Effects of debris on ice-surface melting rates: an experimental study

Abstract

ABSTRACT. Here we report a laboratory study of the effects of debris thickness, diurnally cyclic radiation and rainfall on melt rates beneath rock-avalanche debris and sand (representing typical highly permeable supraglacial debris). Under continuous, steady-state radiation, sand cover >50 mm thick delays the onset of ice-surface melting by >12 hours, but subsequent melting matches melt rates of a bare ice surface. Only when diurnal cycles of radiation are imposed does the debris reduce the longterm rate of ice melt beneath it. This is because debris >50 mm thick never reaches a steady-state heat flux, and heat acquired during the light part of the cycle is partially dissipated to the atmosphere during the nocturnal part of the cycle, thereby continuously reducing total heat flux to the ice surface underneath. The thicker the debris, the greater this effect. Rain advects heat from high-permeability supraglacial debris to the ice surface, thereby increasing ablation where thin, highly porous material covers the ice. In contrast, low-permeability rock-avalanche material slows water percolation, and heat transfer through the debris can cease when interstitial water freezes during the cold/night part of the cycle. This frozen interstitial water blocks heat advection to the ice-debris contact during the warm/day part of the cycle, thereby reducing overall ablation. The presence of metre-deep rock-avalanche debris over much of the ablation zone of a glacier can significantly affect the mass balance, and thus the motion, of a glacier. The length and thermal intensity of the diurnal cycle are important controls on ablation, and thus both geographical location and altitude significantly affect the impact of debris on glacial melting rates; the effect of debris cover is magnified at high altitude and in lower latitudes.

Spatial Variability in Patterns of Glacier Change across the Manaslu Range, Central Himalaya

“…The presence of supraglacial debris allows for glacier tongues to extend to much lower-lying elevations than would otherwise be possible (Owen and Benn, 2005). The effects of debris-cover on glacier mass balance are poorly known, and can act to either retard or exacerbate glacier melting depending on debris thickness, composition, and distribution (Reznichenko et al, 2010;Zhang et al, 2011). Many studies have reported thresholds of debris-thickness, above which the debris acts to insulate the glacier as opposed to amplifying melt rates (Reznichenko et al, 2010).…”

Pond Dynamics and Supraglacial-Englacial Connectivity on Debris-Covered Lirung Glacier, Nepal

“…The porosity and thickness of the surface debris enables a saturated debris layer to develop (Reznichenko et al, 2010;Collier et al, 2014). Rapid downslope transport of surface water occurs in areas of steep relief, but the surface debris may lead to the formation of shallow groundwater lenses in lowslope areas.…”

Climate–Glacier Dynamics and Topographic Forcing in the Karakoram Himalaya: Concepts, Issues and Research Directions

“…Reznichenko et al [124] studied the effects of debris-cover on glacier ice in a laboratory environment. The insulating effect of thicker debris-cover (greater than 5 cm) is only possible when diurnal cycles of radiation forcing are present, as nighttime allows for the absorbed energy to be emitted back to the atmosphere.…”