By Gabriel Lewis, PhD candidate, Department of Earth Sciences, Dartmouth College, New Hampshire (US).
Image from GreenTrACS Video (greentracs.blogspot.com)
The stability of the Greenland Ice Sheet (GrIS) in a warming world is a critical research area with societally important implications for future sea level rise, with projected GrIS sea level contributions of at least 20 cm by 21001,2. Previous studies conclude that GrIS mass loss has been accelerating over the past decade, but spatial and temporal variations in GrIS mass balance remain poorly understood due to a complex relationship among precipitation and temperature changes, surface melt, runoff, ice discharge3, and surface albedo4. Satellite measurements indicate that albedo, the proportion of incoming solar radiation that is reflected by the glacier surface, has been declining over the past decade5,6, but the cause of the GrIS albedo change remains poorly constrained by field data. As fresh snow (albedo > 0.85)7 warms and melts, its albedo decreases due to snow grain growth, promoting solar absorption, higher snowpack temperatures and further melt8. However, dark impurities like soot and dust can also significantly reduce snow albedo, even in the dry snow zone2.
The NSF-funded project, “GreenTrACS: a Greenland Traverse for Accumulation and Climate Studies,” aims to collect continuous records of snow accumulation and meltwater percolation spanning the past 20-40 years from the western Greenland percolation and dry snow zones. During the 1700 km April - June 2016 traverse, we collected continuous ice-penetrating radar and seven 25-30 m firn cores, as well as 373 albedo measurements and snow samples using a FieldSpec® 4 for model calibration and validation (pictures and videos of the traverse are available at www.greentracs.blogspot.com).
Measuring Albedo with the spectrometer. Photo: Forrest McCarthy
We collected in situ albedo measurements over the 350-2500 nm range to compare with regional climate models (RCMs) and the Moderate-Resolution Image Spectroradiometer (MODIS) satellite. My presentation at the 2016 American Geophysical Union Annual Meeting showed that both RCM and MODIS albedo data accurately capture albedo spatial variability and agree well with field measurements (Figure 1). We see a negative correlation between optical grain size and albedo, but no correlation between impurity mass and albedo (Figure 2). These results are consistent with recent studies, where snow grain size has been shown to be 5-10 times more important in albedo reduction than black carbon content or snow density10,11.
Fig. 1. Completed GreenTrACS Year 1 traverse (white) showing 373 integrated snow albedo measurements at 35 locations. Year 2 traverse (grey) indicates proposed 2017 route. Background grid is albedo output from the Modèle Atmosphérique Régional climate model9.
Fig. 2. Negative correlation between optical grain size and albedo (R2 = 0.845, p = 0.005), but no significant correlation between total impurity volume and albedo (R2 = 0.0003, p = 0.96).
Citations: 1 IPCC (2007); 2 Dumont et al., Nature, 7 (2014); 3 Hawley et al., JGR, 375 (2014); 4 Box et al., J. Geophys. Res., 109 (2004); 5 Zwally et al., JGR, 88 (2005, 2011); 6 Stroeve, Remote Sensing of Environment, 262 (1997); 7 Hanna et al., Nature, 498 (2013); 8 Mosley-Thompson et al., J. Geophys. Res., 105 (2001); 9 Fettweis et al., The Cryosphere, 7 (2012); 10 Adolph et al., JGR, 121 (2016); 11 Tedesco et al., The Cryosphere, 477 (2016)
Thanks to Gabe from Dartmouth College, New Hampshire (US) for providing us with this recap!
For more information on ASD's snow and ice research solutions, visit the Ice Characteristics Research and Snow Research web page; For more information on ASD's Goetz Instrument Support Program, visit the Goetz Instrument Program web page.