Introducing surface waves in a coupled wave-atmosphere regional climate model: Impact on atmospheric mixing length

The marine atmospheric boundary layer is strongly influenced by the moving surface in the presence of surface waves; the impact depends on the wave conditions and the interaction with the atmosphere. Previous studies using measurements as well as numerical simulations with large-eddy simulations have shown that surface waves propagating faster than the wind (swell) alter the surface exchange as well as turbulence properties in the atmosphere. This impact is here introduced in a coupled wave-atmosphere regional climate model with a so-called E - l turbulence scheme (where E is the turbulent kinetic energy and l is a mixing length). A wave age dependent coefficient (here called W-mix) is added to the mixing length in the turbulence parameterization. This acts similarly to inducing additional convection, with larger mixing length and increased eddy diffusivity, when we have near neutral stratification and strong swell. For shallow boundary layers the regional coupled climate model shows a larger response to the introduced wave coupling with increased near surface wind speed and smaller wind gradient between the surface and middle part of the boundary layer. The impact for the studied areas is relatively minor for parameters averaged over 1 year, but for limited periods and specific situations the impact is larger. One could expect a larger impact in areas with stronger swell dominance. We thus conclude that the impact of swell waves on the mixing in the boundary layer is not insignificant and should be taken into account when developing wave-atmosphere coupled regional climate models or global climate models.