Errors in daily ablation measurements in northern Greenland, 1993-94, and their implications for glacier climate studies

Ablation-climate studies were made at two locations in northern Greenland in the summers of 1993 and 1994, respectively. Daily ablation was measured at ten stakes within a small area, and the data were compared with each other to detect gross errors. For example, high standard deviations for data taken on the same day, or low correlations between data series at different stakes, indicate erroneous data. After discarding data for one stake in 1993 and two stakes in 1994, random errors in daily ablation data for individual stakes are ± 5 kg m-2 d-1, which is further reduced to only about ± 2 kg m-2 d-1 by averaging over eight or nine stakes. Random errors in calculated energy balances using the present ablation data are much lower than found in earlier studies in West Greenland where ablation was only measured on three stakes without any attempt to detect gross errors. Aside from day-to-day errors, there are ±10% differences in mean ablation at different stakes, which are probably caused by small-scale variations in surface albedo. Such interstake differences give a ±10% uncertainty in positive degree-day factors, which are 9.8±0.9 and 5.9±0.6 kg m-2 d-1 deg-1 for the two sites.