Temporal Variability of Surface Reflectance Supersedes Spatial Resolution in Defining Greenland’s Bare-Ice Albedo

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.

Thin debris layers do not enhance melting of the Karakoram glaciers

The assessment of meltwater sourcing from the clean and debris-covered glaciers is scarce in High Mountain Asia (HMA). The melting rate varies with the debris cover thickness and glacier orientation. The present study quantifies glacier melting rate attributed to varying thickness of debris cover in the Karakoram. We observed daily melting rates by installing ablation stakes over debris-free and debris-covered ice during a field expedition. The stakes were installed on glacier surface with debris cover thickness ranges between 0.5 and 40 cm at selected experimental sites during the ablation period (September and October 2018) and (July to August 2019). We selected three glaciers including Ghulkin, Hinarchi, and Hoper facing east, south, and north, respectively to assess the role of glacier orientation on melting rates. We observed that the debris-free ice melts faster than the debris-covered ice. Intriguingly, a thin debris layer of 0.5 cm does not enhance melting compared to the clean ice which is inconsistent with the earlier studies. The melting rate decreases as the thickness of debris cover increases at all the three selected glaciers. Furthermore, south-facing glacier featured the highest melting (on average ~ 25% more). However, the north and east-facing glaciers revealed almost same melting rates. We observed that the average degree-day factors (DDF) slightly varies within a range of 0.58-0.73 and 0.55-0.68 cm °C^-1 day^-1 for debris-free and 0.5 cm debris-covered ice, respectively, however, DDF largely reduces to 0.13-0.25 cm °C^-1 day^-1 for 40 cm debris-covered ice. We suggest continuous physical glacier ablation observations for various debris cover throughout the ablation zone to better understand the role of debris on melting.

Observing and Modeling Ice Sheet Surface Mass Balance

Surface mass balance (SMB) provides mass input to the surface of the Antarctic and Greenland Ice Sheets and therefore comprises an important control on ice sheet mass balance and resulting contribution to global sea level change. As ice sheet SMB varies highly across multiple scales of space (meters to hundreds of kilometers) and time (hourly to decadal), it is notoriously challenging to observe and represent in models. In addition, SMB consists of multiple components, all of which depend on complex interactions between the atmosphere and the snow/ice surface, large-scale atmospheric circulation and ocean conditions, and ice sheet topography. In this review, we present the state-of-the-art knowledge and recent advances in ice sheet SMB observations and models, highlight current shortcomings, and propose future directions. Novel observational methods allow mapping SMB across larger areas, longer time periods, and/or at very high (subdaily) temporal frequency. As a recent observational breakthrough, cosmic ray counters provide direct estimates of SMB, circumventing the need for accurate snow density observations upon which many other techniques rely. Regional atmospheric climate models have drastically improved their simulation of ice sheet SMB in the last decade, thanks to the inclusion or improved representation of essential processes (e.g., clouds, blowing snow, and snow albedo), and by enhancing horizontal resolution (5-30 km). Future modeling efforts are required in improving Earth system models to match regional atmospheric climate model performance in simulating ice sheet SMB, and in reinforcing the efforts in developing statistical and dynamic downscaling to represent smaller-scale SMB processes.