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Fifteen hours of consecutive swell data from the experiment Flux, État de la Mer, et Télédétection en Condition de Fetch Variable (FETCH) in the Mediterranean show a distinct upward momentum flux. The characteristics are shown to vary systematically with wind speed. A hysteresis effect is found for wave energy of the wind-sea waves when represented as a function of wind speed, displaying higher energy during decaying winds compared to increasing winds. For the FETCH measurements, the upward momentum transfer regime is found to begin for wind speeds lower than about U 5 4ms21 . For the lowest observed wind speeds U , 2.4 m s21 , the water surface appears to be close to dynamically smooth. In this range almost all the upward momentum flux is accomplished by the peak in the cospectrum between the vertical and horizontal components of the wind velocity. It is demonstrated that this contribution in turn is linearly related to the swell significant wave height Hsd in the range 0.6 , Hsd , 1.4 m. For Hsd , 0.6 m, the contribution is zero in the present dataset but may depend on the swell magnitude in other situations. It is speculated that the observed upward momentum flux in the smooth regime, which is so strongly related to the cospectral peak at the dominant swell frequency, might be caused by the recirculation mechanism found by Wen and Mobbs in their numerical simulation of laminar flow of a nonlinear progressive wave at low wind speed

Atmospheric and surface wave data from several oceanic experiments carried out on the Floating Instrument Platform(FLIP) and the Air–Sea Interaction Spar (ASIS) have been analyzed with the purpose of identifying swell-related effects on the surface momentum exchange during near-neutral atmospheric conditions and wind-following or crosswind seas. All data have a pronounced negative maximum in uw cospectra centered at the frequency of the dominant swell n_p, meaning a positive contribution to the stress. A similar contribution at this frequency is also obtained for the corresponding crosswind cospectrum. The magnitude of the cospectral maximum is shown to be linearly related to the square of the orbital motion, being equal to , where H_sd is the swell-significant wave height, the effect tentatively being due to strong correlation between the surface component of the orbital motion and the pattern of capillary waves over long swell waves.

A model for prediction of the friction velocity  from measurements of H_sd, n_p, and the 10-m wind speed U₁₀ is formulated and tested against an independent dataset of ~400 half-hour measurements during swell, giving good result.

The model predicts that the drag coefficient C_D, which is traditionally modeled as a function of U₁₀ alone (e.g., the COARE algorithm), becomes strongly dependent on the magnitude of the swell factor  and that C_D can attain values several times larger than predicted by wind speed–only models. According to maps of the global wave climate, conditions leading to large effects are likely to be widespread over the World Ocean.

A systematic comparison of wind profiles and momentum exchange at a trade wind site outside Oahu, Hawaii and corresponding data from the Baltic Sea is presented. The trade wind data are to a very high degree swell dominated, whereas the Baltic Sea data include a more varied assortment of wave conditions, ranging from a pure growing sea to swell. In the trade wind region swell waves travel predominantly in the wind direction, while in the Baltic, significant cross-wind swells are also present. Showing the drag coefficient as a function of the 10-m wind speed demonstrates striking differences for unstable conditions with swell for the wind-speed range 2 m s⁻¹ < U ₁₀ < 7 m s⁻¹, where the trade-wind site drag values are significantly larger than the corresponding Baltic Sea values. In striking contrast to this disagreement, other features studied are surprisingly similar between the two sites. Thus, exactly as found previously in Baltic Sea studies during unstable conditions and swell, the wind profile in light winds (3 m s⁻¹) shows a wind maximum at around 7–8 m above the water, with close to constant wind speed above. Also, for slightly higher wind speeds (4 m s⁻¹ < U ₁₀ < 7 m s⁻¹), the similarity between wind profiles is striking, with a strong wind-speed increase below a height of about 7–8 m followed by a layer of virtually constant wind speed above. A consequence of these wind-profile features is that Monin–Obukhov similarity is no longer valid. At the trade-wind site this was observed to be the case even for wind speeds as high as 10 m s⁻¹. The turbulence kinetic energy budget was evaluated for four cases of 8–16 30- min periods at the trade-wind site, giving results that agree very well with corresponding figures from the Baltic Sea.

The marine boundary layer is known to be influenced by fast long ocean swell waves travelling away from their generation area, where they were initiated by momentum transferred to the ocean wave field during storms. The atmospheric boundary layer during wind-following swell and various stability states has been investigated using large-eddy simulation (LES) data. The dominant energy-containing motions in the near-neutral atmospheric boundary layer over flat terrain are known to be dominated by near-ground shear-induced regions of high- and low-speed flow. Windfields and momentum fluxes from LES for swell-dominated situations have been used to interpret field measurements suggesting that these motions are disrupted by effects related to the underlying wave field in the presence of swell waves. Statistical analysis and visualization are used to further describe the effects of stratification during swell for convective boundary-layer winds and fluxes. A mechanism for transport of momentum to the upper levels of the boundary layer is suggested from interpretation of LES data. Coherent detached eddies from the directly wave-induced motions near the surface are found to maintain an upward momentum transfer. This mechanism is found to strengthen during stronger swell conditions and also during slightly convective conditions. In this way, it is argued that processes related to both the wave field and surface convection can have a significant influence on the global structure of neutral and convective boundary layers during swell. This has implication for the turbulence length-scales during wind-following swell.

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.

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.

Combination of surface water cooling and a deep ocean mixed layer generates convective eddies scaling with the depth of a mixed layer that enhances the efficiency of the airsea transfer of CO2 (and possibly other gases). This enhancement is explained by the convective eddies disturbing the molecular diffusion layer and inducing increased turbulent mixing in the water. The enhancement can be introduced into existing formulations for calculating the air‐ sea exchange of gases by using an additional resistance, due to large‐scale convection acting in parallel with other processes. The additional resistance is expressed here as 1rwc=g (w*qu*w , where w*u*w characterizes the relative role of surface shear andbuoyancy forces