Introducing the impact of surface waves on the atmosphere in a coupled wave-atmosphere regional climate model

When modelling the atmosphere it is of crucial importance to correctly describe the boundary conditions. The atmospheric-ocean boundary is an important source of turbulence and there is a significant exchange of momentum, heat and moisture. The marine atmospheric boundary layer (MABL) has a considerable impact on global climate atmospheric models since 70 % of the global surface is covered with water. The MABL is strongly influenced by the moving sea surface in the presence of surface waves, where the impact depends on the wave conditions and the interaction with the atmosphere. Previous studies using measurements as well numerical simulations with Large-Eddy Simulations (LESs) have shown that surface waves propagating faster than the wind (swell) alters the near surface exchange as well as turbulence properties in the atmosphere. This is potentially of great climatological importance since the world's oceans are dominated by swell conditions (Semedo et al., 2011).

We have developed a coupled wave-atmosphere regional climate model (RCA-WAM) using the WAM wave model and the RCA regional climate model covering Northern Europe (Rutgersson et al., 2010;2012). The wave-impact on surface friction as well as boundary layer mixing is introduced. The response of the atmosphere to the state of the waves depends in a sensitive manner on how the wave properties are introduced in the coupled system.

In measurements and LES it is shown that the length scale of the turbulence mixing in the atmosphere is influenced by the surface waves (Nilsson et al., 2012). This impact is here introduced in the coupled wave-atmosphere regional climate model with a so called E-l turbulence scheme (where E is the turbulent kinetic energy and l a mixing length). A wave age dependent coefficient (Wmix) 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. The impact from the introduced wave-induced mixing is shown for key coupling parameters, wind speed, wind gradients as well as significant wave height and boundary layer height. 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 magnitude of the additional wave mixing is quite arbitrarily and need further tuning.

The impact for the studied areas is relatively minor for parameters averaged over one year, but for specific situations the impact is larger. One could also expect larger impact in areas with stronger swell dominance. We do thus conclude that 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 RCM or GCMs.

The surface stress can be expressed in terms of the drag coefficient, different studies of the drag coefficient show great scatter. For swell situations in the Baltic Sea a reduced drag coefficient was seen (Carlsson et al., 2009), in contrast new results indicate enhanced momentum transport at the surface for very steep swell conditions (Högström et al., 2012). Having a wave-dependent drag coefficient has a large impact on the coupled model system, for wind speed distribution, heat fluxes as well as secondary parameters as cloud cover and precipitation.

The major problem when developing a coupled model is limitations in the knowledge on how to introduce the interaction processes, we here summarize the potential impact of waves on various properties in the atmosphere and discuss potential impact.