Chemical Characteristics - Marine Surface Films - page 131

The ı rel values measured without rain (crosses and pluses in Figure 7,
corresponding to a slick-free and to a slick-covered water surface, respec-
tively) show that the reduction of the radar backscattering by monomo-
lecular slicks is lower at cross-polarisation than at co-polarisation. This
finding is in contrast to the results presented by Braun et al. (2002) and we
suspect that this is due to an insufficient signal-to-noise ratio encountered
during the herein presented measurements.
4 Summary and Conclusions
We have presented results of our laboratory measurements with artificial
rain impinging on slick-free and slick-covered water surfaces. They show
that the rain-induced roughness is not appreciably affected by slicks,
whereas wind waves are strongly damped.
We have studied the dependence of the wave orientation ratio (measured
with the two-dimensional laser slope gauge) on slick coverage and on wind
speed. It can be inferred from our results that, on a slick-covered water sur-
face, the rain-induced effects have an influence on the orientation of the
wave field, particularly on the high-frequency part (up to 25 Hz), which is
expected to be due to high frequency ring waves generated by the rain.
We also observed that, at wind speeds up to 8 m s -1 , the radar backscat-
tering at co- and cross-polarisation is mainly caused by the rain-induced
effects. Since similar results were obtained at slick-free water surfaces, we
conclude that the coverage of the water surface by a monomolecular slick
has no significant influence on the measured radar backscatter. However,
we expect larger differences are to be found at lower rain rates.
Though our experiments were performed in a wind wave tank, where
volume scattering at rain drops is negligible (cf. Melsheimer et al. 1998),
our results may be transformed to lower radar frequencies. The splash
products may strongly affect the radar backscattering at C-band, but vol-
ume scattering occurs only at very high rain rates. Therefore, our results
are important for the interpretation of SAR images acquired by spaceborne
SAR sensors at C-band (e.g., by those flown on the ERS, Envisat, and Ra-
darsat satellites). That is, our results are suitable for the explanation of ra-
dar signatures of rain cells over the ocean, when its surface is covered by
slicks or by mineral oil spills. However, in order to allow better knowledge
of the influence of oceanic surface films on the imaging of strong rain
events (and vice versa ) additional experiments, particularly with lower rain
rates and at different radar frequencies, are proposed.