Subproject 4: Microphysical model for exchange processes through film-covered surfaces
The results of experimental and theoretical process studies will be synthesized in a micro-physical model. It describes the characteristic scales for gas exchange and the film-influenced surface properties in terms of an effective diffusion coefficient. It takes into account the dependence of the aerodynamic roughness and renewal rates on the surface tension and viscosity. Simplified bulk formulae for the estimation of gas fluxes from wind speed, partial pressure differences and film coverage will be derived which can be used in coupled circulation models or with observed data. The production and consumption of trace gases in surface films can be taken into account with corresponding source and sink terms.
The microphysical model describes the coupled balances of turbulent kinetic energy, momentum and passive tracers (blue boxes). The transport occours diffusion-like with molecular, turbulent and wave contributions. For the transfer velocity, scaling relations suggest a certain dependence on the dissipation rate in the viscose sublayer (εv) (lavender box). It involves the kinematic viscosity (ν) and the Schmidt number (Sc = ν / D). The parameter n is 3 for a smooth and 2 for a rough surface, while k = 0.4 and λ = 11 are more or less constant.