Rain- and meltwater induced basal melting in high-rainfall glaciers

The contribution of rain- and meltwater to glacier ice-mass loss at the ice-bed interface through basal melting is poorly understood but generally believed to be minor. We calculate a rain- and meltwater induced basal melt rate due to heat advection of > 2 m water equivalent yr in the lower ablation zone of the high-rainfall (> 6 m yr) Franz Josef Glacier, Aoteroa New Zealand, which is equivalent to ~10% of the total ablation for the glacier. This is based on glacier dynamics, climate station readings, laboratory experiments and field observations. Conventional "wisdom" assumes that rain- and meltwater enters the glacier at 0°C. However, laboratory experiments simulating rain at 20°C falling onto ice at -5°C at a rate of 10 mm hr for five minutes (which is approximately the mean residence time of rain- and meltwater on the glacier surface) yield a water temperature of 7°C. Additionally, measurements of supraglacial meltwater indicate temperatures close to 1°C upon entry into the glacier. Thus, melting through advection of heat from the relatively warm water to the cold ice as rain- and meltwater well above 0°C travels through the glacier will be an important ablation process within subglacial systems of high-rainfall, high ablation, glaciers such as those located on the windward sides of high mountain ranges including western North America, Patagonia and New Zealand. This means that changes in rainfall may alter the mass balance of a glacier significantly, potentially confounding simplistic temperature-related estimates of climate change.