Chemistry Reference
In-Depth Information
The form of the non-equilibrium net source term was derived from the
spectral model. Also relaxation rates were derived, which in the saturation/
gravity and far capillary ranges led to good results, but produced negative
values in the short gravity/near capillary transition range. Negative relaxa-
tion rates are unacceptable as they will not lead to an equilibrium. Never-
theless, the VIERS-1 spectrum has been validated by X- and C-band scat-
terometry and is similar in shape to other validated spectral models. There-
fore, we accept the extended VIERS model for computation of the spec-
trum but at the same time conclude that it is not yet suitable to yield rela-
xation rates.
The final goal of constructing a consistent slick radar imaging model
could, therefore, only partially be met. It was possible to derive three as-
pects in a consistent manner,
viz.
an equilibrium wave spectrum for a clean
surface, one for a slick-covered surface, and the form of the source term
(quadratic). A fourth aspect, the actual relaxation rates, could only be
meaningfully derived on part of the spectral range. Relaxation rates, there-
fore, had to be supplied externally, which was achieved by using the Plant
and Wright (1977) parameterisation. In combination with a composite ra-
dar backscatter model, this leads to a physically based model that is able to
compute radar contrasts across slicks. Sample results of the model show
that at high radar frequencies, the slick appears extended on the leeward
side on account of the quadratic source term leading to a slower relaxation
on that side. Especially at lower radar frequencies, relaxation effects give
the slick smooth edges and make it appear to be displaced in the wind di-
rection. These effects become larger at lower wind speeds.
The slick imaging model has not been validated with measurements.
However, a provisional comparison with observations from the literature,
e.g. Gade (2006), shows that the increase of damping ratio with Bragg
wave number (Table 1) is indeed observed, as well as the very low sensi-
tivity to polarisation.
Acknowledgements.
The availability of parts of the VIERS-1 software is
gratefully acknowledged. This work was partly funded by the EC MAST
program, contract MAS3-CT95-0035.
6 References
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