Modelling new processes controlling the turbulent fluxes over sea

To describe the air-sea exchange of sensible and latent heat as well as momentum is crucial in modelling the atmosphere as well as the ocean. Large areas over the globe are covered by sea and the major part of the humidity in the atmosphere is evaporated from the sea surface. The turbulent transport of heat and momentum at the ocean surface is a substantial part of the atmospheric forcing of the ocean.

A large number of field- as well as modelling-experiments have been investigating the air sea interaction processes. But there still remain large uncertainties.

Traditionally in numerical models it is assumed that the Monin-Obukhov similarity is valid in the surface layer and that the integrated non-dimensional gradients can be used. Over the ocean it has generally been assumed that the non-dimensional gradients are the same as over land areas. Usually effects of waves are neglected or included in a very crude way.

There are presently a number of groups taking measurements over sea and recently there have been some very interesting new results concerning the conditions over the sea. To be able to fully understand the effect of these new results and the impact to the atmosphere and ocean they are implemented in different types of models and the results analysed. We use 3D regional limited-area climate model RCA developed for northern Europe to investigate the impact on the atmospheric conditions and a process-oriented ocean model for the Baltic Sea, for the ocean conditions.

Turbulent heat fluxes over sea (sensible and latent) are generally calculated using different types of bulk formulation including bulk-coefficients. These coefficients show in most investigations a scatter, which is larger than can be explained by uncertainty in measuring techniques and larger than is usually found over land. New results using data from the measuring station Östergarnsholm indicate that the process of Forced Convection is controlling the heat transport in the near neutral region (Sahlee et al., 2005). The data also shows that Monin-Obukhov similarity theory is not valid and that the stratification is not a controlling parameter for the fluxes. The new theories gives increasing heat transfer coefficients for small air-sea temperature/humidity differences and high wind-speeds. This is opposite from traditional parameterisations using the Richardson number, which gives larger heat transfer coefficients for large temperature differences and low wind speeds.

Waves have for long been assumed to have an impact on the air sea interaction, mainly during growing sea, where some increase of the surface roughness have been seen. More recent results have shown that during the presence of long waves (swell) the structure of turbulence in the near surface region is significantly different. This includes very small non-dimensional wind-gradients, low surface roughness and a failure of Monin-Obukhov similarity theory. The new formulations show a significant effect on the model results, in the ocean model, mainly the wave information is of great importance. For the atmospheric model the effect of Forced convection is controlling the heat fluxes during high wind-speeds. Including the effects of long waves changes the turbulence intensity and thus also fluxes during low wind speeds.