Influences of surface gravity waves on atmospheric boundary layer structure and fluxes

Atmospheric models are strongly dependent on the turbulent exchange of momentum and scalars at the air-sea interface. Surface gravity waves have been shown to influence the exchange process, but these effects are often poorly represented or neglected in weather and climate models. A better understanding of the mechanisms behind wave-related turbulent exchange can be reached from direct measurements and high-resolution numerical modelling (Sullivan et. al. 2008, 2010). Using large eddy simulations (LES) and field measurements we investigate and show how surface gravity waves affect atmospheric boundary layer turbulence and fluxes.

Previous studies have shown that the atmospheric response to fast, long waves propagating away from their generation area, referred to as swell, can become different from conventional shear-driven boundary layers influenced by atmospheric stability. We have here used the LES model from Sullivan et. al. (2008) to examine the dynamics of the marine boundary layer under influence of swell waves (Nilsson et. al. 2012). This model has the capability to resolve a moving sinusoidal wave at its lower boundary and is used to study the atmospheric influence caused by an idealized dominant swell wave and its effects on turbulent flux. The modelling results show that wave-induced motions leads to altered mean wind profiles and increased turbulence length scales in dominant swell conditions. Also turbulent fluxes are affected by the presence of swell. For a more detailed understanding of the mechanisms we investigate vertical momentum flux using a multiresolution analysis technique (Vickers and Mahrt 2003). Preliminary results indicate that upward directed momentum flux is often closely related to the vertical wind variance. In most atmospheric situations this upward momentum flux is however compensated by a larger downward directed momentum flux related to shear-induced streamwise oriented wind streaks. In low wind speed situations with dominant swell waves such motion are however modulated and in extreme cases the upward directed momentum flux can even exceed the downward directed flux, causing a net upward Reynolds-averaged flux.

In addition to numerical simulations we have carried out multiresolution analysis on measured turbulence signals for different atmospheric conditions to characterise air-sea interaction during swell in comparison to other boundary layer processes that are also present over flat terrain and growing sea conditions. Field measurements from several sites are used in the analysis; among them is data from our main observational site Östergarnsholm located on a small flat island in the Baltic Sea. This site has been used in several previous studies of air-sea interaction, coastal meteorology and studies of the atmospheric response to surface waves. The analysis of the measurements support the LES results in that upward momentum flux is related to the vertical wind variance and in low wind speed situations this can dominate the total net flux. For growing sea conditions with higher wind speed and stronger shear the net flux becomes downward, however, due to a much larger downward directed flux component.

One of our long-term research goals is to better understand the role of surface wave processes, then include such effects in large-scale atmospheric models, and evaluate the sensitivity of the atmospheric circulation on such modelling attempts. Similar to the effects of wave-induced mixing in the presence of non-breaking surface waves for the oceanic circulation (Qiao et. al. 2004), we find that swell waves may be responsible for increased mixing of the atmospheric boundary layer. This wave-induced mixing can in near-neutral conditions with swell act to cause similarity to more convective atmospheric states (Nilsson et. al. 2012). A simplified parameterization with an inclusion of a wave-field dependent mixing length formulation has therefore been suggested and implemented in a regional climate model coupled to a wave model (Rutgersson et. al. 2012).

In conclusion we believe a better understanding of surface wave processes is needed to build model parameterizations for the marine atmospheric boundary layer. We have therefore continued with new analysis of field measurements and LES using a multiresolution technique to investigate the atmospheric response to the surface waves that separates the atmospheric and oceanic boundary layers. This new investigation reveals wave signatures in the atmospheric variables and distinguishes some of the effects that surface gravity waves causes for the near surface turbulence structure and fluxes.

References:

Nilsson, E. O., A. Rutgersson, A.-S. Smedman and P. P. Sullivan. 2012. Convective boundary layer structure in the presence of wind-following swell. Quarterly Journal of Royal Meteorological Society. In press

Rutgersson, A., E. Nilsson and R. Kumar. 2012. Introducing surface waves in a coupled wave-atmosphere regional climate model: Impact on atmospheric mixing length. Journal of Geophysical Research – Oceans. Accepted

Sullivan P., J. McWilliams and T. Hristov. 2010. Large-Eddy Simulations of high wind marine boundary layers above a spectrum of resolved waves. 19th AMS symposium on Boundary Layers and Turbulence.

Sullivan PP, JB. Edson, T. Hristov and JC. McWilliams. 2008. Large-Eddy Simulations and Observations of Atmospheric Marine Boundary Layers above Nonequilibrium Surface Waves. J. Atmos. Sci. 65:1225-1244.

Qiao F., Y. Yuan, Y. Yang, Q. Zheng, C. Xia and J. Ma, 2004. Wave-induced mixing in the upper ocean: Distribution and application to a global ocean circulation model. Geophys. Res. Lett. 31, Ll1303.

Vickers D., and Mahrt L. 2003. The Cospectral Gap and Turbulent Flux Calculations. Journal of Atmospheric and Oceanic Technology 20: 660-672.