Journal
JOURNAL OF GEOPHYSICAL RESEARCH-EARTH SURFACE
Volume 118, Issue 3, Pages 1658-1676Publisher
AMER GEOPHYSICAL UNION
DOI: 10.1002/jgrf.20098
Keywords
albedo; blue ice; Snowball Earth; radiative transfer modeling; snow; ice morphology; Antarctica
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Funding
- U.S. NSF [ANT-07-39779, ANT-11-42963]
- Swiss National Science Foundation [124273]
- Directorate For Geosciences
- Office of Polar Programs (OPP) [1142963] Funding Source: National Science Foundation
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Spectral albedo was measured along a 6km transect near the Allan Hills in East Antarctica. The transect traversed the sequence from new snow through old snow, firn, and white ice, to blue ice, showing a systematic progression of decreasing albedo at all wavelengths, as well as decreasing specific surface area (SSA) and increasing density. Broadband albedos under clear-sky range from 0.80 for snow to 0.57 for blue ice, and from 0.87 to 0.65 under cloud. Both air bubbles and cracks scatter sunlight; their contributions to SSA were determined by microcomputed tomography on core samples of the ice. Although albedo is governed primarily by the SSA (and secondarily by the shape) of bubbles or snow grains, albedo also correlates highly with porosity, which, as a proxy variable, would be easier for ice sheet models to predict than bubble sizes. Albedo parameterizations are therefore developed as a function of density for three broad wavelength bands commonly used in general circulation models: visible, near-infrared, and total solar. Relevance to Snowball Earth events derives from the likelihood that sublimation of equatorward-flowing sea glaciers during those events progressively exposed the same sequence of surface materials that we measured at Allan Hills, with our short 6km transect representing a transect across many degrees of latitude on the Snowball ocean. At the equator of Snowball Earth, climate models predict thick ice, or thin ice, or open water, depending largely on their albedo parameterizations; our measured albedos appear to be within the range that favors ice hundreds of meters thick.
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