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

摘要

Large-scale positive degree-day based melt parameterisations for the Greenland ice sheet are highly sensitive to their parameters (standard temperature deviation, snow and ice degree-day factors). In this article, these parametrs have been simulated with a coupled atmosphere-snow regional climate model for the southern part of Greenland during the summer of 1991 forced at the lateral boundaries with ECMWF re-analysis. The calculated (from net ablation, i.e. ablation without refreezing) snow and ice positive degree-day factors vary considerably over the ice sheet. At low elevations, the modelled snow degree-day factor approaches closely the generally accepted value of 3 mm WE d-1 °C-1. Higher up the ice sheet, large values up to 15 mm WE d-1 °C-1 are simulated. In case of ice melt, maximum values until 40 mm WE d-1 °C-1 are found. The snow and ice positive degree-day factor distributions peak respectively at 3 and 8 mm WE 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 2m air temperature standard deviation exhibits a strong seasonal cycle with the highest (between 3.0° and 5.0°C) values in May and June. July shows the lowest temperature fluctuations due to melting of the surface.