Modelling of large-scale melt parameters with a regional climate model in south Greenland during the 1991 melt season

Abstract

Large-scale positive degree-day based melt parameterizations for the Greenland ice sheet are highly sensitive to their parameters (standard temperature deviation, snow and ice degree-day factors). In this paper, these parameters are simulated with a coupled atmosphere-snow regional climate model for southern Greenland during summer 1991, forced at the lateral boundaries with European Centre for Medium-Range Weather Forecasts re-analyses at a high horizontal resolution of 20 km. The calculated (from net ablation, i.e. melt minus refreezing) snow and ice positive degree-day factors vary considerably over the ice sheet. At low elevations, the modelled snow degree-day factor closely approaches the generally accepted value of 3 mm w.e. d-1 °C-1. Higher up the ice sheet, large values up to 15 mm w.e. d-1 °C-1 are simulated. For ice melt, maximum values of 40 mm w.e. d-1 °C-1 are found. The snow and ice positive degree-day factor distributions peak, respectively, at 3 and 8 mm w.e. d-1 °C-1. Refreezing is of small importance close to the ice-sheet margin. Higher up the ice sheet, refreezing considerably lowers the amount of net ablation. The monthly simulated 2 m air-temperature standard deviation exhibits a strong seasonal cycle, with the highest (3.0-5.0°C) values in May and June. July shows the lowest temperature fluctuations, due to the melting of the surface.