Chemistry Reference
In-Depth Information
tailed description of these results the reader is referred to Gade (1996) and
to the papers by Gade et al. (1998a,b,c).
2 Results obtained from measurements with HELISCAT
During the two SIR-C/X-SAR missions in April and October, 1994, radar
backscatter measurements were carried out with a 5-frequency/multi-
polarisation scatterometer flown on a helicopter. This scatterometer, called
HELISCAT, works at 1.25, 2.4, 5.3, 10.0, and 15.0 GHz (L-, S-, C-, X-,
and Ku-band, respectively) and is capable of performing radar backscatter
measurements at different incidence angles.
During all measurements, monomolecular slicks consisting of oleyl al-
cohol (OLA) were deployed, thus allowing for an investigation of the de-
pendence of the damping behaviour on different environmental conditions,
in particular, on wind speed. Even though it is known to simulate biogenic
slicks not optimally, this substance was already deployed during several
earlier campaigns, because its strong damping capabilities make it easy to
be detected by microwave sensors. In addition to OLA, slicks consisting of
oleic acid methyl ester (OLME) and triolein (TOLG) were deployed,
which both represent compounds of natural surface films. Because of their
molecular structure, these substances form monomolecular surface films
(i.e., slicks with a thickness of only one molecule). Depending on wind
and wave conditions, however, these slicks may be locally disrupted, thus
causing a large scatter in the measured radar contrast (Figures 1 and 2).
The measurements were performed shortly after the slick deployment,
when the slicks were fully spread, in order to avoid any data spoiling by
slick aging effects. The measured damping ratios (i.e., the ratios of the
relative backscattered radar power from a slick-free and a slick-covered
water surface) are shown in Figure 1.
It can be concluded that, under all wind speed conditions, the measured
damping ratios increase with increasing Bragg wave number and that the
maximum damping decreases with increasing wind speed. These two find-
ings cannot be explained by pure Marangoni damping theory which de-
scribes the damping of small sinusoidal water surface waves by monomo-
lecular slicks and which predicts a distinct damping maximum at inter-
mediate wave numbers of approx. 100 rad m
-1
(Hühnerfuss 1986). In addi-
tion, wind-induced effects, primarily the energy input into the wave spec-
trum, have to be considered, as well.
