Chemistry Reference
In-Depth Information
tilting) and a specular component (only important at steeper incidence an-
gles) (e.g., Valenzuela 1978).
The results are shown in Fig. 8, for several radar wavelengths and inci-
dence angles. For reference, the Bragg wavelengths of the radar confi-
gurations used are listed in Table 1. Only the results for VV polarisation
are shown, those for HH are quite similar. It can be seen that at high Bragg
wave numbers, the radar contrast is very large; there is practically no radar
reflection from the slick. In C-band, when going from typical airborne in-
cidence angles ( 40°) to the ERS case (23°), the radar backscatter damp-
ing is substantially reduced. This is mostly a consequence of the shape of
the wave spectrum, that has on the slick still appreciable energy at
k
= 87
rad m
-1
but much less at
k
143 rad m
-1
(at 5 m s
-1
wind), but also the re-
sult of the specular contributions to the backscatter, that come into play at
23°. Since the long waves are damped much less than the shorter ones, L-
and P-band contrasts are low, with especially in P-band the maximum con-
trast not being reached on account of the small slick size with respect to
the relaxation distance.
The lower damping at the long water wavelengths also reduces the con-
trast at the shorter radar wavelengths via the two-scale effect: when only
Bragg backscatter is taken into account, the expected C-band contrasts
would be much larger.
Table 1.
Radar configurations used in Fig. 8, their Bragg wave numbers and
maximum damping ratios attained.
Radar
band
Radar wave-
length [cm]
Incidence
angle [°]
Bragg wave-
number [rad m
-1
]
Maximum
damping [dB]
X
3.3
55
312
40
C
5.7
40
143
13.7
C
5.7
23
87
2.7
L
24
55
43
1.5
P
68
55
15
0.3
The finite relaxation rate smoothes the edges of the slick in the modelled
radar image. For strong damping, this effect is stronger on the leeward side
of the slick than on the windward side. This is due to the factor
N
/
N
eq
in
the source term producing smaller relaxation rates at lower absolute spec-
tral levels. Because this form of the source term derives from the analysis
of section 3, this should be a realistic effect. (In reality, one may expect
additional effects from the wind action on the slick distribution on the wa-
ter surface that may also give rise to different radar profiles for the leeward
and windward sides.) Because of this, the apparent size of the slick is in-
creased. For example, if one would take as the criterion for the extent of
