Chemistry Reference
In-Depth Information
This contribution presents some results of research into the modelling of
the radar imaging of slicks, which are here assumed to be in the form of
surface films. The research was aimed at assembling a numerical, physics-
based model to calculate the radar image of a slick on the sea surface, and
at improving the consistency of the elements of this model. The elements
of the model are:
1. Equilibrium surface wave spectra, based on the balancing of the spectral
energy source and sink terms, both for clean and slick-covered water;
2. Relaxation rates prescribing the time scales of evolution of the surface
waves toward the equilibrium;
3. A model that calculates the actual local ocean surface wave spectrum,
based on relaxation towards the equilibrium spectrum; and
4. A radar backscatter model that yields normalised radar cross section
(NRCS), given a local ocean surface wave spectrum.
Concerning the first element, the equilibrium wave spectra, the literature
offers several forms, such as the Phillips, Pierson-Moskowitch or
JONSWAP spectra, the Donelan-Banner-Jähne spectra (Apel 1994) or the
spectra proposed by Romeiser et al. (1997). Most of these are parameter-
ised functions of wind speed, based on measurements, and have only a
limited basis in physical processes. Not all of them adequately describe the
short gravity-capillary waves which are the primary ones that radar is sen-
sitive to, and none of them are amenable to including slicks. Also for the
relaxation rate (the second element), the literature offers several parame-
terisations, such as Hughes (1978), Plant and Wright (1977) or Hsiao and
Shemdin (1983). Again, the effects of slicks cannot be included. In this
work, we have, therefore, sought to extend the ocean backscatter model
called “VIERS-1” of Janssen et al. (1998), who compute a gravity-
capillary equilibrium spectrum based on physics with parameterisations for
'unresolved' physical (sub-) processes. This ocean backscatter model was
originally developed for ERS scatterometry. Our aim was to obtain a con-
sistent modelling of both the equilibrium spectrum and the relaxation rates,
also in the presence of slicks.
Concerning the third element, the relaxation model, a radar imaging
model that was originally developed for bathymetry and ship wakes was
used. This is a quasi-two dimensional wave-current interaction model,
along the lines of e.g. Lyzenga and Bennett (1988) and Vogelzang (1989).
Parts of it have been described in Greidanus (1994). It has been validated
on radar bathymetry measurements and performs quite well (Greidanus et
al. 1997). The basis of the modelling is that water wave energy packets are
traced while their action changes by processes such as wind input, non-
linear interactions and dissipation. This action change is modelled as a re-
laxation toward an equilibrium spectral value. For the present application,
