Measured spectral bidirectional reflection properties of four vegetated surfaces.

Spectral bidirectional reflectance values are presented at the 0.52-microm wavelength based on measured values of the radiation field of four vegetated surfaces: savannah, bog, pasture land, and coniferous forest, which cover a wide range of natural vegetated canopies. The results are given as examples of the full set of bidirectional reflectance values which consists of data at seven wavelengths between 0.43microm and 2.20 microm for each of the four surfaces. (From July 1977, the full set of data is available from the author on request.) The data may be applied for calculations of the radiative transfer in the atmosphere with realistic ground properties instead of isotropic albedo values.

Influence of measured reflection properties of vegetated surfaces on atmospheric radiance and its polarization.

Based on measured values of the spectral bidirectional reflection functions of four vegetated surfaces, the influence of their angular anisotropy on the upward and downward emerging radiance and its polarization is calculated. By means of a realistic model of the atmosphere and with the assumption of completely depolarizing reflection properties of the surfaces, results are obtained in dependence of wavelength and solar zenith angle. The angular anisotropy influences considerably the upward emerging radiance. On the degree of polarization and on the downward emerging radiance the anisotropy has negligible to small influence. Due to the angular anisotropy of the reflection properties the spectral albedo depends strongly on the solar zenith angle. This influences upward and downward radiance as well as its degree of polarization. Therefore, for the interpretation of radiation measurements, those spectral albedo values should be used which correspond to the respective solar zenith angle. This is essential especially at longer wavelengths where vegetated surfaces have high spectral albedos.