19. REMOTE SENSING OF ALBEDO USING THE BRDF IN
RELATION TO LAND SURFACE PROPERTIES
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all 5 bands of the AVHRR and temporal differencing on the hourly
geostationary GOES-8 data. Cloud shadows and pixels immediately adjacent
to clouds were also removed. Atmospheric corrections were performed using
visibility data from ground stations in the area to characterize aerosol optical
depth and the 6S atmospheric radiative transfer code (Vermote et al., 1997).
The AVHRR data display an almost constant solar angle while the viewing
zenith angle varies from one orbit to another with observations located at
constant relative azimuth. The GOES data are characterized by a constant
viewing zenith angle and varying solar zenith and relative azimuth angles.
The resulting multiangular reflectance observations data set was inverted
using the reciprocal version of the RossThick-LiSparse BRDF model
(Wanner et al., 1995; Lucht, 1998) on all pixels with at least 7 observations.
Figure 1 shows, in panel A, the black-sky albedo derived from the BRDF
model parameters for a solar zenith angle of 0 degrees. Since the
observations were made at solar zenith angles between about 40 and 60
degrees, this image represents an extrapolation of the albedo observations to
an angular range not originally sampled, through the inversion of a BRDF
model. Black-sky albedo at all solar angles is relevant to land surface
modeling with respect to diffuse-sky situations.