Chemistry Reference
In-Depth Information
Laboratory measurements of artificial rain impinging on
slick-free and slick-covered water surfaces
Nicole Braun, Martin Gade, and Philipp A. Lange
Institute of Oceanography, University of Hamburg, Hamburg, Germany
Abstract.
Laboratory measurements with artificial rain were carried out in
the wind-wave tank of the University of Hamburg, in order to gain better
understanding of the radar backscattering from a slick-free and slick-
covered water surface, particularly when it is agitated by strong rain. We
used a coherent 9.8 GHz (X band) scatterometer at different polarisations
and at an incidence angle of 28 degrees, a resistance-type wire gauge, and
a two-dimensional laser slope gauge. A water surface area of 2.3 m
2
was
agitated by strong artificial rain with a rain rate of 160 mm h
-1
and rain
drops of 2.9 mm diameter. The wind-speed range used in the present in-
vestigation was between 2 and 10 m s
-1
. A monomolecular surface film
was produced by deploying oleyl alcohol on the water surface. The results
of the analyses of the measured radar Doppler spectra and wave amplitude
and slope spectra are presented. We show that while the wind-induced sur-
face roughness is strongly reduced in the presence of the slick, at a high
rain rate (of 160 mm h
-1
) the surface slick less strongly affects the rain-
induced increase of the surface roughness (i.e., the generation of crowns,
cavities, stalks, ring waves, and secondary drops).
1 Introduction
Radar signatures of rain events are often observed on synthetic aperture
radar (SAR) images of the ocean surface because rain drops impinging on
the sea surface change the surface roughness in at least two ways: (1)
roughness is increased because the rain drops generate ring waves, crowns,
cavities, and stalks, and (2) roughness is decreased because they generate
turbulence in the upper water layer which in turn dampens the wind gener-
ated water waves. Moreover, oceanic surface films (be they of biogenic or
anthropogenic origin) strongly influence the backscattered radar power
(
ı
rel
) by damping the small-scale surface roughness, which is responsible
for the radar backscattering at oblique incidence angles (between 20 and
