The Role of Increased Surface Melt in Movement of the Runoff Limit on Ice Sheets

One uncertainty in ice sheet mass balance is the destination of surface meltwater. The runoff limit is the elevation above which surface melt percolates into the underlying cold snow and refreezes, thus the local mass balance above this elevation remains unchanged due to melt. Below the runoff elevation, the annual quantity of melt is great enough to supply the heat required to raise the snow temperature to the melting point and to satisfy the residual water content requirement of the firn, leaving enough meltwater to travel downslope and out of the glacier system. Passive microwave data from the Scanning Multi-channel Microwave Radiometer (SMMR) and Special Sensor Microwave Imager (SSM/I) have shown that melt is occurring at higher elevations (further inland) on the Greenland Ice Sheet, which may lead to an increase in the elevation of the runoff limit. It is unknown how many years of melt are required to move the location of the runoff limit to a higher elevation. A theoretical model shows this time to be on the order of tens of years, which is much less than the time required to completely fill the underlying firn pore space with superimposed ice (order 100's years). Increased meltwater flow into the underlying firn will also alter the stratigraphy. Subsurface flow in the firn occurs heterogeneously and forms horizontal ice layers and vertical ice pipes on refreezing. These structures (especially ice layers) may have a strong influence on subsequent water flow in the firn and facilitate downslope transport of water along perched horizons, leading to greater runoff of melt. Ground-based and space borne radar together with theoretical modeling are used to determine the combined effect of surface melt features on the elevation of the runoff limit of ice sheets.