Surface radiation balance in the ablation zone of the west Greenland ice sheet

We present 4 years of radiation observations and derived cloud properties from the ablation zone of the west Greenland ice sheet (67°N). Data were collected using three automatic weather stations located at 6, 38, and 88 km from the ice sheet margin at elevations of 490, 1020, and 1520 m asl. This part of Greenland is characterized by a ∼150 km wide tundra, a ∼100 km wide ablation zone and an average equilibrium line altitude of ∼1500 m asl. At the lowest station, snow is redistributed by the wind in crevasses and gullies, leading to very little measured winter accumulation. As a result, glacier ice (albedo ≈ 0.55) is at the surface throughout the melting season. At 1020 m asl, the winter snow cover typically disappears in mid to end June. At 1520 m asl, superimposed ice briefly reaches the surface at the end of the ablation season. The combined effect of decreasing surface albedo and increasing cloud optical thickness as summer progresses causes absorbed shortwave radiation to peak in June at low elevations but progressively later at higher elevations. Incoming longwave radiation peaks in August, in response to increased cloud optical thickness and heating of the atmosphere over the snow and ice‐free surroundings. The ice sheet margin experiences continuous melting in June, July, and August. This limits the emission of longwave radiation, causing net longwave radiation to peak in August, further enhancing melt. The sum of these effects is that summer net radiation increases sharply toward the ice sheet margin. To resolve this correctly requires high‐resolution climate models, in the order of 10 km or better.