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Fluxes and Mixing Processes in the Marine Atmospheric Boundary Layer
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. (Meteorologi, AWEP)
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 [en]
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: urn:nbn:se:uu:diva-195875ISBN: 978-91-554-8606-8 (print)OAI: oai:DiVA.org:uu-195875DiVA: diva2:608612
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
List of papers
1. Flux attenuation due to sensor displacement over sea
Open this publication in new window or tab >>Flux attenuation due to sensor displacement over sea
2010 (English)In: Journal of Atmospheric and Oceanic Technology, ISSN 0739-0572, E-ISSN 1520-0426, Vol. 27, no 5, 856-868 p.Article in journal (Refereed) Published
Abstract [en]

When using the eddy correlation method to measure turbulent scalar fluxes, there is often a spatial separation between the instruments measuring the scalar and the vertical velocity. The attenuation of the flux due to this separation is studied here for marine conditions. Measurements of a two-point covariance between vertical velocity and temperature are compared to covariance measurements from collocated sensors for both horizontal and vertical displacements, with the purpose of finding the approximate functions to describe the flux loss for typical separation distances. On the basis of this study’s measurements, there is only a slight directional dependence (i.e., streamwise or crosswind separation) of the flux loss for sensor separation distances less than 1 m but an increasing dependence with increasing displacement distance. For a vertical displacement, observations from this study confirm that flux loss is less with the scalar sensor positioned below the velocity sensor than at an equal distance above. Furthermore, the data show a clear dependence on atmospheric stability with increasing flux loss for increasing stable stratification, but it is not as large as that found in previous studies of flux attenuation over land. For example, the authors compare estimated flux loss for neutral and moderately stable (z/L = 0.3) stratification at a measuring height of z = 10 m and a sensor displacement r = 0.3 m, where L is the Obukhov length. For neutral (stable, z/L = 0.3) stratification the estimated loss of flux is 3% (5%) of the total flux for horizontal displacement. Whereas for an equal vertical separation the estimates are 2% (4%) when the scalar sensor is placed above the anemometer but less than 1% (2%) if it is placed below. Thus, the authors conclude that placing the scalar sensor below the anemometer minimizes the flux loss due to sensor separation, and that a simple correction function can be used to quantify the mean flux loss due to sensor separation over sea.

Place, publisher, year, edition, pages
45 Beacon Street Boston, MA 02108-3693: American Meteorological Society, 2010
Keyword
Eddy Correlation Method, Scalars, Turbulence, Fluxes, Instrumentation, Sensors, Separation, Attenuation, Flux loss, Skalärer, Turbulens, Flöden, Instrument, Sensorer, Separation, Flödesdämpning, Flödesförslust
National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:uu:diva-125742 (URN)10.1175/2010JTECHA1388.1 (DOI)000277777600005 ()
Available from: 2010-05-27 Created: 2010-05-27 Last updated: 2013-04-02Bibliographically approved
2. Convective boundary-layer structure in the presence of wind-following swell
Open this publication in new window or tab >>Convective boundary-layer structure in the presence of wind-following swell
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.

Keyword
surface gravity waves, large-eddy simulation, turbulence length-scales, air–sea interaction
National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:uu:diva-172538 (URN)10.1002/qj.1898 (DOI)000308657000006 ()
Available from: 2012-04-11 Created: 2012-04-11 Last updated: 2017-12-07Bibliographically approved
3. Turbulent flux characterization using extended multiresolution analysis
Open this publication in new window or tab >>Turbulent flux characterization using extended multiresolution analysis
(English)Manuscript (preprint) (Other academic)
National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:uu:diva-195858 (URN)
Available from: 2013-02-27 Created: 2013-02-27 Last updated: 2013-04-02
4. Air-Sea momentum flux characterization in the presence of wind-following swell
Open this publication in new window or tab >>Air-Sea momentum flux characterization in the presence of wind-following swell
(English)Manuscript (preprint) (Other academic)
National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:uu:diva-195859 (URN)
Available from: 2013-02-27 Created: 2013-02-27 Last updated: 2013-04-02
5. Introducing surface waves in a coupled wave-atmosphere regional climate model: Impact on atmospheric mixing length
Open this publication in new window or tab >>Introducing surface waves in a coupled wave-atmosphere regional climate model: Impact on atmospheric mixing length
2012 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, C00J15- p.Article in journal (Refereed) Published
Abstract [en]

The marine atmospheric boundary layer is strongly influenced by the moving surface in the presence of surface waves; the impact depends on the wave conditions and the interaction with the atmosphere. Previous studies using measurements as well as numerical simulations with large-eddy simulations have shown that surface waves propagating faster than the wind (swell) alter the surface exchange as well as turbulence properties in the atmosphere. This impact is here introduced in a coupled wave-atmosphere regional climate model with a so-called E − l turbulence scheme (where E is the turbulent kinetic energy and l is a mixing length). A wave age dependent coefficient (here called 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. 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 impact for the studied areas is relatively minor for parameters averaged over 1 year, but for limited periods and specific situations the impact is larger. One could expect a larger impact in areas with stronger swell dominance. We thus conclude that the 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 regional climate models or global climate models.

Keyword
boundary layer height, regional climate model, surface waves, turbulent mixing, wave-atmosphere coupling
National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:uu:diva-173771 (URN)10.1029/2012JC007940 (DOI)000305155400001 ()
Available from: 2012-05-05 Created: 2012-05-05 Last updated: 2017-12-07Bibliographically approved

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