Prof. Dr. Bernd Jähne (Senior Professor) The Air-Sea Interaction Group
The exchange of mass, momentum and heat between the atmosphere and the ocean is governed by thin boundary layers at each side of the interface. These boundary layers are a mere 10–350 μm thick in the water, and around a mm in the air. The hydrodynamics in these layers is significantly different from boundary layers at rigid walls because of the free surface and wind-induced waves.
Energy, heat and momentum are transported across the interface between the atmosphere and the ocean.
The physical mechanisms that drive this transport are the subject of our research. On the one hand, we use imaging measurement methods to investigate the water surface without contact, such as the visualization of gases in the wavy boundary layer using Boundary Layer Imaging (BLI), which is based on fluorescence. On the other hand, we use spectroscopic methods of gas analysis. We work both on the ocean and in wind-wave channels worldwide, in which the boundary conditions such as wind speed, air and water temperatures, as well as the coverage of the surface with surface-active substances that inhibit wave formation, can be freely adjusted.
The former Aeolotron Wind-Wave Tank
The Aeolotron wind-wave tank was a large annular wind-wave tank with a diameter of 10 m. Its construction was finished in 1999 and it was operational until mid - 2024. It was named after Aeolus, the Greek god of the wind. Due to its annular shape, the distance the wind can travel over the water (fetch) is virtually unlimited, so that waves in the 1 m deep water section could grow much larger than in the more common linear wind-wave tanks, which we visit for our research in the future. The Aeolotron served to develop imaging techniques of gas concentrations and heat. Also, the transfer velocities of many gases were measured without interference by biological or chemical processes.
Imaging Techniques
The imaging techniques developed and used in our group cover a wide range of studied processes.
- The Imaging Slope Gauge (ISG) resolves the slope of the water surface in a footprint of approx. 20 cm x 22 cm by an intensity coded flashed light source below the water and a high speed camera above, so that processes such as wave creation and wave breaking can be studied.
- Boundary Layer Imaging (BLI) makes the thickness of the water-side boundary layer visible by the combination of a pH-sensitive fluorescent dye and an alkaline gas. Together with the ISG, BLI is a powerful tool to study the relationship between wave shape, size and area affected by wave breaking and the boundary layer thickness.
- Particle Streak Velocimetry (PSV) allows for measurements of the velocity of the air or water right up to the air-water interface. Turbulent, wave-coherent, pressure induced and laminar contributions to the transfer of momentum can be separated.
- The Active Thermography (AT) is used to measure the transfer velocity of heat by heating the water surface periodically with a laser and measuring the temperature response with a highly sensitive infrared camera. The bulk transfer velocity of heat can be converted to that of a gas. Combining AT, BLI and the ISG gives insights into the mechanisms responsible for transporting gas and heat across the air-water interface.
- Laser Induced Fluorescence (LIF) of the gas SO2 in the ultraviolet range allows to measure concentration profiles in the air right down to the water surface with a resolution high enough to resolve the air-side mass boundary layer. In combination with PSV, turbulent and laminar transport mechanisms of gases in the air can be studied.
This video shows rendered BLI and ISG data which were measured in the same footprint. The darker the water surface is, the thinner the mass boundary layer is. Streaky structures, which are parallel to the wind, develop. Occasionally (e.g. frames 130-150; in the upper right corner) a wave breaks without entrainment of bubbles, but removes accumulated gas from the boundary layer and leaves a trail of high turbulence.
Current research project
The Reinhart-Koselleck project “Quantifying the mechanisms of gas exchange between ocean and atmosphere - bridging the gap between laboratory and field using imaging measurements” (01/2021-12/2025) studied important process of exchange of climate and environmentally relevant gases and volatiles between the atmosphere and the ocean. This included carbon dioxide and methane, which remains not yet sufficiently understood. Measurements of gas transfer on the ocean have so far only been carried out in a relatively narrow wind speed range between 4 and 20 m/s with sometimes contradictory results. These field measurements have not yet been able to contribute much to the understanding of the underlying physical mechanisms. There is a lack of reliable results for low wind speeds, as all existing measurement methods are not suitable for this. Laboratory measurements in wind wave tanks, which are an alternative to measurements at sea, have the disadvantage that the shape of the waves generated by the wind on the water surface is very different from that on the open sea, where the wind has plenty of time to build up the waves. Linear tanks only allow a short time for the interaction between wind and waves (fetch) and therefore only generate a young wind sea.
Even in a ring-shaped wind-wave basin with almost infinite interaction times, such as the Heidelberg aeolotron, the waves differ from those on the open sea. Due to the shallow water depth of the aeolotron, the waves travel slower than on the ocean. In this project, a completely new approach was pursued to more realistically simulate oceanic conditions at low and medium wind speeds. Two advanced imaging techniques will be used to measure the gas and heat exchange rates locally within seconds under transient conditions: Active thermography is used to measure the heat exchange rate and a new opto-chemical technique was used to visualize the mass boundary layer on the water surface, which is only fractions of a millimetre thick and from which the local gas exchange rate can be determined.
The second phase of the project has developed a simple technique to measure gas and heat exchange on the ocean in less than a minute with a resolution on the meter scale. The instrument consists only of a thermal imaging camera and determines the transfer rate and the prevailing physical mechanisms based on the spatio-temporal heat patterns on the ocean surface. This also makes it possible to check whether the laboratory measurements have captured all the mechanisms relevant to the ocean. The measurements on the ocean are carried out in collaboration with GEOMAR in Kiel and the Institute for Chemistry and Biology of the Marine Environment at the University of Oldenburg.
Small-Scale Air-Sea Interaction Video Channel
This channel collects videos from the Small-Scale Air-Sea Interaction Group at the Institute of Environmental Physics (IUP) and Interdisciplinary Center for Scientific Computing (IWR) of Heidelberg University, Germany
Publications
Jähne, B. (2024). Digitale Bildverarbeitung und Bildgewinnung. 8 ed. Berlin, Heidelberg: Springer Vieweg doi:10.1007/978-3-662-59510-7.
Jähne, B. (2024). On the crucial role of wind-wave-tunnel studies to reveal the mechanisms of air-sea gas exchange. in EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024 EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024. (Copernicus GmbH), EGU24–13372. doi:10.5194/egusphere-egu24-13372.
Dong, Y., Jähne, B., and Marandino, C. (2024). Cross-linking laboratory and field measurements to quantify the role of bubbles in air-sea CO2 exchange. in EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024 EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024. (Copernicus GmbH), EGU24–732. doi:10.5194/egusphere-egu24-732.
Hofmann, D., and Jähne, B. (2023). Fluorescence imaging of concentration fields of dissolved gases at water interfaces. in OCM 2023 - Optical Characterization of Materials: Conference Proceedings OCM 2023 - Optical Characterization of Materials: Conference Proceedings., ed.J. Beyerer, T. Längle, and M. Heizmann, 149–157. doi:10.5281/zenodo.7768615.
Lu, G.-h., Tsai, W.-t., Garbe, C., and Jähne, B. (2021). Characteristics of streaky thermal footprints on wind waves. J. Geophys. Res.. doi:10.1029/2021JC017385.
Jähne, B. (2020). “What controls air-sea gas exchange at extreme wind speeds? Evidence from laboratory experiments,” in Recent Advances in the Study of Oceanic Whitecaps Recent Advances in the Study of Oceanic Whitecaps., ed.P. Vlahos and E. Monahan (Springer), 133–150. doi:10.1007/978-3-030-36371-0_10.
Nagel, L., Krall, K. E., and Jähne, B. (2019). Measurement of air-sea gas transfer velocities in the Baltic Sea. Ocean Sci., 15, 235–247. doi:10.5194/os-15-235-2019.
Friman, S. I., and Jähne, B. (2019). Investigating SO_2 transfer across the air–water interface via LIF. Exp. Fluids, 60, 65. doi:10.1007/s00348-019-2713-6.
Krall, K. E., Smith, A. W., Takagaki, N., and Jähne, B. (2019). Air–sea gas exchange at wind speeds up to 85 m/s. #os# 15, 1783–1799. doi:10.5194/os-15-1783-2019.