Optical Properties of Snow
COOPERATIVE INST FOR RESEARCH IN ENVIRONMENTAL SCIENCE BOULDER CO
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Measurements of the dependence of snow albedo on wavelength, zenith angle, grain size, impurity content, and cloud cover can be interpreted in terms of single-scattering and multiscattering radiative transfer theory. Ice is very weakly absorptive in the visible minimum absorption at lambda 0.46 micrometer but has strong absorption bands in the near infrared near IR. Snow albedo is therefore much lower in the near IR. The near-IR solar irradiance thus plays an important role in snowmelt and in the energy balance at a snow surface. The near-IR albedo is very sensitive to snow grain size and moderately sensitive to solar zenith angle. The visible albedo for pure snow is not sensitive to these parameters but is instead affected by snowpack thickness and parts-per- million amounts or less of impurities. Grain size normally increases as the snow ages, causing a reduction in albedo. If the grain increases as a function of depth, the albedo may suffer more reduction in the visible or in the near IR, depending on the rate of grain size increase. The presence of liquid water has little effect per se on snow optical properties in the solar spectrum, in contrast to its enormous effect on microwave emissivity. Snow albedo is increased at all wavelengths as the solar zenith angle increases but is most sensitive around lambda 1 micrometer. Many apparently conflicting measurements of the zenith angle dependence of albedo are difficult to interpret because of modeling error, instrument error, and inadequate documentation of grain size, surface roughness, and incident radiation spectrum. Cloud cover affects snow albedo both by converting direct radiation into diffuse radiation and also by altering the spectral distribution of the radiation.