Exploring air-sea gas transfer by active thermography

Abstract

Active thermography has been used for almost 30 years to explore air-sea gas transfer both in laboratory and field experiments [Jahne et al., 1989]. In the early 2000s, some doubt arose whether it was possible to extrapolate heat transfer velocities to gas transfer velocities [Asher et al., 2004]. Because of the large difference in the molecular diffusivity, different mechanisms may govern the transfer of heat and mass. However, in a recent experimental study at the large annular wind-wave tank in Heidelberg, the Aeolotron, Nagel et al. [2014] could show that heat transfer velocities can be scaled to gas transfer velocities with an accuracy of better than 10%, provided the Schmidt number exponent is known. These measurements were performed using the original active thermographic technique proposed by Jahne et al. [1989], by heating a rather large area at the water surface of up to one square meter. This is required to ensure that water parcels stay longer in the heated patch than the response time of heat transfer across the boundary layer.