The inversion of Tv and Ts from radiometric temperature is based on the assumption that the change of radiometric surface temperature with view angles is only caused by the changing fraction of vegetation cover in the field of view of the radiometer. Therefore, a pre-analysis is made to evaluate the quality of ATSR directional surface temperature prior to retrieve Tv and Ts from the brightness measurements. Three cases can be distinguished which will not be used in the inversion of and namely:
Fig. 2 shows there are some clouds dispersed in the IMGRASS ATSR subset. In the case that clouds fall in the field of view of the radiometer with the sensor looking at the surface at nadir view angle, while less or no clouds exist in the forward view, could be obser ved (see Fig. 3a). This situation may also be observed because of heterogeneity, i.e. a large fraction of vegetation is observed at nadir while it is mixed with a large fraction of bare soil in the forward view. Observations meeting this criterion, i.e. are not considered in our study.
This happens when the surface is rather homogeneous with either bare soil or full canopy, under which directional effects in the surface temperature are not significant. Considering the surface types in these two experiments area, full canopies exists most probably in the HEIFE subset and bare soil surface exists in IMGRASS subset for the pixels with 0 < TB0(A,,nadir) -Tbo(A. ,forward) < 0.5K. The threshold T = 0.5 K is the nominal accuracy of surface temperature estimates based on ATSR- radiances. This criterion implies that directional changes smaller than the accuracy of observed brightness temperature are neglected.
The directional brightness temperature measurements collected duirng IMGRASS gave a mean difference of between nadir and forward views (52° was taken in the field measurements) of 2.6 K. Kimes and Kirchner (1983) found 16.2 K differential between the 0 and 80 zenith view angles at noon and 0.9 K differential in the early morning on a cotton canopy with mean height of 44 cm and mean row spacing of 1 m. In their case, the large difference at high solar zenith is due to the large change in the portion of sunlit soil or shaded vegetation with view angle for a relatively higher canopy with row structure. A maximum difference up to 3.5 K for a corn canopy and 1.5 K for grass (with 20 cm height) between 0 and 60° were observed around solar noon by Lagouarde and Kerr (1993). For the HEIFE
subset with corn and bean surface, and for IMGRASS subset with sparse short grass the difference in TBo between nadir and forward view should not be significantly larger than 10K for instance. Much larger difference in TBo between nadir and forward view are probably caused by the fact that there are clouds in the forward view but no or less in the nadir view (see Fig. 3b).
The pixels in the three categories A, B and C described above are not considered in retrieving and from ATSR directional brightness surface temperature measurements in our study.
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