Chemistry Reference
In-Depth Information
the current field is set to zero. The outcome of this calculation is a full
wave spectrum at a series of points along a cross cut over the water sur-
face, that may be input into a radar backscatter algorithm (the fourth ele-
ment) to give a radar backscatter (NRCS) profile. For the radar backscatter
model a “standard” composite surface model was used.
In what follows, the above elements will successively be discussed. In
section 2, the VIERS-1 equilibrium spectrum will be briefly described, fol-
lowed by a discussion of the extensions that have been implemented in or-
der to be able to better use it for the present purpose of slick modelling. In
section 3, an attempt is made to derive relaxation rates from this spectrum,
and the form of the net restoring source term when out of equilibrium is
established. In section 4, the radar image modelling will be discussed. Sec-
tion 5, finally, discusses and summarises the results.
2 Equilibrium spectrum
2.1 The VIERS-1 spectrum
The VIERS-1 model (Janssen et al. 1998) is a physically based theoretical
ocean backscatter model for intermediate incidence angles. It comprises an
ocean wave equilibrium spectral model and a radar backscatter model. It
was developed in particular to model the relation between ambient ocean
parameters and observed radar backscatter from the ERS C-band scattero-
meter, in order to be able to derive more accurate wind fields from that in-
strument. Because a number of physical processes are included in the
modelling, it lends itself for rather general use as a tool in interpreting
maritime radar backscatter. The main effects that are taken into account in
the wave spectral part include: wind speed, fetch (or wave age), air and sea
temperature (and difference), surface tension, viscosity, Marangoni effect
(for surface slicks), wave breaking (dissipation), 3- and 4-wave non-linear
interactions and wind input. The physical modelling, however, is only ap-
plied in the high wave number tail of the spectrum, i.e. in the capillary re-
gime and the transition regime between capillary and gravity waves. At a
low level, many of the effects mentioned are parameterised; for example,
there is a parameter
Į
3
(function of wavelength) that describes the strength
of the 3-wave interactions and an
Į
4
for the 4-wave interactions. While the
parameters have been tuned to X-band scatterometer measurements in
wave tanks and on the open sea, the fit to ERS C-band scatterometer data
is almost as good as the fully empirical CMOD4, and even better at very
low and very high winds.
Here, the VIERS-1 model is only briefly sketched; a more complete de-
scription can be found in Janssen et al. (1998) and Snoeij et al. (1993). The
