681
optically thin and located over a sea ice. The effect of the bright surface, in that case,
dominates.
Figure 10 : Spectral signature over snow and cloud (9v = 0 degree))
Thus, by examining the angular and spectral features of the total reflectance only, the
presence of the stratocumulus clouds, visually observed aboard the aircraft, cannot be
detected with certainity in the POLDER images.
The polarized reflectance, by contrast, exhibits drastically different patterns for snow/ice
and clouds. An arc of relatively high values is characteristic of the clouds, indicating that
different processes are at work to polarize solar ligth incident on the two types of target
(Figure 11).
0.06
0.05
0.04
0.03
0.02
0.01
0
Figure 11 :Polarized reflectance versus scattering angle at 850 nm for snow and cloud targets.
The polarization data over clouds around 140 degrees scattering angle are about 10 times
larger than over snow. This increase is not observed in the measurements over snow-ice.
Over snow/ice, we basically observe the polarization contribution from air molecules
(negligible at 850 nm) and aerosols. The reflectance around 140 degrees, in fact, is a very
well known features. It has already been discuss in section 3. The nature of the target
(snow-ice or clouds), therefore, can be identified by looking at the scattering angle
