Debris, ranging from thin surface dust to medial moraines and thick, continuous layers in ablation zones, partially covers glaciers all around the world. By modifying energy transfer from the atmosphere to the ice, the supraglacial debris layer fundamentally controls sub-debris melt rates. Debris physical properties such as surface roughness (z0) and thermal conductivity (k) have only been derived from local measurements at a few sites, and modelling studies of debris-covered glaciers have often relied on literature values. The correct representation of these properties in energy-balance models is crucial for understanding the climate-glacier dynamics and how debris-covered glaciers will behave in the future. There are several established methods to derive these properties from field measurements, yet relatively few studies undertake to measure properties for their sites, or to evaluate the resulting property values.We undertook an observational campaign to investigate supraglacial debris properties at Pirámide Glacier, in the central Chilean Andes. First, we used established approaches, as well as some variations on those approaches, to derive z0 from wind-temperature tower data and k from thermistor strings in the debris at three glacier locations. Second, we determined locally-optimal k and z0 values to reproduce observed ice melt: we optimised k by simulating energy conduction through the debris with the surface temperature as an input, then optimised z0 by running a complete energy-balance model using the observed surface meteorology. We then conducted point-scale energy-balance modelling using the z0 and k values obtained i) with the derivations from field measurements; ii) through optimisation, or; iii) from the typical values found in literature. This allowed us to evaluate how the different methods perform by comparing the modelled and measured ice melt. Our results show that deriving local debris properties from measurements is challenging and that measured values can differ significantly from common literature values. The values derived from measured data can vary significantly depending on the method employed. It is important to note that these values can also differ significantly from the values required by an energy-balance model to accurately represent sub-debris ice melt. Furthermore, energy-balance models typically assume a representation of heat transfer within the supraglacial debris layer based solely on conduction and require a bulk thermal conductivity value. This highlights the necessity of efforts to reevaluate measurements in the field and reconsider our definition of debris properties in melt modelling.

During dry and hot years in the Swiss Alps, melt water from glaciers can moderate streamflow deficits caused by reduced precipitation and enhanced evapotranspiration rates. However, little is known about how glacier melt water contribution to streamflow varies sub-seasonally and in space, especially further downstream from glacierized catchments, where additional streamflow contributions are modulated primarily by rainfall and the biosphere (vegetation, soils). We study distributed catchment hydrology in Switzerland using a land surface model that constrains energy and mass fluxes using advanced physical representations of both cryospheric and biospheric processes at a 250 m spatial resolution. We simulate catchment runoff in Switzerland during the past 6 years, including two recent severe drought years (2018 and 2022), characterized by particularly warm summers and reduced precipitation. The model is forced with hourly observed meteorological data based on the weather station network SwissMetNet and the precipitation product RhiresD, and uses state-of-the-art land cover, soil characteristics, glacier area and debris thickness as initial conditions. The spatially explicit simulations allow, when temporally aggregated, to trace upstream contributions of individual water balance components for any downstream point in the catchment. We use the model to quantify the amount and timing of glacier melt and how it affects downstream runoff composition, especially during drought conditions, along the river network. We do this across regions from the Swiss Alps’ headwaters to the lowlands in a spatially continuous way. When comparing runoff composition during moderate summer months to periods of drought conditions in the Swiss Alps, our simulations show both an increase in intensity and downstream propagation of ice melt contribution to total runoff. During extreme drought periods, ice melt makes up >70% of streamflow (~doubling the contributions during more moderate periods) in some of the Alps’ headwater regions (>1500 m a.s.l.), and still exceeds 10% of streamflow contribution downstream of the pre-alpine region. Quantifying the timing and amount of glacier melt contributions to downstream water resources under recent drought conditions improves our understanding of potential cryosphere-biosphere interactions and their impacts under future extreme scenarios, when cryospheric runoff contributions may be reduced or completely lost.

The development of a katabatic boundary layer can decouple near surface air temperature changes over glaciers from their surrounding environment during the ablation season, impacting the response of glaciers to ongoing climate change. Current glacier modelling efforts mostly neglect such processes and assume that glacier mass balance will evolve linearly with large-scale (ambient) air temperature changes into the future. Recent work has established that glacier evolution with climate will likely be non-linear, including in its sensitivity to ambient temperature. While past studies have explored this near-surface decoupling at a number of individual sites, the derived patterns have not been generalisable. We compile an extensive new inventory of on-glacier weather station data to explore this phenomena, with over 175 glacier-year sets, including more than 350 individual AWS locations and > 1.3 million hourly air temperature observations. Combining in situ on-glacier and near-glacier meteorological data with reanalysis and surface topography information we are able to explore how the climatic setting and local processes (e.g. wind interactions and local topography) may shape a glacier’s ability to become more or less coupled to the ambient climatic warming. Across all sites studied we find a mean (std.) cold bias of on-glacier vs. ambient temperatures of 1.22±1.35°C and a ratio of above-ice temperature changes compared to ambient, non-glacier conditions of 0.75±0.17 (i.e. a 1°C increase off-glacier equals ~0.75°C change on-glacier). We highlight the relevance of this to glacier modelling applications at select glacier sites and demonstrate hotspots around the world where above-glacier temperature changes during the recent decades are likely to have become decoupled from background warming. Preliminary results show how larger glaciers in maritime climates and those with minimal debris cover are most likely to decouple from ambient warming. However, as glaciers shrink and debris cover expands, the influence of the climatic setting in controlling this decoupling is diminished.

The near-surface boundary layer can mediate the response of mountain glaciers to external climate, cooling the overlying air and promoting a density-driven glacier wind. The fundamental processes are conceptually well understood, though the magnitudes of cooling and presence of glacier winds are poorly quantified in space and time, increasing the forcing uncertainty for melt models. We utilize a new data set of on-glacier meteorological measurements on three neighboring glaciers in the Swiss Alps to explore their distinct response to regional climate under the extreme 2022 summer. We find that synoptic wind origins and local terrain modifications, not only glacier size, play an important role in the ability of a glacier to cool the near-surface air. Warm air intrusions from valley or synoptically-driven winds onto the glacier can occur between similar to 19% and 64% of the time and contribute between 3% and 81% of the total sensible heat flux to the surface during warm afternoon hours, depending on the fetch of the glacier flowline and its susceptibility to boundary layer erosion. In the context of extreme summer warmth, indicative of future conditions, the boundary layer cooling (up to 6.5 degrees C cooler than its surroundings) and resultant katabatic wind flow are highly heterogeneous between the study glaciers, highlighting the complex and likely non-linear response of glaciers to an uncertain future. The presence of a 0 degrees C ice surface cools the near-surface air and generates a unique micro-climate that complicates a glaciers response to future warming. Using a new data series on three glaciers during an extreme summer of 2022, we explore how variable this cooling is in space and time and investigate the factors that can control it. We focus largely on the role of valley and synoptic winds that are found to affect glaciers of varying size and orientation differently, influencing the amount of heat transfer to the ice surface that glaciers receive from outside its own micro-climate. Moreover, we find that the presence of glacier winds can act to enhance or reduce overall heat transfer to the glacier, depending on the wind strength and degree of boundary layer disruption. We highlight the complexities that are ignored in simpler melt modeling frameworks and demonstrate how, especially under extreme summer heat, indicative of future conditions, static parameters to relate glacier melt to temperature are likely to be inappropriate. Glacier size and alignment with valley/synoptic wind gradients control the magnitude of near-surface coolingValley/synoptic winds can occur between 19% and 64% of the time and contribute between 3% and 81% of total sensible heat to the ice surfaceLocalized cooling and turbulence in the boundary layer increase the complexity and non-linearity of glacier response to climate

Glaciers are crucial water resources in the Third Pole (the Tibetan Plateau and its surroundings) and are shrinking in response to climate change. Glacier albedo is an expression of glacier interactions with climate and dust/black carbon, and albedo reduction enhances glacier mass loss, but its changes and potential drivers remain poorly quantified. We leverage satellite observations to explore the variability of glacier albedo and understand its sensitivity to potential drivers and its future evolution. We find that glacier albedo has declined during 2001-2020, but high interannual variability is also an important signal. These variations are highly sensitive to air temperature and snow conditions and to nearby dust/black carbon emission sources. Future changes to these drivers will lead to further decreases of 2.9%-12.5% in glacier albedo by 2100 under different warming scenarios. These findings highlight the importance of albedo in glacier future evolution and the urgency of action to mitigate climate warming.

Mountains are important suppliers of freshwater to downstream areas, affecting large populations in particular in High Mountain Asia (HMA). Yet, the propagation of water from HMA headwaters to downstream areas is not fully understood, as interactions in the mountain water cycle between the cryo-, hydro- and biosphere remain elusive. We review the definition of blue and green water fluxes as liquid water that contributes to runoff at the outlet of the selected domain (blue) and water lost to the atmosphere through vapor fluxes, that is evaporation from water, ground, and interception plus transpiration (green) and propose to add the term white water to account for the (often neglected) evaporation and sublimation from snow and ice. We provide an assessment of models that can simulate the cryo-hydro-biosphere continuum and the interactions between spheres in high mountain catchments, going beyond disciplinary separations. Land surface models are uniquely able to account for such complexity, since they solve the coupled fluxes of water, energy, and carbon between the land surface and atmosphere. Due to the mechanistic nature of such models, specific variables can be compared systematically to independent remote sensing observations - providing vital insights into model accuracy and enabling the understanding of the complex watersheds of HMA. We discuss recent developments in spaceborne earth observation products that have the potential to support catchment modeling in high mountain regions. We then present a pilot study application of the mechanistic land surface model Tethys & Chloris to a glacierized watershed in the Nepalese Himalayas and discuss the use of high-resolution earth observation data to constrain the meteorological forcing uncertainty and validate model results. We use these insights to highlight the remaining challenges and future opportunities that remote sensing data presents for land surface modeling in HMA.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% GLACIER METEOROLOGICAL DATA SWISS ALPS -2022%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Data gathered and structured by Thomas Shaw (WSL, Switzerland (until Oct 2022)). On and off-glacier meteorological data were gathered and analysed as part of a Marie-Curie project 'TEMPEST' (tempestglacier.com).The dataset consists of hourly low-cost AWS (Davis Vantage Pro2) and simple temperature ('T-')logger (Onset TidBitv2) sensor records on three glaciers in the Swiss Alps (Canton Valais). The glaciers are:Haut Glacier d'Arolla (45.967°N, 7.526°E)Glacier d'Otemma (45.956°N, 7.454°E)Glacier du Corbassière (45.975°N, 7.303°E) Data are provided in individual Excel files per glacier that contain all hourly data for the sub-period of comparison (11 August-18 September, 2022).Data are quality controlled and checked for obvious errors. Any uncertain values are set to NaN.Air temperature data at 'T-Logger' stations were corrected for heating errors using the comparison of measurements in artificially (AWS) and naturally ventilated (T-Logger) radiation shields on Arolla and Corbassiere glaciers.A multiple linear regression model was applied to estimate these differences at all T-Loggers on all glaciers as a function of incoming shortwave radiation (MeteoSwiss station-derived) and wind speed (measured at AWS). Each Excel file contains a 'META' tab for simple metadata related to station locations (latitude 'LAT' (°), longitude 'LON' (°), elevation 'ELE' (m a.s.l.) and flowpath length 'FPL' (m)) and a 'DATA' tab for the hourly data. Suffixes to the station names in each column provide the variable measured at that site:'TA' - 2m air temperature (°C)'TA_Hi' - Maximum air temperature for timestep (°C)'TA_Lo' - Minimum air temperature for timestep (°C)'RH' - 2m relative humiditiy (%)'FF' - Wind speed (m s^-1)'FF_Hi' - Maximum wind speed for timestep (m s^-1)'FF_Lo' - Minimum wind speed for timestep (m s^-1)'DIR' - Wind direction (°)'DEW' - Dewpoint temperature (°C)'PRESS' - Air pressure (mbar)'CHILL' - Calculated wind chill temperature (°C)'Heat_idx' - Calculated heat index (°C)'THSW' - A calculated index that uses humidity and temperature like for the Heat Index, but also includes the heating effects of sunshine and the cooling effects of wind (like Wind Chill) to calculate an apparent temperature of what it "feels" like out in the shade Wind speeds and direction measured at off-glacier sites 'OG' are for the lower off-glacier station ('OG_Low'). ------------------------- This work was funded by the EU Horizon 2020 Marie Skłodowska-Curie Actions Grant 101026058.

Abstract The presence of a developed boundary layer decouples a glacier's response from ambient conditions, suggesting that sensitivity to climate change is increased by glacier retreat. To test this hypothesis, we explore six years of distributed meteorological data on a small Swiss glacier in the period 2001–2022. Large glacier fragmentation has occurred since 2001 (−35% area change up to 2022) coinciding with notable frontal retreat, an observed switch from down‐glacier katabatic to up‐glacier valley winds and an increased sensitivity (ratio) of on‐glacier to off‐glacier temperature. As the glacier ceases to develop density‐driven katabatic winds, sensible heat fluxes on the glacier are increasingly determined by the conditions occurring outside the boundary layer of the glacier, sealing the glacier's demise as the climate continues to warm and experience an increased frequency of extreme summers.

High Mountain Asia (HMA) is among the most vulnerable water towers globally and yet future projections of water availability in and from its high-mountain catchments remain uncertain, as their hydrologic response to ongoing environmental changes is complex. Mechanistic modeling approaches incorporating cryospheric, hydrological, and vegetation processes in high spatial, temporal, and physical detail have never been applied for high-elevation catchments of HMA. We use a land surface model at high spatial and temporal resolution (100 m and hourly) to simulate the coupled dynamics of energy, water, and vegetation for the 350 km2 Langtang catchment (Nepal). We compare our model outputs for one hydrological year against a large set of observations to gain insight into the partitioning of the water balance at the subseasonal scale and across elevation bands. During the simulated hydrological year, we find that evapotranspiration is a key component of the total water balance, as it causes about the equivalent of 20% of all the available precipitation or 154% of the water production from glacier melt in the basin to return directly to the atmosphere. The depletion of the cryospheric water budget is dominated by snow melt, but at high elevations is primarily dictated by snow and ice sublimation. Snow sublimation is the dominant vapor flux (49%) at the catchment scale, accounting for the equivalent of 11% of snowfall, 17% of snowmelt, and 75% of ice melt, respectively. We conclude that simulations should consider sublimation and other evaporative fluxes explicitly, as otherwise water balance estimates can be ill-quantified.

Ice cliff distribution plays a major role in determining the melt of debris-covered glaciers but its controls are largely unknown. We assembled a data set of 37,537 ice cliffs and determined their characteristics across 86 debris-covered glaciers within High Mountain Asia (HMA). We find that 38.9% of the cliffs are stream-influenced, 19.5% pond-influenced and 19.7% are crevasse-originated. Surface velocity is the main predictor of cliff distribution at both local and glacier scale, indicating its dependence on the dynamic state and hence evolution stage of debris-covered glacier tongues. Supraglacial ponds contribute to maintaining cliffs in areas of thicker debris, but this is only possible if water accumulates at the surface. Overall, total cliff density decreases exponentially with debris thickness as soon as the debris layer reaches a thickness of over 10 cm.

The compiled dataset is a series of summer meteorological observations on the Swiss Haut Glacier d'Arolla (45.97°N, 7.52°E) to support the analysis presented in the manuscript: %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% "The Decaying Near-Surface Boundary Layer of a Retreating Alpine Glacier", submitted to Geophysical Research Letters. Thomas E. Shaw1, Pascal Buri1, Michael McCarthy1, Evan S. Miles1, Álvaro Ayala2, Francesca Pellicciotti1 1 Swiss Federal Institute, WSL, Birmensdorf, Switzerland2 Centro de Estudios Avanzados en Zonas Áridas, La Serena, Chile %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% The following files are provided:1) 6 x xlsx files "Arolla_Meteorological_Data_[YEAR].xlsx" contains within are tabs for: i) The station locations and elevations (tab "[YEAR]_Info"). ii) All AWS/Tlogger data in hourly format (tab "[YEAR]_Met_Data"). iii) Only the hourly air temperature data for on-glacier sites (tab "[YEAR]_Ta"). 2) Glacier outlines (.shp) for years 1850 (GLAMOS), 1973 (GLAMOS), 1994 (Carenzo, 2012), 1999 (Carenzo et al., 2012), 2010 (GLAMOS), 2022 (Digitised from PlanetScope imagery).3) Debris cover area (.shp) derived by applying an NDSI classification of cloud filtered, summer Landsat scenes in Google Earth Engine following the approach of Scherler et al. (2018). For the meteorological data in 1), the following variables are provided:"TA" = near surface air temperature (°C)."RH" - relative humidity (%)."SWIN" - Shortwave incoming radiation (Wm^-2)."SWOUT" - Shortwave outgoing radiation (Wm^-2)."LWIN" - Longwave incoming radiation (Wm^-2)."LWOUT" - Longwave outgoing radiation (Wm^-2)."FF" - Wind speed (m s-1)."DIR" - Wind direction (°)."PP" - precipitation (mm)."DEW" - dew point temperature (°C). The term "OG" refers to an off-glacier station, which are numbered accordingly. If no variable names are given as a header in the "Met_Data" tab, then the data are air temperature values. Data are filtered for obvious errors and errors are then removed. Data are not gap-filled as this would affect the analysis presented about patterns in air temperature data. Data were checked and compiled by Thomas Shaw (WSL) - thomas.shaw@wsl.chData were measured by ETH (2001-2010) and WSL as part of the Marie-Curie Project 'TEMPEST' (2021-2022). Details of data collection and analysis can be found in:Strasser et al. (2004) - 2001.Carenzo (2012) - 2001-2010.Shaw et al. (N.D.) 2021-2022. %% CITED WORK Carenzo, M. (2012). Distributed modelling of changes in glacier mass balance and runoff (Issue 20616). ETH Zurich. Scherler, D., Wulf, H., & Gorelick, N. (2018). Global Assessment of Supraglacial Debris-Cover Extents. Geophysical Research Letters, 45(21), 11,798-11,805. https://doi.org/10.1029/2018GL080158 Shaw, T. E., Buri, P., McCarthy, M., Miles, E. S., Ayala, Á., & Pellicciotti, F. (2023). The Decaying Near-Surface Boundary Layer of a Retreating Alpine Glacier. Geophysical Research Letters, 50, 1–12. https://doi.org/10.1029/2023GL103043 Strasser, U., Corripio, J. G., Pellicciotti, F., Burlando, P., Brock, B. W., & Funk, M. (2004). Spatial and temporal variability of meteorological variables at Haut Glacier d'Arolla (Switzerland) during the ablation season 2001: Measurements and simulations. Journal of Geophysical Research, 109, D03103. https://doi.org/10.1029/2003JD003973 ------------- This work was funded by the EU Horizon 2020 Marie Skłodowska-Curie Actions Grant 101026058.

Understanding the response of Himalayan glaciers to global warming is vital because of their role as a water source for the Asian subcontinent. However, great uncertainties still exist on the climate drivers of past and present glacier changes across scales. Here, we analyse continuous hourly climate station data from a glacierized elevation (Pyramid station, Mount Everest) since 1994 together with other ground observations and climate reanalysis. We show that a decrease in maximum air temperature and precipitation occurred during the last three decades at Pyramid in response to global warming. Reanalysis data suggest a broader occurrence of this effect in the glacierized areas of the Himalaya. We hypothesize that the counterintuitive cooling is caused by enhanced sensible heat exchange and the associated increase in glacier katabatic wind, which draws cool air downward from higher elevations. The stronger katabatic winds have also lowered the elevation of local wind convergence, thereby diminishing precipitation in glacial areas and negatively affecting glacier mass balance. This local cooling may have partially preserved glaciers from melting and could help protect the periglacial environment. High-elevation meteorological observations and reanalysis data indicate local cooling and drying near Himalayan glaciers due to enhanced katabatic winds in response to global warming.

Microbial communities are found throughout the biosphere, from human guts to glaciers, from soil to activated sludge. Understanding the statistical properties of such diverse communities can pave the way to elucidate the common mechanisms ...Multiple ecological forces act together to shape the composition of microbial communities. Phyloecology approaches—which combine phylogenetic relationships between species with community ecology—have the potential to disentangle such forces but are often ...

Ice cliffs are important contributors to the mass balance of debris-covered glaciers, especially in High Mountain Asia where they can account for one sixth of the melt of debris-covered glacier tongues, despite covering less than 10% of their area. These features have been shown to evolve, appear and disappear rapidly from year to year, with high variability in relative area and number. It has been hypothesized that ice cliffs expand and melt more rapidly during the monsoon (June-September), but there are very few observations during this period. Here, we use arrays of time-lapse cameras to reconstruct the geometry of four ice cliffs at a weekly timestep over a period of four to six months at two monsoon-affected sites: Langtang Glacier in Nepal, and 24K Glacier in South-Eastern Tibet. We use Structure-from-Motion photogrammetry to derive point clouds and Digital Elevation Models (DEMs) of the glacier surface, using the stable background terrain to constrain viewing geometries and DEM errors. This technique (time-lapse photogrammetry) enables a high accuracy, quantitative measurement of processes occurring at the cliff-scale (elevation uncertainties stay below 30cm at a distance of 300m from the cameras) and at high temporal resolution over the monsoon season, when dense cloud cover and intense precipitation prevent field- or satellite-based observations. We derive the melt patterns of these cliffs from the differencing of the weekly DEMs by accounting for glacier flow. We compare the observed melt patterns with the modeled energy-balance at the cliff surface and use these observations to quantify the influence of debris slumping and redistribution, as well as supraglacial ponds and streams on the melt patterns of these cliffs. The results highlight the seasonal variations of cliff melt, which occurs at up to 8 cm/day during the monsoon, twice as high as observed in the pre- and post-monsoon period. Our energy-balance results indicate that melt rates are driven by incoming long- and shortwave radiation, and are thus highly dependent on the cliff slope and aspect, as substantiated by our photogrammetric measurements. The observations also demonstrate the competitive influence of debris, which progressively reburies the cliff by accumulating at its surface, and supraglacial streams and ponds, which maintain the cliff slope by preventing debris accumulation at the base. These results will help in understanding the surface evolution of debris-covered glaciers and enable a better representation of ice cliff melt and evolution in glacio-hydrological models.

Glaciers are key components of the Asian water towers and provide water to large downstream communities for domestic, agricultural and industrial uses. In the Nepal Himalaya, the Indian Summer Monsoon dominates climate, and results in a complex meteorology and simultaneous accumulation and ablation that complicate the quantification of snow processes. Assessing solid precipitation input, especially in the upper accumulation area (> 6000 m a.s.l.), remains key to understanding recent mass losses. Catchment-scale glacio-hydrological modelling in the Himalaya has to date mostly relied on temperature-index or intermediate-complexity enhanced temperature-index methods, but recent studies have shown that such approaches can lead to inaccurate amounts of melt, especially at high elevations where refreezing, sublimation and avalanches influence the snow depth variability. The Trakarding–Trambau Glacier system experienced significant mass loss over the last decades, and recent field measurements of meteorology and glacier change present the opportunity to examine these problems with physically-based and spatially-resolved atmospheric and glacio-hydrological modelling. We combine a novel non-hydrostatic atmospheric model (NHM; atmospheric core of the cryosphere-oriented regional climate model NHM-SMAP) and an advanced land surface model at cloud-permitting hyper-resolution (~ 100 m) to explore the role of snow processes in the water balance of this glacierized catchment. We force the land-surface model of the catchment with dynamically downscaled, hourly outputs from NHM for the 2018-2019 hydrological year. We evaluate the NHM output using available in-situ meteorological observations and evaluate the land surface model skills and process representation with in-situ mass balance observations, remotely sensed surface elevation change and snow cover. Coupling of the two types of models is unprecedented in the Himalaya, and holds promise to reveal processes that cannot be explicitly assessed by simpler models or forcing data. We investigate the contribution of sublimation and precipitation partition to the glacier mass balance and catchment runoff, and analyze the difference in mass balance and its drivers between the debris-covered and debris free-glaciers. To place this very novel type of simulations into the context of current research, we compare our NHM-forced simulations with simulations forced by station data and ERA5-Land reanalysis. Finally, we evaluate the effect of spatial resolution (50 m, 100 m, 200 m) on model performance and process representation. Our results highlight the potential of sophisticated models based on the calculations of energy and mass fluxes to unravel the complex processes that shape the response of Himalayan catchments, and provide an assessment of their skills as a function of spatial resolution.

Supplementary Material for 'Sub-seasonal variability of supraglacial ice cliff melt rates and associated processes from time-lapse photogrammetry'Table S1: Optimized accuracy of the different camera parameters for DEM processing with Agisoft Metashape. SiteNumber of cameras Optimized position accuracy (m) Camera pose parameters accuracy (°) X, Y Z Yaw Pitch Roll 24K 4 0.1 0.5 5 5 5 Langtang 8 0.1 0.

Glaciers are key components of the mountain water towers of Asia and are vital for downstream domestic, agricultural, and industrial uses. The glacier mass loss rate over the southeastern Tibetan Plateau is among the highest in Asia and has accelerated in recent decades. This acceleration has been attributed to increased warming, but the mechanisms behind these glaciers' high sensitivity to warming remain unclear, while the influence of changes in precipitation over the past decades is poorly quantified. Here, we reconstruct glacier mass changes and catchment runoff since 1975 at a benchmark glacier, Parlung No. 4, to shed light on the drivers of recent mass losses for the monsoonal, spring-accumulation glaciers of the Tibetan Plateau. Our modeling demonstrates how a temperature increase (mean of 0.39 ∘C ⋅dec-1 since 1990) has accelerated mass loss rates by altering both the ablation and accumulation regimes in a complex manner. The majority of the post-2000 mass loss occurred during the monsoon months, caused by simultaneous decreases in the solid precipitation ratio (from 0.70 to 0.56) and precipitation amount (-10%), leading to reduced monsoon accumulation (-26%). Higher solid precipitation in spring (+18%) during the last two decades was increasingly important in mitigating glacier mass loss by providing mass to the glacier and protecting it from melting in the early monsoon. With bare ice exposed to warmer temperatures for longer periods, icemelt and catchment discharge have unsustainably intensified since the start of the 21st century, raising concerns for long-term water supply and hazard occurrence in the region.

Data and MATLAB code to support the article Glacier contributions to river discharge during the current Chilean megadrought in Earth's Future by McCarthy and others

Abstract The current Chilean megadrought has led to acute water shortages in central Chile since 2010. Glaciers have provided vital fresh water to the region's rivers, but the quantity, timing and sustainability of that provision remain unclear. Here we combine in‐situ, remote sensing and climate reanalysis data to show that from 2010 to 2018 during the megadrought, unsustainable imbalance ablation of glaciers (ablation not balanced by new snowfall) strongly buffered the late‐summer discharge of the Maipo River, a primary source of water to Santiago. If there had been no glaciers, water availability would have been reduced from December through May, with a 31 ± 19% decrease during March. Our results indicate that while the annual contributions of imbalance ablation to river discharge during the megadrought have been small compared to those from precipitation and sustainable balance ablation, they have nevertheless been a substantial input to a hydrological system that was already experiencing high water stress. The water‐equivalent volume of imbalance ablation generated in the Maipo Basin between 2010 and 2018 was 740 × 106 m3 (19 ± 12 mm yr−1), approximately 3.4 times the capacity of the basin's El Yeso Reservoir. This is equivalent to 14% of Santiago's potable water use in that time, while total glacier ablation was equivalent to 59%. We show that glacier retreat will exacerbate river discharge deficits and further jeopardize water availability in central Chile if precipitation deficits endure, and conjecture that these effects will be amplified by climatic warming.

Melt from supraglacial ice cliffs is an important contributor to the mass loss of debris-covered glaciers. However, ice cliff contribution is difficult to quantify as they are highly dynamic features, and the paucity of observations of melt rates and their variability leads to large modelling uncertainties. We quantify monsoon season melt and 3D evolution of four ice cliffs over two debris-covered glaciers in High Mountain Asia (Langtang Glacier, Nepal, and 24K Glacier, China) at very high resolution using terrestrial photogrammetry applied to imagery captured from time-lapse cameras installed on lateral moraines. We derive weekly flow-corrected digital elevation models (DEMs) of the glacier surface with a maximum vertical bias of ±0.2 m for Langtang Glacier and ±0.05 m for 24K Glacier and use change detection to determine distributed melt rates at the surfaces of the ice cliffs throughout the study period. We compare the measured melt patterns with those derived from a 3D energy balance model to derive the contribution of the main energy fluxes. We find that ice cliff melt varies considerably throughout the melt season, with maximum melt rates of 5 to 8 cm d−1, and their average melt rates are 11–14 (Langtang) and 4.5 (24K) times higher than the surrounding debris-covered ice. Our results highlight the influence of redistributed supraglacial debris on cliff melt. At both sites, ice cliff albedo is influenced by the presence of thin debris at the ice cliff surface, which is largely controlled on 24K Glacier by liquid precipitation events that wash away this debris. Slightly thicker or patchy debris reduces melt by 1–3 cm d−1 at all sites. Ultimately, our observations show a strong spatio-temporal variability in cliff area at each site, which is controlled by supraglacial streams and ponds and englacial cavities that promote debris slope destabilisation and the lateral expansion of the cliffs. These findings highlight the need to better represent processes of debris redistribution in ice cliff models, to in turn improve estimates of ice cliff contribution to glacier melt and the long-term geomorphological evolution of debris-covered glacier surfaces.