Climatic controls on albedo across the Western Greenland Ice Sheet Percolation Zone

Recent studies have argued that the average albedo of the Greenland Ice Sheet (GrIS) has been decreasing over recent decades, especially in western Greenland. Spatial and temporal variations in GrIS albedo and their causes remain poorly understood due to a complex relationship between snow accumulation, optical grain size, and both mineral dust and black carbon concentrations. MODerate resolution Imaging Spectroradiometer (MODIS) satellite measurements and regional climate models (RCMs) indicate albedo decline over the past decade, but the cause and extent of GrIS albedo change remains poorly constrained by field data. Previous studies have focused on albedo in the wet or dry snow zones, whereas here we analyze data from the rapidly changing western GrIS percolation zone. As fresh snow (albedo > 0.85) warms and melts, its albedo decreases due to snow grain growth, promoting solar absorption, higher snowpack temperatures and further melt.

Here we assess the relative importance of climatic factors such as cloud cover and blocking high-pressure systems on GrIS albedo. We validate RCMs and MODIS measurements with in situ measurements of broadband albedo (350-2500 nm at 3-8 nm resolution) and snow optical grain size at 55 locations as part of the 4000-km Greenland Traverse for Accumulation and Climate Studies (GreenTrACS). Preliminary results indicate that stronger Greenland blocking has reduced precipitation over the western GrIS, extending the snow grains' surface exposure time and lowering broadband albedo. Additionally, we find that decreased western GrIS cloud cover from 1996-2015 has caused a significant increase in shortwave radiation, driving an increase in temperature and surface grain size, which lowers the summertime albedo and contributes to increased melting. This observed decrease in precipitation, coupled with systematic albedo reduction, has significantly reduced GrIS SMB over the past two decades and could continue to do so if Greenland blocking remains strong in the future.