Quantifying meltwater refreezing along a transect of sites on the Greenland ice sheet

On the Greenland ice sheet, a significant quantity of surface meltwater refreezes within the firn, creating uncertainty in surface mass balance estimates. This refreezing has the potential to buffer seasonal runoff to future increases in melting, but direct measurement of the process remains difficult. We present a method for quantifying refreezing at point locations using in situ firn temperature observations. A time series of sub-hourly firn temperature profiles were collected over the course of two melt seasons from 2007 to 2009 along a transect of 11 sites in the accumulation zone of Greenland. Seasonal changes in temperature profiles combined with heat flux estimates based on high-temporal-resolution temperature gradients enable us to isolate the heat released by refreezing using conservation of energy. Our method is verified from winter data when no refreezing takes place, and uncertainty is estimated using a Monte Carlo technique. While we limit our method to a subsection of firn between depths of 1 and 10 m, our refreezing estimates appear to differ significantly from model-based estimates. Furthermore, results indicate that a significant amount of refreezing takes place at depths greater than 1 m and that lateral migration of meltwater significantly complicates the relationship between total surface melt and total refreezing.