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Convective boundary-layer structure in the presence of wind-following swell
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. (Meteorologi, AWEP)
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. (awep)
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. (awep)
2012 (English)In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 138, no 667, 1476-1489 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2012. Vol. 138, no 667, 1476-1489 p.
Keyword [en]
surface gravity waves, large-eddy simulation, turbulence length-scales, air–sea interaction
National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
URN: urn:nbn:se:uu:diva-172538DOI: 10.1002/qj.1898ISI: 000308657000006OAI: oai:DiVA.org:uu-172538DiVA: diva2:514905
Available from: 2012-04-11 Created: 2012-04-11 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Fluxes and Mixing Processes in the Marine Atmospheric Boundary Layer
Open this publication in new window or tab >>Fluxes and Mixing Processes in the Marine Atmospheric Boundary Layer
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Atmospheric models are strongly dependent on the turbulent exchange of momentum, sensible heat and moisture (latent heat) at the surface. Oceans cover about 70% of the Earth’s surface and understanding the processes that control air-sea exchange is of great importance in order to predict weather and climate. In the atmosphere, for instance, hurricane development, cyclone intensity and track depend on these processes.

Ocean waves constitute an obvious example of air-sea interaction and can cause the air-flow over sea to depend on surface conditions in uniquely different ways compared to boundary layers over land. When waves are generated by wind they are called wind sea or growing sea, and when they leave their generation area or propagate faster than the generating wind they are called swell. The air-sea exchange is mediated by turbulent eddies occurring on many different scales. Field measurements and high-resolution turbulence resolving numerical simulations have here been used to study these processes.

The standard method to measure turbulent fluxes is the eddy covariance method. A spatial separation is often used between instruments when measuring scalar flux; this causes an error which was investigated for the first time over sea. The error is typically smaller over ocean than over land, possibly indicating changes in turbulence structure over sea.

Established and extended analysis methods to determine the dominant scales of momentum transfer was used to interpret how reduced drag and sometimes net upward momentum flux can persist in the boundary layer indirectly affected by swell. A changed turbulence structure with increased turbulence length scales and more effective mixing was found for swell.

A study, using a coupled wave-atmosphere regional climate model, gave a first indication on what impact wave mixing have on atmosphere and wave parameters. Near surface wind speed and wind gradients was affected especially for shallow boundary layers, which typically increased in height from the introduced wave-mixing. A large impact may be expected in regions of the world with predominant swell. The impact of swell waves on air-sea exchange and mixing should be taken into account to develop more reliable coupled Earth system models.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. 51 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1024
Keyword
Waves, Growing sea, Swell, Marine boundary layer, Air-sea interaction, Mixing, Momentum flux, Wind stress, Flux attenuation, Sensor separation, Large eddy simulation, Multiresolution analysis, Coupling, Wave model, Climate model, Wave age
National Category
Meteorology and Atmospheric Sciences Climate Research
Research subject
Meteorology
Identifiers
urn:nbn:se:uu:diva-195875 (URN)978-91-554-8606-8 (ISBN)
Public defence
2013-04-19, Axel Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2013-03-25 Created: 2013-02-28 Last updated: 2013-04-02Bibliographically approved

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Publisher's full texthttp://onlinelibrary.wiley.com/doi/10.1002/qj.1898/pdf

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Nilsson, ErikRutgersson, AnnaSmedman, Ann-Sofi

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