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FREQUENCY
Lines of £ 1 standard deviation above and below the
average spectra of dry winter snow, 48 samples of
4 winter seasons.
Figure 1 :
layers, well isolated from the soil, are composed of fine grain cristals which
rather act as absorbing than as scattering layers , thus increasing the brightness
temperature. Corresponding to the ratio of wavelength and cristal sizes involved
and due to the Rayleigh scattering properties the scattering is particularly
pronounced for the short wavelengths. The direct measurement at 36 GHz or the
difference of brightness temperatures between 36 and e.g. 21 GHz yields the
water equivalent of the snow cover , unambiguously up to 20 cm.
Figure 2 shows the * standard deviation lines for wet snow as a
function of the frequency, again for vertical and horizontal polarization and
50? incidence angle. The brightness temperature tends to increase with frequency,
this in contrast to the winter snow. This behaviour can be explained by the
existence of water in the liquid phase as droplets and bridges between the snow
cristals. Water has a strong Debye-relaxation around 10 GHz, resulting in a
strong decrease of the dielectric constant between 5 GHz and 20 GHz. Therefore
water inclusions are acting as much more efficient scatterers at the lower
frequencies. The decrease of the 5 GHz brightness temperature as a function of
increasing volumetric water content has been shown earlier (Schanda and
Maetzler, 1981).
Unfortunately there exists an unavoidable ambiguity in the radiometric
discrimination between wet snow and soil (Maetzler et al., 1982). However the
backscatter properties of Radar waves allow this discrimination.
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