Abstract
Ice surface albedo is a primary modulator of melt and runoff, yet our understanding of how reflectance varies over time across the Greenland Ice Sheet remains poor. This is due to a disconnect between point or transect scale albedo sampling and the coarser spatial, spectral and/or temporal resolutions of available satellite products. Here, we present time-series of bare-ice surface reflectance data that span a range of length scales, from the 500 m for Moderate Resolution Imaging Spectrometer’s MOD10A1 product, to 10 m for Sentinel-2 imagery, 0.1 m spot measurements from ground-based field spectrometry, and 2.5 cm from uncrewed aerial drone imagery. Our results reveal broad similarities in seasonal patterns in bare-ice reflectance, but further analysis identifies short-term dynamics in reflectance distribution that are unique to each dataset. Using these distributions, we demonstrate that areal mean reflectance is the primary control on local ablation rates, and that the spatial distribution of specific ice types and impurities is secondary. Given the rapid changes in mean reflectance observed in the datasets presented, we propose that albedo parameterizations can be improved by (i) quantitative assessment of the representativeness of time-averaged reflectance data products, and, (ii) using temporally-resolved functions to describe the variability in impurity distribution at daily time-scales. We conclude that the regional melt model performance may not be optimally improved by increased spatial resolution and the incorporation of sub-pixel heterogeneity, but instead, should focus on the temporal dynamics of bare-ice albedo.
Highlights
Melt and runoff from the Greenland Ice Sheet has been greater this century than during the previous 350 years [1]
To assess the importance of temporal variation in areal mean reflectance of the bare-ice zone of western Greenland, we explored the efficacy of simulations of cumulative ablation against observed ablation over the ROI by reporting the mean absolute error (MAE), the root mean squared error (RMSE) and the Nash-Sutcliffe Efficiency criterion (NSE: [88]) and Willmott Index of Agreement (d: [89]) which are predictive skill metrics commonly used in hydraulic modelling (Table 1)
At the highest spatial resolution, varying but consistently positive skewed reflectance distributions were observed throughout the 2016 peak melt season
Summary
Melt and runoff from the Greenland Ice Sheet has been greater this century than during the previous 350 years [1]. Each have contributed to record runoff, ice sheet mass loss, and global sea-level rise [10]. The bare-ice ablation area contributes up to 80% of total runoff [11], with the majority of melt driven by shortwave radiation fluxes [12,13]. Recent satellite-based studies have reported long-term darkening of the ablation zone [16,17,18], underpinned by spatially variable inter- and intra-annual evolution of bare-ice reflectance [14,19,20]. The detail of short-term spatiotemporal reflectance dynamics in Greenland’s bare-ice regions remains poorly resolved and crudely represented in regional climate models. This, represents an important research goal for accurate projection of the ice sheet’s surface mass balance and contribution to sea-level rise [21,22]
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