Chemistry Reference
In-Depth Information
Infrared imaging: a novel tool to investigate the influence
of surface slicks on air-sea gas transfer
Uwe Schimpf
1
, Nelson M. Frew
2
and Bernd Jähne
1,3
1
Institute of Environmental Physics, University of Heidelberg, Germany
2
Woods Hole Oceanographic Institution, Woods Hole, MA, USA
3
Interdisciplinary Center for Scientific Computing, University of
Heidelberg, Germany
Abstract.
The influence of surface films on air-sea gas exchange at low
and moderate wind speeds is investigated. Observations were made in the
small Heidelberg circular wind-wave facility and in coastal and offshore
waters south of Cape Cod, New England. The passive controlled flux tech-
nique was used to investigate the micro turbulence very near the water sur-
face, which controls the rate of transfer of momentum, heat, and mass
across the air-sea interface. The analysis of infrared image sequences al-
lows the estimation of the net heat flux at the water surface, the skin-bulk
temperature difference across the thermal sublayer and thus the heat trans-
fer velocity. Using Schmidt number scaling, estimates of the gas transfer
velocity are obtained. Experimental evidence shows that increased surface
film concentrations suppress near surface turbulence and thus decrease the
gas exchange compared to a slick-free ocean interface. If a surfactant is
present, turbulent mixing is dampened and direct renewal of the surface is
inhibited. A surface slick changes the hydrodynamic boundary conditions
in that the length scales of near surface turbulence controlling air sea gas
exchange are modified. The micro-scale temperature fluctuations at the
water surface indicate that at low wind speeds the transport process is
dominated by large-scale turbulence, whereas at higher wind speeds the
smallest observed scales dominate the transport.
1 Introduction
The oceans cover two-thirds of the earth's surface. Driven by the wind and
heated by solar radiation, this moving bulk of water directly affects earth's
climate (e.g., the impact of the North Atlantic circulation on West
Europe's climate). These huge amounts of flowing water have an enor-
