice temperature was described in the model. The ice velocity simulation was coupled with a description of ice viscosity under the influence of ice temperature. Our simulation results showed that the debris could increase the ice velocity and ablation in the middle of the glacier. Especially in the surface from 3200 to 3400 m.a.s.l, where the debris had great influence for ice velocity and temperature. From simulation results, we inferred that the ice thickness in this region would change obviously. Comparing with the measured thickness changes by ground-penetrating radar (GPR) between 1981 and 2008, the simulation result is consistent with measured results. Therefore, supraglacial lakes are easily developed in this region because of the high ice temperature and uneven ice surface. And the Landsat 8 remote-sensing image had verified this conclusion.
During the last few decades, the lake-terminating glaciers in the Himalaya have receded faster than the land-terminating glaciers as proglacial lakes have exacerbated the mass loss of their host glaciers. Monitoring the impacts of glacier recession and dynamics on lake extent and water volume provides an approach to assess the mass interplay between glaciers and proglacial lakes. We describe the recession of Longbasaba Glacier and estimate the mass wastage and its contribution to the water volume of its proglacial lake. The results show that the glacier area has decreased by 3% during 1988–2018, with a more variable recession prior to 2008 than in the last decade. Longbasaba Lake has expanded by 164% in area and 237% in water volume, primarily as a result of meltwater inflow produced from surface lowering of the glacier. Over the periods 1988–2000 and 2000–18, the mass loss contributed by glacier thinning has decreased from 81 to 61% of the total mass loss, accompanied by a nearly doubled contribution from terminus retreat. With the current rate of retreat, Longbasaba glacier is expected to terminate in its proglacial lake for another four decades. The hazard risk of this lake is expected to continue to increase in the near future because of the projected continued glacier mass loss and related lake expansion.
Supraglacial debris affects glacier mass balance as a thin layer enhances surface melting, while a thick layer reduces it. While many glaciers are debris-covered, global glacier models do not account for debris because its thickness is unknown. We provide the first globally distributed debris thickness estimates using a novel approach combining sub-debris melt and surface temperature inversion methods. Results are evaluated against observations from 22 glaciers. We find the median global debris thickness is ∼0.15 ± 0.06 m. In all regions, the net effect of accounting for debris is a reduction in sub-debris melt, on average, by 37%, which can impact regional mass balance by up to 0.40 m water equivalent (w.e.) yr-1. We also find recent observations of similar thinning rates over debris-covered and clean ice glacier tongues is primarily due to differences in ice dynamics. Our results demonstrate the importance of accounting for debris in glacier modeling efforts.
Supraglacial debris covers 7% of mountain glacier area globally and generally reduces glacier surface melt. Enhanced energy absorption at ice cliffs and supraglacial ponds scattered across the debris surface leads these features to contribute disproportionately to glacier-wide ablation. However, the degree to which cliffs and ponds actually increase melt rates remains unclear, as these features have only been studied in a detailed manner for selected locations, almost exclusively in High Mountain Asia. In this study we model the surface energy balance for debris-covered ice, ice cliffs, and supraglacial ponds at a set of automatic weather station records representing the global prevalence of debris-covered glacier ice. We generate 5000 random sets of values for physical parameters using probability distributions derived from literature, which we use to investigate relative melt rates and to isolate the melt responses of debris, cliffs and ponds to the site-specific meteorological forcing. Modelled sub-debris melt rates are primarily controlled by debris thickness and thermal conductivity. At a reference thickness of 0.1 m, sub-debris melt rates vary considerably, differing by up to a factor of four between sites, mainly attributable to air temperature differences. We find that melt rates for ice cliffs are consistently 2-3x the melt rate for clean glacier ice, but this melt enhancement decays with increasing clean ice melt rates. Energy absorption at supraglacial ponds is dominated by latent heat exchange and is therefore highly sensitive to wind speed and relative humidity, but is generally less than for clean ice. Our results provide reference melt enhancement factors for melt modelling of debris-covered glacier sites, globally, while highlighting the need for direct measurement of debris-covered glacier surface characteristics, physical parameters, and local meteorological conditions at a variety of sites around the world.
Debris-covered glaciers account for almost one-fifth of the total glacier ice volume in High Mountain Asia; however, their contribution to the total glacier melt remains uncertain, and the drivers controlling this melt are still largely unknown. Debris influences the properties (e.g. albedo, thermal conductivity, roughness) of the glacier surface and thus the surface energy balance and glacier melt. In this study we have used sensitivity tests to assess the effect of surface properties of debris on the spatial distribution of micrometeorological variables such as wind fields, moisture and temperature. Subsequently we investigated how those surface properties drive the turbulent fluxes and eventually the conductive heat flux of a debris-covered glacier. We simulated a debris-covered glacier (Lirung Glacier, Nepal) at a 1 m resolution with the MicroHH model, with boundary conditions retrieved from an automatic weather station (temperature, wind and specific humidity) and unmanned aerial vehicle flights (digital elevation map and surface temperature). The model was validated using eddy covariance data. A sensitivity analysis was then performed to provide insight into how heterogeneous surface variables control the glacier microclimate. Additionally, we show that ice cliffs are local melt hot spots and that turbulent fluxes and local heat advection amplify spatial heterogeneity on the surface. The high spatial variability of small-scale meteorological variables suggests that point-based station observations cannot be simply extrapolated to an entire glacier. These outcomes should be considered in future studies for a better estimation of glacier melt in High Mountain Asia.
Temperatures from a bore hole through an active rock glacier in the eastern Swiss Alps are presented and thermal conditions within the slowly creeping permafrost are analyzed. Present mean annual temperature in the uppermost part of the permafrost is −3°C. Permafrost is 52 m thick and reaches heavily fissured bedrock. Thermal conductivity as determined in situ from seasonal temperature variations and measured in a cold laboratory using frozen samples is close to 2.5–3.0 W m−1 °C−1. Vertical heat flow is anomalously high (around 150 mW m-2), probably due to heat advection from circulating ground water or air within the fissured bedrock zone. Beneath this zone, which could in fact represent a non-frozen intra-permafrost layer or “talik”, relic permafrost from past centuries may possibly exist as indicated by a corresponding heat-flow inversion. Given the current temperature condition at the surface of the rock glacier and the fact that the twentieth century is among the warmest in post-glacial time, permafrost conditions may be assumed to have existed during the whole of the Holocene and, hence, during the entire time of rock-glacier formation.
Quantitative investigations have been made of ice-cored dirt cones on Bersaerkerbræ in north-east Greenland. Experiments were also undertaken to evaluate field observations. Measurements included: maximum cone dimensions, sediment thickness and particle size, cone growth rates, slope angles and the temperature distribution within the debris layer and ice core. Particle size, which has not been stressed in previous studies, and related liquid consistency limits, appear as the dominant controls in cone formation, independent of debris thickness within the observed range of 10 mm to 125 mm. A threshold grain-size for dirt-cone inception was found, between 0.2 mm and 0.6 mm. The growth of cones was usually not more than 50% of the ablation over “clean” ice. Temperature measurements within dirt cones has enabled heat-flow studies to be made, evaluating the thermal conductivity of a sediment layer and the heat transfer involved in melting the ice core. A simple model of dirt-cone dynamics is proposed, characterized by negative feedbacks and describing a steady-state system.
Temperatures from a bore hole through an active rock glacier in the eastern Swiss Alps are presented and thermal conditions within the slowly creeping permafrost are analyzed. Vertical heat flow is anomalously high (around 150 MW m-2), probably due to heat advection from circulating ground water on air within the fissured bedrock zone. Beneath this zone, which could in fact represent a non-frozen intra-permafrost layer or "talik', relic permafrost from past centuries may possibly exist as indicated by a corresponding heat-flow inversion. Given the current temperature conditions at the surface of the rock glacier and the fact that the twentieth century is among the warmest in post-glacial time, permafrost conditions may be assumed to have existed during the whole of the Holocene and, hence, during the entire time of rock-glacier formation. -from Authors
Temperature measurements made during summer within supraglacial debris on Khumbu Glacier, Nepal show a sttong diurnal signal that diffused downward into the debris with decreasing amplitude and increasing lag. Surface temperatures during the day were up to 35°C higher than the air temperature; energy transfer into the debris was dominated by the solar radiative flux. Temperature profiles through the debris indicate that heat flow deeper than about 0.2 m was primarily by conduction. The thermal conductivity k of the debris, estimated from a calculated thermal diffusivity and a representative volumetric heat capacity, was 0.85 ± 0.20 W rn 1 K" 1 at one site and 1.28 ± 0.15 W m" 1 K" 1 at another. At the first site the debris was 0.40 m thick and the average temperature gradient dT/dz = 19 K m' 1 ; the average flux of energy through the debris was sufficient to melt 4—6 mm of ice per day. The debris was thicker (estimated to be 2.5 m) and the temperature gradient lower (4.5 K m' 1) at the second site, and the calculated ice-melt was less than 2 mm day" 1 .
Temperatures from a bore hole through an active rock glacier in the eastern Swiss Alps are presented and thermal conditions within the slowly creeping permafrost are analyzed. Present mean annual temperature in the uppermost part of the permafrost is -3· C. Permafrost is 52 m thick and reaches heavily fissured bedrock. Thermal conductivity as determined ill silu from seasonal temperature variations and measured in a cold laboratory using frozen samples is close to 2.5-3.0 W m-I ·C-l . Vertical heat flow is anomalously high (around 150 mW m-2), probably due to heat advection from circulating ground water or air within the fissured bedrock zone. Beneath this zone, which could in fact represent a non-frozen intra-permafrost layer or "talik", relic permafrost from past centuries may possibly exist as indicated by a corresponding heat-flow inversion. Given the current temperature condition at the surface of the rock glacier and the fact that the twentieth century is among the warmest in post-glacial time, permafrost conditions may be assumed to have existed during the whole of the Holocene and, hence, during the entire time of rock-glacier formation.
There are many supraglacial ponds on debris-covered glaciers in the Nepal Himalayas. The heat absorbed at the surface of a pond was estimated from heat budget observations on the Lirung Glacier in Langtang Valley, Nepal. The results indicated an average heat absorption of 170 W m-2 during the summer monsoon season. This rate is about 7 times the average for the whole debris-covered zone. Analysis of the heat budget for a pond suggests that at least half of the heat absorbed at a pond surface is released with the water outflow from the pond, indicating that the water warmed in the pond enlarges the englacial conduit that drains water from the pond and produces internal ablation. Furthermore, the roof of the conduit could collapse, leading to the formation of ice cliffs and new ponds, which would accelerate the ablation of the debris-covered glacier.
Coarse blocks are a widespread ground cover in cold mountain areas. They have been recognized to exert a cooling influence on subsurface temperatures in comparison with other types of surface material and are employed in man- made structures for ground cooling and permafrost protection. The contrast in heat transfer between the atmosphere and the ground caused by thermally driven convection in winter and stable stratification of interstitial air during summer is usually invoked to explain this “thermal diode” effect. Based on measurements and model calculations, we propose an additional cooling mechanism, which is independent of convection, and solely functions based on the interplay of a winter snow cover and a layer of coarse blocks with low thermal conductivity. The thermal conductivity of a block layer with a porosity of 0.4 is reduced by about an order of magnitude compared to solid rock. We use a simple and purely conductive model experiment to demonstrate that low-conductivity layers reduce the temperature below the winter snow cover as well as mean annual ground temperatures by comparison with other ground materials. Coarse block layers reduce the warming effect of the snow cover and can result in cooling of blocky surfaces in comparison with surrounding areas in the order of one or several degrees. The characteristics of this mechanism correspond to existing measurements.
Surface glacier debris samples and field spectra were collected from the
ablation zones of Nepal Himalaya Ngozumpa and Khumbu glaciers in
November and December 2009. Geochemical and mineral compositions of
supraglacial debris were determined by X-ray diffraction and X-ray
fluorescence spectroscopy. This composition data was used as ground
truth in evaluating field spectra and satellite supraglacial debris
composition and mapping methods. Satellite remote sensing methods for
characterizing glacial surface debris include visible to thermal
infrared hyper- and multispectral reflectance and emission signature
identification, semi-quantitative mineral abundance indicies and
spectral image composites. Satellite derived supraglacial debris mineral
maps displayed the predominance of layered silicates, hydroxyl-bearing
and calcite minerals on Khumbu Himalayan glaciers. Supraglacial mineral
maps compared with satellite thermal data revealed correlations between
glacier surface composition and glacier surface temperature. Glacier
velocity displacement fields and shortwave, thermal infrared false color
composites indicated the magnitude of mass flux at glacier confluences.
The supraglacial debris mapping methods presented in this study can be
used on a broader scale to improve, supplement and potentially reduce
errors associated with glacier debris radiative property, composition,
areal extent and mass flux quantifications.
In areas of high relief, many glaciers have extensive covers of supraglacial debris in their ablation zones, which alters both rates and spatial patterns of melting, with important consequences for glacier response to climate change. Wastage of debris-covered glaciers can be associated with the formation of large moraine-dammed lakes, posing risk of glacier lake outburst floods (GLOFs). In this paper, we use observations of glaciers in the Mount Everest region to present an integrated view of debris-covered glacier response to climate change, which helps provide a long-term perspective on evolving GLOF risks.
During the 2005-2007 June-September ablation seasons, meteorological conditions were recorded on the lower and upper parts of the debris-covered ablation zone of Miage Glacier, Italy. In 2005, debris temperature and subdebris ice melt were also monitored at 25 points with debris thickness of 0.04-0.55 m, spread over 5 km2 of the glacier. The radiative fluxes were directly measured, and near-closure of the surface energy balance is achieved, providing support for the bulk aerodynamic calculation of the turbulent fluxes. Surface-layer meteorology and energy fluxes are dominated by the pattern of incoming solar radiation which heats the debris, driving strong convection. Mean measured subdebris ice melt rates are 6-33 mm d-1, and mean debris thermal conductivity is 0.96 W m-1 K-1, displaying a weak positive relationship with debris thickness. Mean seasonal values of the net shortwave, net longwave, and debris heat fluxes show little variation between years, despite contrasting meteorological conditions, while the turbulent latent (evaporative) heat flux was more than twice as large in the wet summer of 2007 compared with 2005. The increase in energy output from the debris surface in response to increasing surface temperature means that subdebris ice melt rates are fairly insensitive to atmospheric temperature variations in contrast to debris-free glaciers. Improved knowledge of spatial patterns of debris thickness distribution and 2 m air temperature, and the controls on evaporation of rainwater from the surface, are needed for distributed physically based melt modeling of debris-covered glaciers.
Monitoring of air and ground temperature at Plateau Mountain (South-Western Alberta) at short intervals (20 minutes) for two years shows vastly different thermal regimes in and beneath coarse blocky materials as opposed to mineral soils and rocks lacking substantial interconnecting voids. The dominant process of heat transfer in the upper layers is by rapid air movement through the voids to at least 50 cm depth as compared with slow conduction through the individual grains. Thermal response to a change in air temperature (positive or negative) is immediate and substantial, so it is not merely the result of the Balch effect. Rain and snow can also penetrate more deeply. These blocky materials are called kurums in Russia. Mean annual ground temperatures are 4-7 °C cooler in the blocky materials than in the adjacent mineral soils in cold climates, but this would be different in warmer climates. The ground temperature envelope is cone-shaped rather than bell-shaped, and this difference also occurs in mineral soils under a thin cover of blocks. There is also a smaller geothermal gradient within the zone affected directly by cooling/heating due to air movement. These processes appear to explain the occurrence of permafrost and substantial ice bodies in block fields such as rock glaciers below the limit of regional continuous permafrost in adjacent rocks and mineral soils. They also affect permafrost mapping and heat flow modelling, but offer a means of cooling near-surface soils.
Debris-covered glaciers respond differently to any given set of climatic conditions than clean glaciers. This difference stems from change in ablation rate caused by a debris cover, approaching zero ablation with sufficiently thick protective cover, a mechanism not yet considered in the context of ice sheet growth. Critical to applying the mechanism to ice sheets is supply of debris. We postulate that periods after major interglacials offer the best conditions for dirty advances. This is because the volume of debris, previously deposited and weathered in interglacial time, the latter a function of exposure length, should be at a maximum. Advances of dirty ice sheets generate landforms and in positions deviating from clean-ice advances under similar climatic conditions. Hence, inferences of both ice sheet properties inferred from such dirty ice advances and the climate conditions related to such advances must be cautious. Furthermore, modeling of past ice sheets must consider the effect of debris cover whenever indications exist for such a cover. r 2002 Elsevier Science Ltd and INQUA. All rights reserved.
Generalized numerical models of sub-debris ice ablation are preferable to empirical approaches for predicting runoff and glacier response to climate change, as empirical methods are site-specific and strongly dependent upon the conditions prevailing during the measurement period. We present a modified surface energy-balance model to calculate melt beneath a surface debris layer from daily mean meteorological variables. Despite numerous simplifications, the model performs well and modelled melt rates give a good match to observed melt rates, suggesting that this model can produce reliable estimates of ablation rate beneath debris layers several decimetres thick. This is a useful improvement on previous models which are inappropriate for thick debris cover.
This paper presents a simple model to estimate ice ablation under a thick supraglacial debris cover. The key method employed in the model is to establish a link between the debris heat flux and the debris temperature at a certain depth when the heat transfer in the debris is described by a diffusion process. Given surface temperature, debris thermal properties and relevant boundary conditions, the proposed model can estimate mean debris temperature at interfaces of different debris layers using an iterative procedure, and then the heat flux for ice ablation. The advantage of the proposed model is that it only requires a few parameters to conduct the modeling, which is simpler and more applicable than others. The case study on Koxkar glacier, west Tien Shan, China, shows, in general, that the proposed model gives good results for the prediction of debris temperatures, except for an apparent phase shift between modeled and observed values. We suggest that this error is mainly due to complex phase relations between debris temperature and debris heat flux. The modeled ablation rates at three experimental sites also show good results, using a direct comparison with observed data and an indirect comparison with a commonly used energy-balance model.
Permafrost is known to respond to changes in atmospheric temperatures but our knowledge of the scale and the processes involved are still not fully understood, especially not when the surface is other than bedrock. In order to gain a better understanding of the influence of the surface and ground characteristics on the thermal regime of the active layer, boreholes were drilled in the high alpine discontinuous permafrost in the Murtel-Chastelets area, Eastern Swiss Alps, and instrumented with chains of thermistor arrays. Five shallow boreholes were drilled during the summer of 2002, adding to the network of the existing four boreholes. Data from the first year of this multi-annual study demonstrate a pronounced thermal offset for loose material composed of gravel or boulders. Also, the thermal diffusivity was calculated over the summer period, adding to the understanding of the spatial discontinuity in the Alpine permafrost.
Here we report a laboratory study of the effects of debris thickness, diurnally cyclic radiation and rainfall on melt rates beneath rock-avalanche debris and sand (representing typical highly permeable supraglacial debris). Under continuous, steady-state radiation, sand cover >50mm thick delays the onset of ice-surface melting by >12 hours, but subsequent melting matches melt rates of a bare ice surface. Only when diurnal cycles of radiation are imposed does the debris reduce the longterm rate of ice melt beneath it. This is because debris >50 mm thick never reaches a steady-state heat flux, and heat acquired during the light part of the cycle is partially dissipated to the atmosphere during the nocturnal part of the cycle, thereby continuously reducing total heat flux to the ice surface underneath. The thicker the debris, the greater this effect. Rain advects heat from high-permeability supraglacial debris to the ice surface, thereby increasing ablation where thin, highly porous material covers the ice. In contrast, low-permeability rock-avalanche material slows water percolation, and heat transfer through the debris can cease when interstitial water freezes during the cold/night part of the cycle. This frozen interstitial water blocks heat advection to the ice-debris contact during the warm/day part of the cycle, thereby reducing overall ablation. The presence of metre-deep rock-avalanche debris over much of the ablation zone of a glacier can significantly affect the mass balance, and thus the motion, of a glacier. The length and thermal intensity of the diurnal cycle are important controls on ablation, and thus both geographical location and altitude significantly affect the impact of debris on glacial melting rates; the effect of debris cover is magnified at high altitude and in lower latitudes.
A simple model suggests that the ablation under a debris layer could be estimated from meteorological variables if the surface temperature data of the layer are available. This method was tested by analyzing the data obtained from experiments with artificial debris layers. Fairly good agreement was obtained between the estimated and the experimental data.
Glaciers and Glaciation is the classic textbook for all students of glaciation. Stimulating and accessible, it has established a reputation as a comprehensive and essential resource.
In this new edition, the text, references, and illustrations have been thoroughly updated to give today's reader an up-to-the minute overview of the nature, origin, and behavior of glaciers and the geological and geomorphological evidence for their past history on earth.
The first part of the book investigates the processes involved in forming glacier ice, the nature of glacier/climate relationships, the mechanisms of glacier flow, and the interactions of glaciers with other natural systems such as rivers, lakes, and oceans.
In the second part, the emphasis moves to landforms and sediment, the interpretation of the earth's glacial legacy, and the reconstruction of glacial depositional environments and palaeoglaciology.
Reports on ablation research carried out on the Rakhiot Glacier, Punjab, Himalaya. Specifically, detailed measurements of ablation rates on debris covered and debris free surfaces allow specification of relationships between ablation and debris cover thickness. Direct ablation measurements indicate a sharp increase in ablation with debris cover thickness increasing from 0.0 to 10 mm followed by a decrease in ablation with debris cover thickness increasing beyond 10 mm. Field observations reveal a critical thickness of 30 mm indicating that at any greater debris thickness ablation is suppressed from that expected on debris-free ice. A comparison with previous research indicates similar hyperbolic trends in the relationship between debris cover thickness and ablation, however, the intensity of these trends differ with global location. -Authors
The effect of a thin layer of fine debris on the melting of snow and ice was investigated over Dokriani Glacier at an altitude of about 4000 m in the Garhwal Himalayas (31°49-31 52 N, 78 47-78 51 E). Such investigations were made during summer 1995, 1997 and 1998. The average melt rate with respect to unit temperature or degree-day factor for clean and dusted snow was computed to be 5.8 and 6.4 mm °C-1 day-1, whereas for clean ice and debris-covered ice the value of this factor was 7.3 and 8.0 mm °C-1 day-1, respectively. Melt rate of clean ice was about 1.26 times greater than that for clean snow under similar weather conditions. The effect of debris layer on the melt rate of snow was more prominent than that for ice. The presence of debris on the ice increased the melt rate by about 8.5%, whereas for snow it increased by about 11.6%. Information on such factors is useful for runoff modelling of glacierized drainage basins.
In order to examine the role of supraglacial debris during ablation, a field experiment was carried out on a snowpatch beside the Rikha Samba Glacier, Hidden Valley. Black phyllite sand was scattered on nine test fields varying in thickness from about 0.5 cm to 8 cm. The thickness of the debris cover and the amount of surface ablation by melting of snow were measured carefully at 5 points in each test field. The results are as follows : (1) The ablation rate was accelerated under a thin debris layer and was retarded under a thick one as compared with that of a natural snow surface. The critical thickness of the debris layer was 1.6 cm. (2) The acceleration of ablation was greatest under a debris cover 0.5 cm thick.
A simple model suggests that the ablation under a debris layer could be estimated from meteorological variables if the surface temperature data of the layer are available. This method was tested by analyzing the data obtained from experiments with artificial debris layers.-from Authors