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170 CHAPTER 5. SMALL-SCALE AIR-SEA INTERACTION Recently, the intermittent nature of the gas exchange process has come into the focus of research. Of particular interest is the microscale wave breaking, a process that causes enhanced near-surface turbulence. It is known that microscale wave breaking enhances gas transfer. Zappa et al. [2001] found, e. g., a good correlation between the measured transfer velocity and the fraction of the water surface at which surface renewal occurred by microscale wave breaking. Details of the mechanisms that allow a physically-based parameterization are, however, not yet known. The intermittency of the gas transfer is of much importance when integrating gas transfer rates form short time and spatial scales as determined by active thermography to larger scales. Because the relation between wind speed and the transfer velocity and possible other parameters is certainly nonlinear, not only the mean values but also the variations must be known for a proper integration. A particularly difficult and interdisciplinary problem is the influence of organic films on air-sea gas transfer [Frew, 1997]. Surfactants attenuate short wind waves and thus changes the hydrodynamic boundary conditions at the water surface. In effect, the gas transfer rate is significantly reduced. Interestingly this complex process can be modeled by simply relating the gas transfer rate to the mean square wave slope [Frew, 1997; Bock et al. , 1999; Frew et al. , 2004]. This close correlation between the mean square slope and the gas transfer rate was already pointed out by Jähne et al. [1987] long before systematic studies of the influence of surface films on gas transfer were performed. However, a more detailed modeling is still lacking. Main methods By using novel visualization techniques and image sequence processing techniques, the complex small-scale air-sea interaction processes can be tackled experimentally for the first time in both laboratory and field experiments. Equally important are spectroscopic techniques that allow simultaneous gas exchange measurements with many tracer. The techniques are briefly described in the following. UV-spectroscopy for volatile species dissolved in water UV-spectroscopy in contrast to IRspectroscopy offers the significant advantage that it is not required to degas water samples in order to measure gases and volatile species dissolved in water, because water is transparent in the required wavelength range from 200–300 nm. Thus this technique can be used to measure concentrations both the water and air space. The technique adds a number of volatile species for gas exchange measurements, especially species containing aromatic rings, such as fluorobenzenes, thiophenes, and pyrenes. For further details see section 5.1.3. Wave slope and height imaging A detailed investigation of the dynamics of short wind waves required simultaneous imaging of the wave height and slope. This is because the energy of gravity waves is contained in the wave height whereas the energy of the capillary waves is proportional to the wave slope. Therefore a novel instrumentation was developed that is capable to measure the wave slope and height simultaneously by making use of the fact that light at different wavelengths in the near infrared shows significant differences in absorption. In this way the imaging wave slope gauge could be extended for simultaneous wave height measurements (section 5.1.7 and Jähne et al. [2005c]). Active thermography Using heat as a proxy tracer for gases has two significant advantages. Firstly, the concentration of the tracer at the water surface can directly be measured with infrared cameras. Secondly, the heat flux can be controlled by applying infrared radiation onto the water surface. Periodically varying infrared radiation was applied to the water surface to create a corresponding variation in the surface temperature of the water. For slow variations of the infrared radiation - slower than the time constant for the exchange process - the transfer velocity can be measured directly by dividing the known flux density by the measured temperature variation. If the heat flux is switched off, the temperature increase will decay with the characteristic time constant t⋆ (surface renewal time) for the transport process across the aqueous heat boundary layer. From this time constant the transfer velocity k can be computed directly using the relation t⋆ = D/k 2 , where D is the diffusion coefficient for heat [Jähne et al. , 1989]. By measuring the phase shift and amplitude damping of the thermal boundary to periodically changing IR radiation, it is also possible to distinguish between different models for the exchange process and to investigate the intermittency of the process. Because the infrared image sequences contain significant contrast, the surface flow field including its vorticity and local convergence and divergence zones can also be computed from image sequences [Garbe et al. , 2003]. Moreover, the spatial structure of the micro-turbulence at the water surface can be studied [Schimpf et al. , 2004].
5.1. SMALL-SCALE AIR-SEA INTERACTION 171 Fluorescence imaging of gas tracer concentration fields The fluorescence spectrum of dissolved dyes generally shows a bandwidth between 30 and 100 nm. A second dye is used to attenuate the emitted fluorescent light in this wavelength range differently for different wavelengths. With this double-dye technique, the shape of the observed fluorescence spectrum depends on the path length the light travels through the water before it is measured. Because the fluorescence is excited from the air space, the path length directly corresponds to the distance from the water surface. For a given wavelength, the fluorescent light received is then integrated over a characteristic depth ˆz = α −1 (λ), where α(λ) is the wavelength-dependent absorption coefficient of the absorbing dye solution. If α(λ) is known, the depth-dependent concentration can be computed from the measured spectra as a linear inverse problem. This technique is investigated for the quenching of the fluorescence of a organic rutheniumcomplex by oxygen (section 5.1.6) and pH-dependent fluorescent dyes (section 5.1.8). 3-D flow measurements close to and at the water surface The aqueous viscous boundary layer at a wind-driven water surface has a thickness of at most a millimeter. Therefore almost no flow measurements are available from this layer at a free water surface. We try to tackle this difficult experimental problem with two techniques. First, thermal image sequences from the water surface contain enough structures to computed 2-D flow fields (section 5.1.2). Secondly, a modified particle tracking technique is used for depth-resolved flow measurements. Again an absorbing dye is used to code the depth of the tracked particles and to measure the velocity component perpendicular to the water surface from the brightness change of the particle (section 5.1.5). Main activities Our current main activities include 1. the development of a novel optic technique for simultaneous imaging of the height and slope of short wind waves (section 5.1.7), 2. the development of depth-resolving imaging technique to measure concentration fields of gases close to the water surface by using a double dye laser induced fluorescence techniques (sections 5.1.6 and 5.1.8), 3. detailed studies of the Schmidt number dependency of air-water gas transfer (section 5.1.3), 4. investigations of the air-sea gas exchange by active thermography (section 5.1.1), 5. analysis of flow fields by passive and active thermography thermography (section 5.1.2), 6. 3-D flow measurements within the aqueous viscous boundary layer (section 5.1.5), and 7. 3-D flow measurements within porous gravel layers (section 5.1.4). Funding 1. DFG-Schwerpunktprogramm 1114 “Mathematische Methoden der Zeitreihenanalyse und digitalen Bildverarbeitung” 2. DFG-Schwerpunktprogramm 1147 “Bildgebende Strömungsmesstechnik” 3. Graduiertenkolleg 1114 “Optische Messtechniken <strong>für</strong> die Charakterisierung von Transportprozessen an Grenzflächen” (TU Darmstadt and U Heidelberg) 4. DFG Ja395/10: ”Mechanismen des Gasaustausches zwischen Atmosphäre und Ozean: Laborversuche und Modellierung” 5. DFG Ja395/13: ”Impact of Wind, Rain, and Surface Slicks on Air-Sea CO2 Transfer Velocity - Tank Experiments” 6. Bundesanstalt <strong>für</strong> Wasserbau
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Institut für Umweltphysik und Arbe
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Contents 1 Introduction/Overview of
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CONTENTS 7 3.2.1 Glaciological pilo
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Introduction In Heidelberg, environ
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Overview Summary Atmospheric resear
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2.1. TROPOSPHERIC RESEARCH GROUP 15
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36 CHAPTER 2. ATMOSPHERE AND REMOTE
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2.3. RADIATIVE TRANSFER 45 2.3 Radi
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2.3. RADIATIVE TRANSFER 47 Funding
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2.3. RADIATIVE TRANSFER 49 2.3.2 Ox
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2.3. RADIATIVE TRANSFER 51 2.3.4 At
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2.3. RADIATIVE TRANSFER 53 Rothman,
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2.7. CARBON CYCLE GROUP 105 2.7 Car
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2.7. CARBON CYCLE GROUP 107 Diploma
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2.7. CARBON CYCLE GROUP 109 2.7.2 S
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2.7. CARBON CYCLE GROUP 111 2.7.4 I
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7.5. INVITED TALKS 221 Invited Talk
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