Quantifying short-term backwasting rates of a supraglacial ice cliff at Machoi Glacier in the Indian Himalaya

Abstract Highly dynamic, ephemeral ice cliffs are melting hotspots on debris-covered glaciers. While the seasonal evolution of Himalayan ice cliffs is well documented, short-term changes on hourly to daily scales and their driving factors are rarely investigated. This study reports hourly backwasting rates of a supraglacial ice cliff at Machoi Glacier ( $34.29^{\circ}\,\mathrm{N}$ , $75.53^{\circ}\,\mathrm{E}$ ) in the western Himalaya, measured over 3 days in June 2022 using a terrestrial laser scanner (TLS). An energy-balance model, incorporating the ice cliff’s topography, solar positions and radiation components, analyses the drivers of variability in backwasting rates. Within a single day (29 June), we observed very large variability in hourly mean backwasting rates, rising from $0.38 \pm 0.05\ \mathrm{cm\,hr}^{-1}$ (1430–1530 hours) to 1.06 ± $0.13\ \mathrm{cm\ h}^{-1}$ (1530–1630 hours), driven by direct solar radiation (solar elevation angle ∼50 ∘ ). Subsequently, rates declined to $0.68 \pm 0.03\ \mathrm{cm\ h}^{-1}$ (1730–1830 hours) influenced by diffuse shortwave and net longwave radiation. The mean daily backwasting rate ( $7.7 \pm 0.13\ \mathrm{cm\ d}^{-1}$ ) resulted in the complete melting of the ice cliff within 2 months. This study highlights the potential of TLS to estimate short-term variations in ice cliff dynamics and controlling processes.