Investigating the transfer process across the free aqueous boundary layer by the controlled flux method

Theory and experimental results of a new method are described directly investigating the transfer processes across the aqueous viscous boundary layer. The method is based on a known and controllable flux density being applied at the interface. Then the local transfer velocity can be determined by monitoring the tracer concentration at the water surface within minutes. Moreover, the time constant for the transport across the boundary layer ("surface renewal time") can be measured directly. Comparison of the theoretical and measured frequency response of the boundary layer yields significant deviations. The technique is put into operation for heat transfer measurements. Direct comparisons with gas exchange measurements in several wind/wave facilities verify that the gas transfer velocity can be accurately extrapolated from the heat transfer measurements. A new way is opened both for detailed studies of the transfer processes in wind/wave facilities and the urgently needed direct parameterization of the transfer velocity as a function of windshear, wave parameters, and water turbulence in natural systems as rivers, lakes and the ocean. This paper includes (as a first example) measurements on the fetch dependency of the transfer process.