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spective) and suggests that the straining of the surface current field, not the
wind, was responsible for the presence of this roughness patch.
Regions of suppressed roughness occurred at both edges of the internal
wave event. These suppressed regions tend to be more contiguous. At the
leading edge, two subnormal bands occurred on both sides of the enhanced
roughness (at
t
|3190 s). The water temperature showed a prominent fluc-
tuation signature (Fig. 1b provides a better perspective) associated with
this roughness enhancement event. The temperature dropped significantly
(at
t
|3250 s) following the subsequent roughness suppression. The sec-
ond group of roughness suppression that occurred at the trailing edge was
more spread out. A long contiguous region near
t
|3600 s was coincident
with a plateau of the water temperature time series. Surface wind speed in
this segment was above average of the whole episode. The significance of
these correlations of the surface temperature microstructure and the surface
roughness may provide interesting insights of the dynamics of short waves
on the ocean surface. Because surface temperature is a much easier quan-
tity to measure than the near-surface current, it may serve as a surrogate
parameter to represent the near surface turbulence and current strain. The
correlation to the surface roughness then allows radar or other remote sen-
sors to monitor the near-surface turbulence structure.
The dimensionless spectral density (the degree of saturation,
B
(
k
)=
k
F
1
(
k
), defined by Phillips, 1985) of the three classes and two sub-
classes are shown in Figs. 2d and 2e in linear and logarithmic scales, re-
spectively. The maximum of
B
(
k
) occurred at
k
|7 rad cm
-1
. Using the
mean spectral density of the class II segments as the normalization factor,
the normalized spectra are shown in Fig. 2f. The envelope formed by the
super- and sub-normal spectra is a representation of the roughness contrast
of the ocean surface. Environmental parameters contributing to the rough-
ness contrast include wind-induced turbulence fluctuation and surface or
internal wave modulation. The range of spectral variations in the short
gravity wave range (1d
k
d5 rad cm
-1
) was relatively constant, and in-
creased steadily toward short capillary waves with higher wave numbers.
The same analysis is applied to three other cases without internal waves.
The mean wind speeds were 1.3 (case 2422), 3.5 (case 1310) and 5.7 (case
1223) m s
-1
, as compared to 2.1 m s
-1
for the internal wave case (case
1413) just described. The spectral variability as a function of wave number
is calculated by the ratio of
B
III
(
k
)/
B
I
(
k
), where subscripts I and III denote
class I and class III, respectively. The results are plotted in Fig. 3, showing
a monotonic increase in the variation level with increasing wind speed and
wave number. The spectral variability can be interpreted as a measure of
the modulation amplitude. The modulation level of the internal wave seg-
