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Abstract. Coseismic avalanches and rockfalls, as well as their simultaneous air blast and muddy flow, which were induced by the 2015 Gorkha earthquake in Nepal, destroyed the village of Langtang. In order to reveal volume and structure of the deposit covering the village, as well as sequence of the multiple events, we conducted an intensive in situ observation in October 2015. Multitemporal digital elevation models created from photographs taken by helicopter and unmanned aerial vehicles reveal that the deposit volumes of the primary and succeeding events were 6.81 ± 1.54 × 10 6 and 0.84 ± 0.92 × 10 6 m 3 , respectively. Visual investigations of the deposit and witness statements of villagers suggest that the primary event was an avalanche composed mostly of snow, while the collapsed glacier ice could not be dominant source for the total mass. Succeeding events were multiple rockfalls which may have been triggered by aftershocks. From the initial deposit volume and the area of the upper catchment, we estimate an average snow depth of 1.82 ± 0.46 m in the source area. This is consistent with anomalously large snow depths (1.28-1.52 m) observed at a neighboring glacier (4800-5100 m a.s.l.), which accumulated over the course of four major snowfall

We conducted a mass-balance study of debris-free Trambau Glacier in the Rolwaling region, Nepal Himalaya, which is accessible to 6000 m a.s.l., to better understand mass-balance processes and the effect of precipitation on these processes on high-elevation Himalayan glaciers. Continuous in situ meteorological and mass-balance observations that spanned the three melt seasons from May 2016 are reported. An energy- and mass-balance model is also applied to evaluate its performance and sensitivity to various climatic conditions. Glacier-wide mass balances ranging from −0.34 ± 0.38 m w.e. in 2016 to −0.82 ± 0.53 m w.e. in 2017/18 are obtained by combining the observations with model results for the areas above the highest stake. The estimated long-term glacier mass balance, which is reconstructed using the ERA-Interim data calibrated with in situ data, is −0.65 ± 0.39 m w.e. a−1 for the 1980–2018 period. A significant correlation with annual precipitation (r = 0.77, p < 0.001) is observed, whereas there is no discernible correlation with summer mean air temperature. The results indicate the continuous mass loss of Trambau Glacier over the last four decades, which contrasts with the neighbouring Mera Glacier in balance.

Scientifically valuable information can be learned by listening to the tiny vibrations emanating from a glacier with seismometers. However, this approach has never been employed to better understand glaciers protected from heat by a debris mantle, despite being common in the Himalayas, one of the most glacierized regions in the world. Here we installed a seismic network at a series of challenging high-altitude sites on a glacier in Nepal. Our results show that the diurnal air temperature modulates the glacial seismic noise. The exposed surface of the glacier experiences thermal contraction when the glacier cools, whereas the areas that are insulated with thick debris do not suffer such thermal stress. Thus, the unprotected ice surface bursts with seismicity every night due to cracking, which gradually damages and weathers the ice. This is the first time such processes have been observed at relatively warm temperatures and outside of the polar regions.Plain Language Summary It has been realized that much scientifically valuable information can be learned by listening to the tiny vibrations emanating from a glacier with sensitive sensors. However, due to their remoteness and the difficulties in accessing glacial environments, this approach has rarely been employed to better understand these important systems. For example, debris-covered glaciers, which are protected from heat by a debris mantle, remain to be studied despite being common in the Himalayas, one of the most glacierized regions in the world. Here we installed a seismic network at a series of challenging high-altitude sites on a glacier in Nepal. Our results show that the diurnal air temperature modulates the glacial seismic activity. A debris mantle dampens the diurnal amplitude of temperature and thus protects the ice from cyclic mechanical damage, whereas debris-free (exposed) ice experiences intensive near-surface fracturing early in the morning. This implies that the unprotected ice surface bursts with seismicity every night due to cracking, which gradually damages and weathers the ice. This is the first time such processes have been observed outside of the polar regions. These findings are in agreement with the personal experiences of climbers who felt and heard loud cracks on high-altitude glaciers at night.

Ice cliffs can act as “hot spots” for melt on debris-covered glaciers and promote local glacier mass loss. Repeat high-resolution remote-sensing data are therefore required to monitor the role of ice cliff dynamics in glacier mass loss. Here we analyze high-resolution aerial photogrammetry data acquired during the 2007, 2018, and 2019 post-monsoon seasons to delineate and monitor the morphology, distribution, and temporal changes of the ice cliffs across the debris-covered Trakarding Glacier in the eastern Nepal Himalaya. We generate an ice cliff inventory from the 2018 and 2019 precise terrain data, with ice cliffs accounting for 4.7 and 6.1% of the debris-covered area, respectively. We observe large surface lowering (&gt;2.0 m a−1) where there is a denser distribution of ice cliffs. We also track the survival, formation, and disappearance of ice cliffs from 2018 to 2019, and find that ∼15% of the total ice cliff area is replaced by new ice cliffs. Furthermore, we observe the overall predominance of northwest-facing ice cliffs, although we do observe spatial heterogeneities in the aspect variance of the ice cliffs (ice cliffs face in similar/various directions). Many new ice cliffs formed across the stagnant middle sections of the glacier, coincident with surface water drainage and englacial conduit intake observations. This spatial relationship between ice cliffs and the glacier hydrological system suggests that these englacial and supraglacial hydrological systems play a significant role in ice cliff formation.

The diurnal cycle of precipitation is important to the hydroclimate in the Himalayas in summer; however, features of the diurnal cycle that affect precipitation from the foothills to glacierized, high‐elevation areas are poorly understood. We investigated the diurnal cycle of precipitation using 3 years of in situ observations recorded close to a glacier at 4,806 m asl, and 17 years of data from the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) and the Integrated Multi‐satellitE Retrieval for Global Precipitation Measurement (IMERG). The mechanisms that drive the diurnal cycle were examined using hourly European Center for Medium‐Range Weather Forecasts Re‐Analysis fifth generation (ERA5) data. In situ observations showed that the diurnal precipitation cycle has daytime and nighttime peaks, which both consist of high rainfall frequency and low rainfall intensity. In addition, twice‐daily maxima exist in the TRMM PR data, particularly over two rain bands at around 500–1,000 m asl, and at ∼2,000 m asl. Convective‐type rainfall, with a lower rain‐top height, occurs in the daytime, whereas stratiform‐type rain, with a higher rain‐top height, occurs at night, particularly over terrain at elevations above ∼1,500 m asl. Land surface processes likely cause the two peaks in the diurnal cycle. Daytime surface heating drives upslope flows that promote condensation. At night, surface cooling over the plain to the south of the Himalayas causes low‐level monsoon flows to accelerate, creating a nocturnal jet, which results in large‐scale moisture flux convergence over the southern slopes.

Recent observations suggest that the nocturnal thermal fracturing of ice occurs at relatively warm temperatures (above −15 °C) at a high‐altitude Himalayan glacier system unless the ice is shielded by a debris mantle. Here we estimate the stresses induced by diurnal temperature variations using viscous, elastic, and two viscoelastic models, and various thicknesses of the debris mantle. Only the elastic and visco‐elastic models are in agreement with the observations. The timing and amplitudes of the stresses in the upper 15 cm of the glacier are different among the models despite the ability of each approach to predict a diurnal increase in tension exceeding the critical threshold proposed for crevasse formation. For example, the elastic stress is several times larger than the viscous stress at the ice surface (650 vs. 250 kPa) and reaches its peak up to 5–6 hr later in the night. The time lag is in line with the seismic records, suggesting that the viscous model is not appropriate. Furthermore, a debris layer of ≥50 cm in thickness suppresses the diurnal fluctuations in thermal stress and therefore protects the ice from mechanical damage. We suggest that high‐amplitude diurnal cooling and weak ice properties due to weathering are essential factors that influence thermal fracturing in the Himalayan environment. The ongoing expansion of seismic networks into cryospheric regions, which will be capable of detecting local thermal‐contraction‐induced cracks, in combination with the fact that such cracks can erode and weaken the ice, and thereby serve as meltwater and heat channels, warrants further research to better understand these near‐surface processes and to monitor ice properties.

Abstract. Co-seismic avalanches and rock falls, and their simultaneous air blasts, which were induced by the 2015 Gorkha earthquake in Nepal, destroyed the village of Langtang. In order to reveal volume and structure of the deposit covering the village, and sequence of the multiple events, we conducted an intensive in-situ observation in October 2015. Multi-temporal digital elevation models created from photographs taken by helicopter and unmanned aerial vehicles reveal that the deposit volumes of the primary and succeeding events were 6.81 × 106 m3 and 0.84 × 106 m3, respectively. Visual investigations of the deposit and witness statements of villagers suggest that the primary event was an avalanche composed mostly of snow, while the contribution of collapsed glacier ice could account for a few percent of the total mass. Succeeding events were multiple rock falls which may have been triggered by aftershocks. From the initial deposited volume and the upper catchment area, we estimate an average snow depth of 1.56 m in the source area using density assumptions of snow and ice. This is consistent with anomalously large snow depths (1.28–1.52 m) observed at a neighboring glacier (4800–5100 m a.s.l.), which accumulated over the course of four major snowfall events since October 2014. Considering long-term observational data, probability density functions, and elevation gradients of precipitation, we conclude that this anomalous winter snow was an extreme event with a return interval of at least 100 years, which amplified or even caused the disaster induced by the 2015 Gorkha earthquake in Nepal.