Effects of bubbles, cracks, and volcanic tephra on the spectral albedo of bare ice near the Transantarctic Mountains: Implications for sea glaciers on Snowball Earth

• Published in: Journal of geophysical research. Earth surface 2013-09, Vol.118 (3), p.1658-1676
• Main Authors: Dadic, Ruzica, Mullen, Peter C., Schneebeli, Martin, Brandt, Richard E., Warren, Stephen G.
• Format: Article
• Language: English
• Subjects: Albedo; Antarctica; blue ice; Broadband; Bubbles; Climate models; Density; Earth; Earth science; Equator; Firn; Geophysics; Glaciers; Hills; Ice; Ice thickness; Marine; Mathematical models; Mountains; Parametrization; Porosity; radiative transfer modeling; Snow; snow/ice morphology; Snowball Earth; Sublimation; Wavelengths
• ISSN: 2169-9003; 2169-9011
• DOI: 10.1002/jgrf.20098

Spectral albedo was measured along a 6 km 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 6 km 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. Key Points Broadband albedos for Allan Hills clear sky conditions are 0.57-0.80. Seasonal temperature cycles modify the ice morphology and increase the albedo. Broadband albedo is parameterized as function of density for use in GCMs.