Simulating the growth of supra-glacial lakes at the western margin of the Greenland ice sheet
Abstract. We present a method of modelling the growth of supra-glacial lakes at the western margin of the Greenland ice sheet, based on routeing runoff estimated by a Regional Climate Model (RCM) across a digital elevation model (DEM) of the ice sheet surface. Using data acquired during the 2003 melt season, we demonstrate that the model is 18 times more likely to correctly predict the presence or absence of lakes identified in MODIS satellite imagery within an elevation range of 1000 to 1600 metres above sea level (m.a.s.l.) than it is to make incorrect predictions. Our model does not, however, simulate processes leading to lake stagnation or decay, such as refreezing or drainage – a process which affects approximately 17% of lakes in our study area (Selmes et al., 2011). This likely explains much of why our model over-predicts cumulative area by 32% although other factors including uncertainty in the DEM and in the MODIS derived observations used for validation contribute to this error. Simulated lake filling tends to lead observations by approximately 5 days which could be related to a filling period required to saturate cracks, crevasses and other porous space within the ice. We find that the maximum modelled lake covered ice sheet area is 6% and suggest that this is a topographic limitation for this sector. We can take this as an upper bound; given the absence of drainage in the model. In 2003, the difference between RCM estimates of runoff and the maximum volume of water simulated to be stored in lakes was 12.49 km3. This can be taken as a measure of potential water available for lubrication and is calculated to be 1.86 m3 per square metre of ice. This study has proved a good first step towards capturing the variability of supra-glacial lake evolution with a numerical model; we are optimistic that the model will develop further into a useful tool for use in analysing the behaviour of supra-glacial lakes on the Greenland ice sheet in the present day and beyond.