Boundary layers at a dynamic interface: Air‐sea exchange of heat and mass

Abstract

AbstractExchange of mass or heat across a turbulent liquid‐gas interface is a problem of critical interest, especially in air‐sea transfer of natural and anthropogenic gases involved in the study of climate. The goal in this research area is to determine the gas flux from air to sea or vice versa. For sparingly soluble nonreactive gases, this is controlled by liquid phase turbulent velocity fluctuations that act on the thin species concentration boundary layer on the liquid side of the interface. If the fluctuations in surface‐normal velocity and gas concentration are known, then it is possible to determine the turbulent contribution to the gas flux. However, there is no suitable fundamental direct approach in the general case where neither nor can be easily measured. A new approach is presented to deduce key aspects about the near‐surface turbulent motions from measurements that can be taken by an infrared (IR) camera. An equation is derived with inputs being the surface temperature and heat flux, and a solution method developed for the surface‐normal strain experienced over time by boundary layers at the interface. Because the thermal and concentration boundary layers experience the same near‐surface fluid motions, the solution for the surface‐normal strain determines the gas flux or gas transfer velocity. Examples illustrate the approach in the cases of complete surface renewal, partial surface renewal, and insolation. The prospects for use of the approach in flows characterized by sheared interfaces or rapid boundary layer straining are explored.