Approach To Derivation Of Avhrr Albedo Algorithm

The albedo of a surface is the ratio of upwelling radiative flux density to downwelling radiative flux density at the surface, and radiation propagating in all directions over the upward and downward hemispheres, respectively, is included. While the surface strongly controls the albedo, it also depends on the angular distribution of the downwelling radiation, which is different, for instance, for clear and cloudy skies. These comments apply to monochromatic radiation. They also apply to a broad spectral band such as the visible, near infrared, or solar (shortwave) bands, if the radiative fluxes are integrated over the spectral band. Surface broadband albedos are affected not only by the spectral reflectance properties of the surface but by the spectral distribution of the downwelling radiation.

In general the reflectance of a surface depends on the view direction, sun direction, and wavelength, and is formally described by the spectral bidirectional reflectance distribution function (BRDF). The direction from which AVHRR views a particular surface point varies with a cycle of about ten days (Figure 1). At mid-latitudes, at the start of the cycle the views are from roughly west at a zenith angle at the surface of approximately 60°, progress through nadir, and finish being from roughly east at 60° zenith angle. For the mid-afternoon satellite passes the variation of view direction corresponds to a shift from reflection in the backward direction to reflection in the forward direction, and commonly introduces a variation in reflectance of as much as 30 to 50%. Figure 2 shows a time series of AVHRR channel 1 reflectance at Tinga Tingana in the Strzelecki Desert in South Australia. The target is clear for the entire 40-day series. The site is very sparsely vegetated and has very little rainfall and can be presumed to be unchanging through the period shown. Cyclic variations in reflectance of relative amplitude 20% are clear and can be ascribed to the varying view geometry.

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