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= Experiments With Monomolecular Surface Films =
=== Investigations on Wave Damping and Radar Backscattering ===
We performed measurements of the damping of small gravity and gravity-capillary water surface waves
covered with monomolecular organic films of different viscoelastic properties. The wind-speed dependence
of the radar cross sections for X and Ka band was measured with upwind looking microwave antennas. We
show that Marangoni damping theory, which describes the damping of water surface waves by viscoelastic
surface films, is not the only damping mechanism in wind wave tank experiments where the wind sea is not
fully developed. The other source terms of the action balance equation, i.e., the energy input into the
water waves from the wind, the nonlinear wave-wave interaction, and the dissipation by wave breaking,
are affected differently by the various substances. It is hypothesized that this difference is caused
by the different intermolecular interactions of the film molecules causing different viscoelastic
properties. Polarization ratios (i.e., the ratios of the radar backscatter at vertical and horizontal
polarization) higher than those predicted by simple Bragg scattering theory for X band at low wind
speeds and different incidence angles are explained within a (three-scale) composite-surface model.
The dependence of the polarization ratio on the coverage of the water surface with a slick is explained
qualitatively by means of the composite-surface model. Our measurements prove that wind wave tank
measurements in the presence of monomolecular surface films are useful for the verification of theories
concerning radar backscattering, wave damping, and wind-wave and wave-wave interactions.
'''References:'''
* Gade, M., W. Alpers, H. Hühnerfuss, and P.A. Lange, 1998: Wind-wave tank measurements of wave damping and radar cross sections in the presence of monomolecular surface films, ''J. Geophys. Res., 103,'' 3167-3178.
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=== Wave Damping Measurements ===
{{attachment:UHH_WWK_Damp_theocurv.gif|Theoretical damping ratios according to Marangoni damping theory|width="500"}} <
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~-Theoretical damping curves for the deployed substances, according to Marangoni damping theory.-~
The damping ratio, ''y(f)'', is the ratio of the viscous damping coefficients at slick-covered and slick-free
water surfaces:
{{attachment:UHH_WWK_Damp_ratio.gif|Damping ratio|height="55"}}
{{attachment:UHH_WWK_Damp_XY.gif|Damping ratio|height="55"}}<
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{{attachment:UHH_WWK_Damp_wavespec.gif|Wind-wave spectra at slick-free and slick-covered water surfaces|width="500"}} <
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~-Wave height spectra for slick-free and slick-covered water surfaces; solid lines: 8 m/s wind speed, slick-covered water surfaces; dashed-dotted line: 8 m/s wind speed, slick-free water surface; dotted line: 5 m/s wind speed, slick-free water surface, i.e., the same friction velocity, u* , as in the slick-covered cases.-~
The table below shows the rheological parameters for the deployed substances, i.e., the absolute
value ''E'' and phase ''theta'', of the complex dilational modulus, the frequency of maximum damping
ratio (''f_M'', "Maragoni frequency"), and the maximum damping ratio, ''y_max''.
{{attachment:UHH_WWK_Damp_table.gif|Rheological parameters of the deployed substances|height="150"}}
In order to simulate different states of the development of biogenic slicks on the ocean surface, we
varied the dropping rate, i.e., the amount of dissolved substance that was deployed (lower left figure).
The change of the (relative) wave height spectra is depicted in the left Panel as a function of increasing
dropping rate. The measurements were carried out at a wind speed of 8 m/s when PME was deployed at 5.5 m
fetch. The vertical dotted line indicates the frequency of the maximum damping ratio (Marangoni frequency).
With increasing dropping rate the spectral peak frequency increases, and the dip around the Marangoni
frequency becomes more pronounced. At a dropping rate of 2.79 mL/min (corresponding to 0.2 mmol/min of PME)
the wind waves almost vanish, as can be inferred from the decrease in the total spectral power. Further
increase in the dropping rate does not cause significant changes in the wave height spectra due to the
saturation of the water surface with the deployed substance.
{{attachment:UHH_WWK_Damp_cascade.gif|Cascade plot of wave spectra|width="234"}}
{{attachment:UHH_WWK_Damp_windprof.gif|Wind profiles with and without slicks|width="500"}}
In two measurement series the substances were deployed at two different points, i.e., directly at the wind
entrance (Point 1) and at a fetch of 5.5 m (Point 2). The reference wind speed was increased from 2 m/s up
to 10 m/s in steps of 2 m/s. The respective wind profiles recorded at 15.5 m fetch are shown in the upper
right figure. The red crosses, solid black diamonds, and blue circles depict the wind speeds measured over
a slick-free water surface, a water surface covered with CEM3AB deployed at point 1, and a water surface
covered with CEM3AB deployed at point 2, respectively. Note that the slick coverage caused higher wind
speeds above the water surface (i.e., steep profiles). At 10 m/s the CEM3AB slick deployed at point 1 was
already disrupted by the strong wind action.
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=== Radar Cross Sections (RCS) ===
The presence of monomolecular films at the air/water interface leads to a reduction of the spectral power
density of the Bragg waves and therefore to a reduction of the mean backscattered radar power. This reduction
has been measured at X band at incidence angles of 36° (Panels a) and b)) and 54° (Panels c) and d)) and at
Ka band at 53° (Panel e)). The corresonding intrinsic frequencies of the Bragg waves are 9.3 Hz (X band, 36°),
12.2 Hz (X band, 54°), and 61.8 Hz (Ka band, 53°). According to Marangoni damping theory (see the theoretical
curves above), the longer X band Bragg waves at 36° incidence angle are more affected by the different damping
behavior of the deployed substances than the shorter ones at 54° incidence angle. As a consequence, the
decrease in the RCS is more pronounced at 36° incidence angle and the measured curves exhibit a stronger
decrease (at wind speeds of 7–9 m/s). However, Marangoni damping theory cannot explain the strong RCS
reduction at Ka band.
{{attachment:UHH_WWK_Damp_X36.gif|X-band radar cross sections at 36deg incidence angle|width="400"}}
{{attachment:UHH_WWK_Damp_X54.gif|X-band radar cross sections at 54deg incidence angle|width="400"}}<
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{{attachment:UHH_WWK_Damp_Ka53.gif|Ka-band radar cross sections at 53deg incidence angle|width="400"}}
Complementing wind-wave tank measurements give rise to the explanation that two different kinds of ripple
waves are responsible for the radar backscattering, namely, freely propagating waves and waves bound to the
longer decimeter waves. If these two kinds of waves are damped in a different way (via their different
generation mechanisms), the result should be a change of the measured damping behavior, which was observed
in our measurements.
{{attachment:UHH_WWK_Damp_Xdrop.gif|X-band radar cross sections at 36deg incidence angle|width="750"}}
The backscattered radar power was measured as a function of the dropping rate. The results for PME and
OLA at different wind speeds are depicted in the above figures. Obviously, the backscattered radar power
at low wind speeds is more sensitive to the dropping rate when OLA is deployed (right panel). A low amount
of OLA causes a strong damping at 6–7 m/s wind speed, while at higher wind speeds the dependency for both
substances is similar (the absolute values of the dropping rate cannot be compared because of the different
chemical structure of the monomolecular films).
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=== X-Band Polarization Ratios ===
The wind dependence of the polarization ratio, sVV /sHH, at incidence angles of 36° and 54° is depicted
for slick-free and slick-covered water surfaces in the figures below, where the solid horizontal lines
denote the theoretical values according to simple Bragg scattering theory (SBS), and the dashed horizontal
lines denote the same values according to a composite surface model [CSM].
{{attachment:UHH_WWK_Damp_Xpolrat.gif|X-band radar cross sections at 36deg and 54deg incidence angle|width="750"}}
In the slick-free case (Panel a)) and for low to moderate wind speeds (up to 6 m/s) the measured ratios
agree with those calculated by the composite surface model. However, at higher wind speeds (Uref > 6 m/s)
they decrease, a finding that can be explained by the increased dominance of wedges and spilling breakers
at high wind speed, which in turn causes an increased backscatter at HH polarization. Therefore, the
polarization ratio decreases, which is in agreement with the experimental results. The abrupt strong
decrease in the polarization ratios at high wind speeds is due to the dispersing of the slicks; therefore,
the values become comparable to those measured on a slick-free water surface. Again, at high wind speeds
(above 11 m/s), backscattering by wedges and spilling breakers, which was prevented by the slicks at low
wind speeds, leads to low values of the polarization ratio.
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