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Analysis of passive microwave satellite observations over the Greenland ice sheet reveals a significant increase in surface melt over the period 1979-2005. Since 1979, the total melt area was found to have increased by +1.22 x 10(7) km(2). An improved version of the cross-polarized gradient ratio (XPGR) technique is used to identify the melt from the brightness temperatures. The improvements in the melt retrieval XPGR algorithm as well as the surface melt acceleration are discussed with results from a coupled atmosphere-snow regional climate model. From 1979 to 2005, the ablation period has been increasing everywhere over the melt zone except in the regions where the model simulates an increased summer snowfall. Indeed, more snowfall in summer decreases the liquid water content of the snowpack, raises the albedo and therefore reduces the melt. Finally, the observed melt acceleration over the Greenland ice sheet is highly correlated with both Greenland and global warming suggesting a continuing surface melt increase in the future.

[1] The objective of this paper is to present the validation over Greenland of a thermodynamic snow-ice model that was complemented with the snow metamorphism and albedo parameterizations of the Centre d'Etudes de la Neige (Grenoble, France) in order to make the surface albedo variable and interactive. Special attention is given to the surface albedo since it is the most important parameter in energy exchanges with the atmosphere for snow and ice melt. The development of an integrated (snow, ice, and water) albedo model takes into account the different surface types observed on an ice sheet, and the snow albedo is calculated from the simulated surface snow grains. The validation for a polar site has been done at ETH Camp (West Greenland, 1155 m above sea level) during the 1990 and 1991 summer seasons. Although both ablation seasons differed greatly (in 1990 it showed a negative mass balance while in 1991 it ended with a positive mass balance), a single model configuration was able to provide good results for both 1990 and 1991. These simulations show that the snow metamorphism laws included enable an accurate simulation of the surface albedo and henceforth of the surface mass balance for a polar site, provided that the snow model is correctly forced at its surface level and that meltwater retention, percolation, and drainage are well represented in the snow model. The model results are also compared with two other modeling approaches, and differences between the three snow models are detailed.

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 kin. 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) degreesC(-1). Higher up the ice sheet, large values up to 15 mm w.e. d(-1) degreesC(-1) are simulated. For ice melt, maximum values of 40 mm w.e. d(-1) degreesC(-1) are found. The snow and Ice positive degree-day factor distributions peak, respectively, at 3 and 8 mm w.e. d(-1) degreesC(-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 in air-temperature standard deviation exhibits a strong seasonal cycle, with the highest (3.0-5.0degreesC) values in May and June. July shows the lowest temperature fluctuations, due to the melting of the surface.

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.