Quantifying the Mechanisms of Air-Sea Gas Exchange and Bridging Laboratory and Field by Imaging Measurements

• Applicant: Professor Dr. Bernd Jähne
• Subject Area: Oceanography ; Image and Language Processing, Computer Graphics and Visualisation, Human Computer Interaction, Ubiquitous and Wearable Computing
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 454778408

The important process of air-sea gas exchange still lacks sufficient understanding. This is mainly caused by limitations in current measuring techniques. Field measurements are available only in a rather narrow range of wind speeds from 4--20 m/s and still show conflicting results. No reliable estimates are possible for lower wind speeds, because all techniques (dual-tracer, eddy covariance and active thermography) are not suitable to be used at such low wind speeds. Field measurements could only make minor contribution to the mechanisms of air-sea gas exchange so far.This is much better possible with lab measurements in wind-wave-tunnels. Here the problem is that the conditions in these facilities deviate significantly from those at the open ocean. The common linear facilities have a short interaction length between wind and waves and thus generate only young wind seas far away from a wind sea in equilibrium with the wind (fetch gap). Even in an annular facility with an infinite fetch as the Heidelberg Aeolotron, the wind field is different from the ocean: because of the limited water depth waves cannot travel fast enough (wave age gap).Here a radically new approach is proposed to overcome all these limitations and to perform gas exchange measurements in the laboratory which sufficently realistic simulate all relevant oceanic conditions at low and medium wind speeds. New imaging techniques are applied to measure the gas transfer velocity locally and instantaneously in the Heidelberg Aeolotron under non-stationary conditions. In this way the whole fetch range can be covered including decaying wind seas, when the wind speed is lowered. The wave age gap can be overcome by using heavier gases (argon and krypton) in the atmosphere of the Aeolotron. The imaging techniques include active thermography to measure the heat transfer across the aqueous viscous boundary layer and a novel opto-chemical technique to image the mass boundary layer and to measure the gas transfer velocity. Also the influence of surfactants, which is very important for lower wind speeds, will be studied in detail.The anticipated outcome is two-fold: Firstly, a quantitative description of the mechanisms of air-sea gas exchange for oceanic conditions and therefore a physically-based relation of the gas transfer velocity as a function of the parameters controlling it. This will include the range of gas transfer velocities that are possible in different wind speed regimes.Secondly, a simple technique to measure the gas transfer in the field. This device will consist of only a thermal imager and infers the gas transfer velocity and mechanism from the thermal patterns in space and time at the sea surface. Thus, it will also be possible to verify whether the laboratory measurements have covered all relevant oceanic conditions.

• DFG Programme: Reinhart Koselleck Projects