10. EXPLOITATION AND EVALUATION OF RETRIEVAL
ALGORITHMS FOR GEOSTATIONARY SATELLITE DATA
PROCESSING
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vegetation in the observed radiances. It therefore represents a challenging
radiative transfer problem.
A typical Northern Hemisphere mid-latitude pixel has been considered,
covered by a homogeneous canopy with a LAI of two. The spectral
characteristics of the leaves and the soil are shown in Figure 2. A month
(April) of observations has been simulated during which the LAI increases
linearly from 2 to 3. The transfer of radiation in the canopy is computed with
the model of Gobron et al. (1996). The corresponding top-of-atmosphere
reflectance in the AVHRR channels 1 and 2 and SEVIRI RED and NIR
channels have been modeled with the 6S code (Vermote et al. 1997),
assuming noise-free instruments and perfectly calibrated data. The impact of
this LAI increase on radiances, as observed by the two satellites, is then
compared.
Figure 3 shows the simulated top-of-atmosphere reflectances. The
SEVIRI reflectances are plotted only when the Sun zenith angle is smaller
than 70 degrees. The SEVIRI and AVHRR NIR mean level reflectances
differ by about 25%. AVHRR channel 2 spectral response extends indeed up
to 1
and is therefore largely affected by the water vapor absorption band
centered around 0.9
This difference illustrates the importance of the
water vapor absorption on the observed radiances in AVHRR channel 2.