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 1-2 mm in the air. The hydrodynamics in these layers is significantly different from boundary layers at rigid walls since the orbital motion of the waves is of the same order as the velocities in the boundary layer. In our research, we use non-invasive imaging techniques to understand the processes that govern the thickness of these layers. While wind is the main driver of the exchange processes by producing shear at the water surface, waves and ultimately turbulence in the water, other factors, such as the contamination of the water surface with surface active material (surfactants), the length the wind is blowing over the water surface (fetch) or the entrainment of bubbles and the production of spray by breaking waves also play a role.
The Aeolotron wind-wave tank is the largest operational annular wind-wave tank in the world with a diameter of 10 m. Its construction was finished in 1999, and it is 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 1m deep water section can grow much larger than in the more common linear wind-wave tanks, which have a limited length of typically below 40 m. The Aeolotron is chemically clean, so that very sensitive chemical conditions can be met which is important for the imaging techniques of gas concentrations developed in our group. Also, the transfer velocities of many gases can be measured in parallel without interference by biological or chemical processes. The Aeolotron's construction also allows the use of artificial or natural sea water. Wind is produced by 4 axial fans with 2.2 kW of power each, with the highest wind speed being about 23 m/s. The Aeolotron is thermally insulated, allowing for the measurement of heat exchange. The 1.4 m high air space can be flushed with fresh air with a time constant of approx. 2 min, so that gases that have accumulated in the air space can be flushed out of the system very fast. A large window in the bottom of the tank and several other windows in the side walls and roof allow optical access for imaging systems.
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.
Reinhart-Koselleck-Project "Quantifying the Mechanisms of Gas Exchange between Ocean and Atmosphere - Bridging Laboratory and Field by Imaging Measurements" (01/2021-12/2025)
The important process of the exchange of climatically and environmentally relevant gases and volatiles between the atmosphere and the ocean, such as carbon dioxide and methane, is 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 partially contradicting results. Thus far, these field measurements were unable to contribute much to the understanding of the underlying physical mechanisms. Reliable results for low wind speeds are missing, since all existing measurement techniques are not applicable for this. Laboratory measurements in wind-wave tanks, which are an alternative to measurements on the ocean, have the disadvantage that the shape of the wind-generated waves on the water surface is very different from those on the open ocean, where the wind has plenty of time to build up the waves. Linear tanks allow only a short time for the interaction between wind and waves (fetch), and thus generate only a young wind sea. Even in an annular wind-wave tank with virtually infinite interaction times, like the Heidelberg Aeolotron, the waves differ from those on the open ocean. Because of the shallow water depth of the Aeolotron, the waves travel slower than on the ocean.
In this project, a radically new approach will be taken to simulate oceanic conditions at low and medium wind speeds in the Heidelberg Aeolotron in an appropriately realistic way for the first time. Two advanced imaging techniques will be used to measure the gas and heat exchange rates locally within seconds under non-stationary conditions in the Heidelberg Aeolotron: Active thermography will be used to measure the heat exchange rate and a new opto-chemical technique will visualize the mass boundary layer at the water surface, which is only fractions of a millimeter thick, from which the local rate of gas exchange can be determined. By using these fast new techniques, the processes controlling gas and heat transfer can be studied in the presence of growing and decaying wave fields for the first time. High wave ages, which have not been studied before in a wind-wave tank, will become accessible by using gases heavier than air as the Aeolotron's atmosphere. At low wind speeds, the important influence of surface-active materials on the gas transfer, which are produced as waste products in the metabolism of marine organisms, will also be studied in detail. With these measurements, a physically based description of the mechanisms of gas exchange under oceanic conditions finally will be possible.
In the second phase of the project, a simple technique to measure gas and heat exchange on the ocean in less than a minute with meter-scale resolutions will be developed. The instrument consists just of a thermal imaging camera and determines the transfer rate and the dominant physical mechanisms from the spatio-temporal thermal patterns on the ocean surface. This will also enable the verification, that laboratory measurements have included all mechanisms relevant to the ocean. The measurements on the ocean will be performed in cooperation with GEOMAR in Kiel and the Institute of Chemistry and Biology of the Sea at the University of Oldenburg.
Air-Sea Gas Exchange at High Wind Speeds DFG project JA395/17-3 (12/2020 - 12/2022)
In the previous proposals JA 395/17-1 and 17-2, air-sea gas transfer at high wind speeds was studied in the high-speed wind-wave tank of Kyoto University and the Miami SUSTAIN wind-wave tank. Beyond a wind speed of 33 m/s, a new regime is established where the gas transfer velocities increase stronger than with the friction velocity cubed. It was possible to separate bubble-induced gas transfer from transfer across the free surface. Bubble-induced gas exchange is not significant at all in fresh water. In seawater at a wind speed of 80 m/s and for gases with low solubility such as He and SF6, it is 1.7 times higher than the transfer across the free water surface. For CO2 and DMS, the bubble effect is not dominant even at the highest wind speeds.
This project will study the cause of the new found steeply increasing gas transfer velocities at high wind speeds. The hypothesis is that it is either caused by strongly enhanced turbulence associated with surface fragmentation processes, a significantly enlarged exchange surface area or a combination of both. Short fetch wind-wave tunnel studies are not representative for oceanic conditions. Indeed, bubble concentrations are about ten times higher at the infinite fetch annular wind-wave tank, the Heidelberg Aeolotron. This is an indication that the start of the steeply increasing high wind speed regime may be shifted down to about two times lower wind speeds.
Measurements in the Heidelberg Aeolotron can narrow the gap between linear wind-wave tank and oceanic conditions significantly: A) Using fast gas exchange measurements with a temporal resolution of only 10 s, the "fetch gap" can be closed by performing gas exchange measurements directly after turning on the wind. The wave field develops in about 10 min, so that the whole fetch range up to infinity can be covered. B) Using an SF6 atmosphere, the "wave-age gap" can be reduced. Then 2.2 times higher friction velocities are achieved at the same wind speed. Thus, high wind speeds are effectively scaled down by the same factor, resulting in more than twice the wave age than in other laboratory facilities. The experiments in the Aeolotron will also include highly soluble gas tracers to measure the air-side transfer velocities in order to better understand how different parts of the surface (breaking waves with white caps, closed bubble surfaces, spray droplets and water surface disturbed by bursting bubbles and impacting droplets) contribute to gas exchange.
|Aeolotron Air-Sea Interaction Laboratory
Institute of Environmental Physics
Im Neuenheimer Feld 229