Lock-in thermography at the ocean surface: a local and fast method to investigate heat and gas exchange between ocean and atmosphere

Heat is used as a proxy tracer for gases to study the transport processes across the sea-surface interface to obtain a detailed insight into the diffusive and turbulent processes controlling the transport. A carbon dioxide laser forces a periodically varying heat flux density onto the water surface and the amplitude damping and phase shift of the sea surface temperature is measured from infrared image sequences. The transport process can be treated by linear system theory and the relation between the input signal (periodically varying surface flux density) and the output (surface temperature) is estimated. Within the framework of the SOPRAN initiative three field experiments in the Baltic Sea were conducted. The locally derived heat transfer rates are scaled to gas transfer rates, which are in good agreement with empirical gas transfer wind speed relationships for moderate winds speeds. At high wind speed, the transfer rates are lower, which is explained by the fact that heat transport is insensitive to bubble-mediated gas transfer, i.e. it measures only a part of the transfer process directly at the water surface. Together with eddy covariance measurements a significant improvement of the parameterization of heat and gas transfer velocities can be expected.