High-resolution modelling of the seasonal evolution of surface water storage on the Greenland Ice Sheet

摘要

Seasonal meltwater lakes on the Greenland Ice Sheet form when surface runoffis temporarily trapped in surface topographic depressions. The developmentof such lakes affects both the surface energy balance and dynamics of theice sheet. Although areal extents, depths and lifespan of lakes can beinferred from satellite imagery, such observational studies have a limitedtemporal resolution. Here, we adopt a modelling-based strategy to estimatethe seasonal evolution of surface water storage for the ~ 3600 km2 Paakitsoq region of W. Greenland. We use a high-resolution time-dependent surface mass balance model to calculate surface melt, asupraglacial water routing model to calculate lake filling and a prescribedwater-volume-based threshold to predict rapid lake drainage events. Thisthreshold assumes that drainage will occur through a fracture if V = F_a ⋅ H, where V is lake volume, H is the local ice thickness and F_ais the potential fracture area. The model shows good agreement betweenmodelled lake locations and volumes and those observed in nine Landsat 7 ETMimages from 2001, 2002 and 2005. We use the model to investigate the lakewater volume required to trigger drainage, and the impact that varying thisthreshold volume has on the proportion of meltwater that is stored insurface lakes and enters the subglacial drainage system. Model performanceis maximised with values of F_a between 4000 and 7500 m2. For thesethresholds, lakes transiently store < 40% of available meltwaterat the beginning of the melt season, decreasing to ~ 5 to10% by the middle of the melt season; over the course of a melt season,40 to 50% of total meltwater production enters the subglacial drainagesystem through moulins at the bottom of drained lakes.