Laboratory Studies of Surface Fluxes at Slick Edges

Martin Gade, Philipp Lange
Institut für Meereskunde, Universität Hamburg, Hamburg, Germany
gade@ifm.zmaw.de

Stanislav Ermakov
Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia

ABSTRACT

Marine surface films have a strong impact on sea surface fluxes such as the transfer of energy, momentum, mass, and heat, and thereby strongly influence remote sensing signals acquired with sensors working at optical, infrared, and microwave frequencies. However, since the appearance of slicks at the sea surface usually is inhomogeneous and patchy, not only the processes inside the slick-covered areas, but also at the slick edges are of scientific interest and need to be understood. In order to gain insight into the generation of small-scale surface roughness at the leeward and windward slick edges we have conducted laboratory experiments with monomolecular (oleyl alcohol) surface films on wind-roughened water surfaces. The aim of our studies was to gain better knowledge of the processes that are involved in the generation of small-scale waves and turbulence in the presence of slicks, and of their influence on the radar backscattering from the water surface.

The experiments were conducted in the wind-wave tank of the University of Hamburg at wind speeds between 3 m/s and 9 m/s and with slick patches of different sizes (i.e., different slick deployment duration). We used upwind-looking X-band and W-band microwave sensors to study the temporal and spatial distribution of small-scale surface roughness along the profiles of longer waves. Simultaneous video and wave slope measurements were performed with a CCD camera and a laser slope gauge, respectively. In a second measurement campaign, we used an acoustic Doppler velocimeter (ADV) to measure sub-surface velocity pulsations at different depths.

Time series of the X-band radar backscatter and Doppler shift are used to demonstrate that at low wind speeds the ratio of bound and freely propagating Bragg waves depends on the location along the slick profile. The 5 cm range resolution of the W band radar allows for studying the temporal and spatial development of small-scale roughness associated with wave breaking inside and outside the slicks. These results are complemented by measurements of the turbulent diffusion of wave-generated sub-surface turbulence at either sides of the slick patches. Summarising the results from the remote sensing and in-situ sensors we provide better insights into the slick spreading dynamics and into the mechanisms of the radar backscatter from a wavy water surface.

The work is supported by INTAS (Project : 03-51-4987, “SIMP”)

Last Update: 2005-03-16