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5.1. SMALL-SCALE AIR-SEA INTERACTION 169
5.1 Small-Scale Air-Sea Interaction
Names of group members
Prof. Dr. B. Jähne, head of group
Dr. G. Balschbach, staff
Dipl.Phys. K. Degreif, PhD student
Dipl.Chem. A. Falkenroth, PhD student
Dr. C. S. Garbe, Postdoc
Dipl.Phys. M. Jehle, PhD student
Dipl.Phys. M. Klar, PhD student
Dipl.Phys. C. Popp, PhD student
R. Rocholz, diploma student
Dipl.Phys. M. Schmidt, PhD student
Dr. U. Schimpf, Postdoc
T. Schwarz, diploma student
Abstract Research in small-scale air-sea interaction at the IUP concentrates on air-sea gas exchange
and the dynamics of wind waves. The basic mechanisms are studied in laboratory experiments in a
unique annular wind-wave facility (The Heidelberg Aeolotron) and field experiments using imaging
techniques for quantitative visualization of the water surface structure (wind waves), the flow field,
concentration fields by laser-induced fluorescence, and the water surface temperature by passive and
active thermography.
Scientific Objectives The basic scientific objective is a better physical understanding of air-sea
gas transfer, the dynamics of short wind waves, and the micro turbulence at the ocean surface.
Currently the following topics are investigated:
1. The influence of the diffusion coefficient (or the Schmidt number Sc) on the transfer velocity
of gases. Together with the transfer velocity itself, this is a sensitive measure to distinguish
different models.
2. The influence of wind waves on air-sea gas transfer by enhancing the turbulence near the water
surface.
3. The influence of chemical reactivity on gas transfer. Here the hydration reaction of CO2 is
currently of most interest, because today only some model computations but no detailed measurements
are available.
4. Analysis of the spatial and temporal structure of the turbulence in the water-sided viscous
boundary layer as the driving force for air-water gas exchange.
Overarching topic is the study of small-scale air-sea interaction, especially the mechanisms of
air-sea gas exchange and the dynamics of wind waves.
Background Despite intensive research, knowledge about air-sea interaction processes has not been
significantly improved during the last decade. Many basic questions are still open. The relation of
the air-sea gas exchange rate with wind speed is discussed controversial. Both field and laboratory
experiments show large deviations up to a factor of two from a simple relation between the wind
speed and the gas transfer velocity [Jähne & Haußecker, 1998; Jähne, 2001]. It is obvious that
other parameters, especially wave-induced near-surface turbulence and surface active films are of
importance, but detailed modeling of these processes is still lacking. Therefore, despite their known
significant limitations, semi-empirical relations between the wind speed and the gas transfer velocity,
as established by Liss & Merlivat [1986] or Wanninkhof [1992], are still the state of the art.
Another uncertainty in the estimation of transfer velocity is the dependence on the Schmidt number
Sc. It is common to assume that the transfer velocity k is proportional to Sc −1/2 . However, for quite
some time it has been known [Jähne, 1980] that the exponent is about 2/3 for a smooth water surface
at low wind speeds and gradually decreases to 1/2 for a rough and wavy water surface. The exact
shape of this transition, and especially how it is influenced by surface films in coastal zones and on
which parameters it depends, is not known. This lack of knowledge causes a considerable uncertainty
in estimating the transfer velocity. A change of the exponent from 2/3 to 1/2 without a change of
other parameters causes an increase of the transfer velocity by about a factor of two.