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

Abstract

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 db 2 kg m−2d−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 stuthes 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 interstate 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.

Exploring uncertainty in glacier mass balance modelling with Monte Carlo simulation

“…Since the measurements are not error free (e.g., Braithwaite et al, 1998), the tuning will result in a systematic over-or underestimation of C b . Consequently, we have assigned a systematic uncertainty to this parameter and we selected the same value (0.0005) as used for the parameter sensitivity testing by Klok and Oerlemans (2002).…”

Near surface meltwater storage in low-density bare ice of the Greenland ice sheet ablation zone

“…In sum, the weathering crust represents an understudied component of the Greenland Ice Sheet hydrologic system with potential implications for surface mass and energy balance budgets (van den Broeke et al, 2008;Wheler and Flowers, 2011), retrieval of mass changes from surface height change observations (Braithwaite et al, 1998;LaChapelle, 1959;Müller and Keeler, 1969), and controls on the supraglacial hydrograph (Karlstrom et al, 2014;Munro, 2011). Our study presents the first measurements of weathering crust physical properties and documents non-trivial meltwater storage within the low-5 density, near surface ablating weathering crust in a mid-elevation Greenland Ice Sheet supraglacial catchment.…”

“…As 30 a result, mass change during periods of weathering crust development may be underpredicted or, during periods of weathering crust removal, overpredicted, if determined solely from ice surface elevation changes (Braithwaite et al, 1998;The Cryosphere Discuss., https://doi.org/10.5194/tc-2017-107 Manuscript under review for journal The Cryosphere Discussion started: 17 July 2017 c Author(s) 2017. CC BY 4.0 License.…”

“…First, subsurface meltwater generation within the weathering crust does not lower the ice sheet surface (Braithwaite et al, 1998;LaChapelle, 1959;Müller and Keeler, 1969).…”

“…Subsequent lateral drainage of this meltwater through the permeable weathering crust to supraglacial channels reduces near surface ice density, by removing mass with no detectable change in glacier surface height (Hoffman et al, 2014;Müller and Keeler, 1969). As such, mass change during periods of weathering crust development or removal cannot be resolved solely from ice surface elevation changes (Braithwaite et al, 1998;van den Broeke et al, 2008;Müller and Keeler, 1969). 20 Weathering crust formation has primarily been studied in Antarctica, where subsurface radiative heating produces subsurface melt in otherwise melt-free environments (Brandt and Warren, 1993;Budd, 1967;Hoffman et al, 2014;Liston et al, 1999;Liston and Winther, 2005).…”