Description: Ice cliffs locally enhance the melt of debris-covered glaciers. These features are widespread across such glaciers and are therefore important contributors to their mass balance. However, little is known about their formation or distribution across glaciers, making it difficult to account for their melt contribution at the glacier to the regional scale. Here, we took advantage of semi-automated mapping approaches to assemble a dataset of 37,537 ice cliffs and determine their characteristics across 86 debris-covered glaciers in High Mountain Asia (HMA). We complemented this dataset with the analysis of 202 cliff formation events from multi-temporal UAV observations for a subset of these glaciers. We find that, for this large population of debris-covered glaciers, 38.9% of the cliffs are stream-influenced, 19.5% pond-influenced and 19.7% are crevasses. Surface velocity stands out as the main predictor of cliff distribution at both local and glacier scale, indicating its dependance on the dynamic state and hence evolution stage of debris-covered glacier tongues. As a result, total cliff density decreases exponentially with debris thickness as soon as debris gets thicker than 10 cm. This effect is somewhat compensated by the influence of supraglacial ponds, that contribute to maintaining cliffs in areas of thicker debris, when water can accumulate at the surface. This leads us to propose a conceptual model which links the distribution and ice cliff type to the glacier evolution stage and its pattern of debris cover in a broad context of glacier slow down and debris thickening.

Description: The cryosphere is a crucial water storage and buffer in the Amu Darya River basin, which receives little precipitation downstream of the Pamir and Hindu Kush mountains. Lowland agriculture and livelihoods heavily rely on the water released from the cryosphere, especially during the summer period. While the Pamir glaciers exhibited near-neutral mass balance in recent decades, the future of the region’s mountain water supply depends on how snow and glaciers will respond to continued climate change.We apply a state-of-the-art land surface model to the Kyzylsu catchment, a recently established research catchment in the basin’s glacierized headwaters, for the recent past and for four decades into the future. We force the model with an ensemble of CMIP6 SSP scenarios downscaled to 100m spatial resolution. The model is constrained and evaluated with in-situ observations and remote sensing data. We study the present and future of the catchment’s water and ice mass balances and in particular the energy driven partitioning of fluxes into runoff and vapor. We investigate whether the catchment’s cryosphere is at a tipping point towards a negative mass balance, as it has been hypothesized for the wider region, as well as whether peak water will be reached by the middle of the 21st century. This study provides an important insight into the efficacy of mechanistic models to reveal the hydrological functioning of the region’s headwater catchments and to project their future evolution.

Description: High Mountain Asia (HMA) hosts the largest mass of ice outside the Polar Regions and provides water to large downstream communities. Glacier change has been highly diverse across the region over the last decades, with glaciers in the Pamirs experiencing near-neutral mass balance while fast rates of mass loss are observed in the Southeastern Tibetan Plateau (STP). In a previous modelling study in the STP, we found that precipitation phase changes associated with climate warming were a major accelerator of glacier losses, but this mechanism of mass loss acceleration has yet to be explored across the rest of HMA. High-elevation measurements of precipitation phase are rare, especially along glacierized elevations, which lie at the solid-mixed-liquid boundary during summer months. Therefore, accurate representation of precipitation phase in models is essential for quantifying glacier accumulation rates and projecting change.Here we use downscaled and bias-corrected ERA5-Land reanalysis to show the high sensitivity of the solid precipitation ratio to the choice of parameterisation and its impacts on the seasonality of simulated snowfall accumulation at three glacierized catchments covering distinctive climates in HMA (Kyzylsu in the Northern Pamirs, Trakarding-Trambau in the Nepalese Himalayas and Parlung No.4 in the STP). We then apply a land-surface model at high spatio-temporal resolution (100m, hourly) to show the effect of the precipitation phase parametrization on the simulated glacier-mass balance, snowpack dynamics and catchment hydrology. Our results highlight the need to better constrain precipitation phase in glaciological studies and to consider the uncertainty associated with the choice of parameterisation.

Description: Debris cover significantly influences glacier melt rates, but our knowledge of debris physical properties, their variability and their effects on ablation is limited. In-situ measurements on debris-covered glaciers are relatively sparse and therefore literature values are often used in modelling studies. To address this issue, we have compiled DebDab, a database of debris cover properties that includes debris thickness, bulk thermal conductivity, surface roughness, albedo, emissivity, porosity and measured subseasonal melt rates from published literature and datasets. The first version contains debris thickness measurements for 73 glaciers, over 7000 individual debris thickness measurements and more than 1700 melt rate measurements. The other properties have more than 400 entries of which only 45% are actual measurements. DebDab is openly available as a community resource and will continue to develop in the coming years, and we welcome community contributions of their measurements of debris thickness and properties. We hope that it will be useful for physical parameter probability distributions, model uncertainty assessments, and as validation data for debris thickness estimation and melt modelling. To demonstrate its utility, we used DebDab to explore the sensitivity of point scale melt simulations to debris properties variability, and in particular to investigate (1) the effect of changing debris thickness on each debris property; (2) the relationship between debris properties; and (3) whether these relationships are the same for different climatic regions worldwide.

Description: Since 2010, central Chile has experienced one of the driest decadal-scale droughts of the past thousand years, with annual precipitation deficits of 20-40%. In the Andes, this ‘megadrought’ has manifested in acute reductions in snowpack; streamflow; lake, reservoir and glacier volume; and soil moisture, with knock-on effects especially for agriculture and vegetation productivity. While many studies have characterised such impacts via remote sensing, in-situ measurements and conceptual hydrological modelling, fewer have taken a mechanistic approach, leaving room to improve process understanding and disentangle the complex response of the mountain water cycle. Here we present preliminary results from a modelling study of the propagation of the megadrought in the Cajon del Maipo, to the east of Santiago. We simulate water, energy and carbon fluxes both before and during the drought, using the land-surface model Tethys-Chloris, at 250 m spatial and hourly temporal resolution. We force the model with a new meteorological dataset, CHELSA high-res, and evaluate its performance using in-situ streamflow and glacier mass balance data, as well as snow cover and leaf-area index data from satellite images. We explore in particular how reduced snow cover has affected vegetation dynamics and downstream hydrology to gain insights relevant for a snow-scarce future.

Description: Debris, ranging from surface dust to medial moraines and thick, continuous layers covering ablation zones, partially covers glaciers all around the world. This supraglacial layer fundamentally modifies the energy transfer from the atmosphere to the ice, thus directly controlling sub-debris melt rates. Debris physical properties such as albedo, surface roughness, and thermal conductivity have only been measured directly or derived from local measurements at a few sites, so studies have relied on literature values. As we now understand, debris properties are highly heterogeneous, potentially implying high uncertainty in melt rates and debris thicknesses derived from literature debris properties.To meet this data gap and evaluate its impact on modeled melt, we undertook an observational campaign to investigate supraglacial debris properties at Pirámide Glacier, in Chile, in a climate distinct from previous studies. First, we derived aerodynamic surface roughness from wind-temperature tower data in three glacier locations and thermal conductivity from thermistor strings in the debris layer colocated with the wind-temperature towers; we measured ablation with stakes adjacent to the towers for validation; and sampled debris thickness across the glacier surface.We then conducted point-scale energy-balance modeling using both locally-derived debris properties and typical literature values to investigate the impact of using literature values when modeling glacier melt. While deriving local debris properties from measurements is challenging, we show that they can differ significantly from literature values; thus, their use in melt models can result in considerable differences in estimates of sub-debris ice ablation.

Description: The Peruvian Andes contains the largest mass of glaciers in the tropics and previous work identified glacier melt as a key contributor to sustaining dry season water resources. Snow, however, has been neglected to date, and modelling and hydro-chemical analyses have been unable to resolve the snow cover dynamics nor fully distinguish between the separate contributions of snow and glaciers to runoff. To provide these insights we run the fully distributed, hourly glacier-hydrological model TOPKAPI-ETH from 2008-2018 over the upper Rio Santa catchment in the Cordillera Blanca. The model parameters are derived from ground-based data and evaluated against independent snow cover and glacier mass balance estimates from remote sensing, alongside gauged runoff. Glacier melt is important in the dry season and in the Blanca (eastern) side of the catchment, where even the catchments with the smallest glacier-covered area benefit from dry season runoff. However, our results highlight the underappreciated importance of snow for discharge. Snowmelt is a strikingly consistent contributor to runoff temporally and spatially: its proportional contribution is largest at the beginning of the dry season and lowest at the beginning of the wet season. Off-glacier snowfall is significant in the wet season. However, this melts quickly, so that accumulation is limited to high elevations and the dry season snow-cover reduces to on-glacier areas. Snow cover durations are in the order of hours to days, contrasting with the seasonal snowpack typical of mid-latitude climates. Paradoxically ephemeral snow cover provides a reliable source of runoff in the tropical Andes.