Physics – Optics
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.c72a..03w&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #C72A-03
Physics
Optics
1863 Snow And Ice (1827), 3307 Boundary Layer Processes, 3359 Radiative Processes, 3360 Remote Sensing, 4540 Ice Mechanics And Air/Sea/Ice Exchange Processes
Scientific paper
The radiative properties of snow depend strongly on wavelength because the absorption coefficient of ice varies by eight orders of magnitude from the ultraviolet to the infrared. The reflectance, transmittance, absorptance, and emissivity of snow are determined by the distances that photons travel through ice between air-ice interfaces (i.e., between opportunities for scattering). Thus the grain size is the most important variable. To characterize the size of a nonspherical snow grain by a single number, the most relevant dimension is not the long dimension but rather the short dimension, which is proportional to the volume-to-area ratio. This effective optical grain size normally increases during destructive metamorphism, leading to increased path-lengths of photons through snow grains, more absorption and lower albedo. Alternatively, sorting of drift snow by wind can result in a concentration of the smallest grains at the top surface, raising the albedo. The flux-penetration depth of radiation into snow can be several centimeters for visible light but less than a millimeter in the near-infrared, so the near-infrared reflectance is sensitive only to the surface grain size. A spectral signature thus results from layered snowpacks, which has been observed in measurements of spectral albedo on the Antarctic Plateau, where grain size increases with depth. Snow cover on sea ice is often thin and vertically inhomogeneous, but very little snow is needed to effectively hide the underlying ice. Just 7 mm of snow can raise the albedo of thick ice from 0.5 to 0.8. The angular distribution of the reflected radiation, knowledge of which is needed for remote sensing of snow in the solar spectrum, is affected not only by grain size but also by surface roughness, particularly sastrugi. However, the effects of sastrugi are mostly restricted to viewing-zenith angles greater than 50 degrees, so near-nadir viewing is recommended. References: Brandt, R.E., and S.G. Warren, 1993: Solar heating rates and temperature profiles in Antarctic snow and ice. Journal of Glaciology, 39, 99-110. Grenfell, T.C., and S.G. Warren, 1999: Representation of a nonspherical ice particle by a collection of independent spheres for scattering and absorption of radiation. J. Geophys. Res., 104, 31697-31709. Grenfell, T.C., S.G. Warren, and P.C. Mullen, 1994: Reflection of solar radiation by the Antarctic snow surface at ultraviolet, visible, and near-infrared wavelengths. J. Geophys. Res., 99, 18669-18684. Masonis, S.J, and S.G. Warren, 2001: Gain of the AVHRR visible channel as tracked using bidirectional reflectance of Antarctic and Greenland snow. International Journal of Remote Sensing, 22, 1495-1520. Warren, S.G., 1984: Optical constants of ice from the ultraviolet to the microwave. Applied Optics, 23, 1206-1225. Warren, S.G., C.S. Roesler, and R.E. Brandt, 1997: Solar radiation processes in the East Antarctic sea ice zone. Antarctic J. U.S., 32, 185-187. Warren, S.G., R.E. Brandt, and P. O'Rawe Hinton, 1998: Effect of surface roughness on bidirectional reflectance of Antarctic snow. J. Geophys. Res. (Planets), 103, 25789-25807.
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