Variations in near‐surface debris temperature through the summer monsoon on Khumbu Glacier, Nepal Himalaya

Debris surface temperature is a function of debris characteristics and energy fluxes at the debris surface. However, spatial and temporal variability in debris surface temperature, and the debris properties that control it, are poorly constrained. Here, near-surface debris temperature (Ts) is reported for 16 sites across the lower elevations of Khumbu Glacier, Nepal Himalaya, for the 2014 monsoon season. The debris layer at all sites was ≥1 m thick. We confirm the occurrence of temporal and spatial variability in Ts over a 67-day period and investigate its controls. Ts was found to exhibit marked temporal fluctuations on diurnal, short-term (1–8 days) and seasonal timescales. Over the study period, two distinct diurnal patterns in Ts were identified that varied in timing, daily amplitude and maximum temperature; days in the latter half of the study period (after Day of Year 176) exhibited a lower diurnal amplitude (mean = 23°C) and reduced maximum temperatures. Days with lower amplitude and minimum Ts were concurrent with periods of increased seasonal variability in on-glacier air temperature and incoming shortwave radiation, with the increased frequency of these periods attributed to increasing cloud cover as the monsoon progressed. Spatial variability in Ts was manifested in variability of diurnal amplitude and maximum Ts of 7°C to 47°C between sites. Local slope, debris clast size and lithology were identified as the most important drivers of spatial variability in Ts, with inclusion of these three variables in the stepwise general linear models resulting in R2 ≥0.89 for six out of the seven sites. The complexity of surface energy fluxes and their influence on Ts highlight that assuming a simplified relationship between air temperature and debris surface temperature in glacier melt models, and a direct relationship between debris surface temperature and debris thickness for calculating supraglacial debris thickness, should be undertaken with caution.