Chemical Characteristics - Marine Surface Films - page 125

Fig. 1. Frequency dependence of the normalised orientation ratio, NOR (see (1)),
measured with the two-dimensional laser slope gauge at different wind speeds
(columns) without rain (upper row) and with rain at a rain rate of R=160 mm h -1
(lower row). The water surface was not covered with any slick.
In Figure 3 it is demonstrated how the strong rain decreases the effect of
strong wave orientation, particularly at the frequency of the dominant wind
wave: for each wind speed a sharp maximum of the NOR was observed
without wind (upper row in Figure 3), but only a weak maximum was ob-
served in the presence of strong rain (lower row in Figure 3). Even at high
wind speeds (9 m s -1 ) and at a rain rate of 160 mm h -1 the orientation of the
wave field along the wind direction is still reduced.
The results shown in Figure 4 were obtained when the water surface was
covered by an OLA slick. At 3 m s -1 (leftmost column), which is below the
threshold wind speed for the first excitation of wind waves on a slick-
covered water surface, only slight differences are caused by the rain ( Fig-
ures 4 a and e). However, at 5 m s -1 ( Figures 4 b and f) we measured
strong differences: whereas a pronounced orientation of the wave field is
observed from the maximum in Figure 4 b (corresponding to the frequency
of the dominant wave field), a less pronounced maximum was measured